current limiting lithium battery charging (by means of cable)

[prelude]

So, as some might know I am NOT a big fan of lithium batteries but I can not deny the one (and only important to me at least) feature they have is that they are lighter and weight of the vehicle is always a challenge. This means that I have been looking into what I can and cannot do with them and how I would integrate them into my system.

My understanding is that:

lithium likes to live between 20% and 80% SoC for longevity
has an internal BMS that looks after the health of the cells
the BMS does NOT limit current except for over current conditions
will kill your alternator if you hook it up directly
can handle fairly large currents but not winch-power kinda currents
prefers lower charging currents (ie 1/3 of the max current usually)


I currently have a CTEK setup with which I am most happy. It is a lot lighter than the equivalent redarc or victron stuff, it can handle huge currents and it has an integrated mppt charger which is nice. Most setups with lithium I see only use a DC-DC charger and are not directly connected to the main battery/alternator. I guess that is to alleviate the problem of the alternator getting stuffed or the charge current into the lithium battery going way over. Having said that, I would like to charge my batteries with near as much as my CTEK setup can deliver which is a theoretical maximum of 140A. I'll settle for 100.

Looking into what my (upgraded) high power alternator can deliver, with 100A of draw into the battery system I would still have around 100A left for the rest of the vehicle which in pretty much all conditions should be enough. It should also prevent the thing going up in smoke... The CTEK DC-DC charger can provide only 20A of charge current so a DC-DC only setup is not ideal imo. Hooking up the smartpass however would, in an ideal system with no losses, push the maximum current from the alternator/starter battery into the lithium battery causing the BMS to shut down I should think, the smartpass to be overloaded eventually and undoubtedly damage the batteries and alternator in no time.

With all this in mind I have been looking into a way to limit the current to my set goal of 100A and it struck me that cable diameter which is usually our enemy can actually be a friend here :)


The internal resistance of a victron lithium battery (I used that as a respected brand example) is 0.9mOhm or 0.0009 ohm. For the sake of simplicity we'll just say it's zero since it is so near as to make no difference.
the cable run from my main battery to the CTEK setup is 5 meters
I am assuming zero ohm for the connectors, fuses, internal resistance of the CTEK, etc.
100A maximum charging current as mentioned
14V at the alternator


Under normal circumstances one would choose a very beefy cable to handle such currents without too much loss, something like 95mm2 cable or 3/0 AWG. However, if I were to pick 35mm2 or 2 AWG cable the internal resistance would create around 5% drop in voltage, which lowers the voltage over the lithium battery terminals and thereby effectively limiting the current draw. Regarding safety 95A is the max a 35mm2 cable is rated for and if I look at the numbers I would be dissipating some 70watts over the entire cable run. Over such a long and thick cable it would hardly get warm so I think that should be fine.

So far this has been the only "simple" solution I can think of that does not negate the advantages of lithium over lead acid. I know sterling power (and other brands for that matter) have DC-DC chargers with more power then the CTEK but they weigh a LOT more which kinda defeats the purpose (reminder, my main concern was weight)

All this is of course theoretical although the physics and maths are correct afaict but not owning a lithium battery and not being able to find charging voltage vs current tables I am not sure if such a voltage drop would be sufficient.

Anyone here with some experience, figures or other thoughts ?

Cheers,
-P

[drivesafe]

Hi Prelude, first off, a lithium battery will NOT damage your alternator.

Next, because you have a vehicle with a SMART alternator, you will not be able to charge the lithium battery directly from the alternator. You MUST use a DC/DC charger, and this must be matched to the lithium battery’s maximum continuous charge current.

If you try to charge the lithium battery directly from the alternator, your lithium battery will not get charged much over 20% capacity and even if you fully charge the lithium battery with a battery charger before you go for a drive, when you go for a drive, you will continuously discharge the lithium battery.

Beginning a trip with a fully charged 100Ah lithium battery, after just 2 hours of driving, your lithium battery will have been discharged down to around 20%.

[prelude]

Thanks for your reply :)

I am not sure my P38 already has a smart alternator, I believe it to be of the "stupid" kind still? It always measures 14volts at the terminals in any case. Further, I do not doubt a smart alternator will withstand it (since it would regulate back) but both victron and sterlingpower seem to suggest it to be a risk. Although victron uses a simple alternator in their video, sterling used a bosch unit that did not blow up in the video but did get very hot indeed. (he used a FLIR camera)

Lithium to Alternator Direct Charge Test : Why current limiting is vital - YouTube (https://www.youtube.com/watch?v=ShtGB07fCSs)
How to not blow up your Alternator when charging Lithium - YouTube (https://www.youtube.com/watch?v=jgoIocPgOug)

Regarding the voltage. a fully charged lithium battery should read 13.2v so I can see how with a bit of cable loss here and there a lithium battery could discharge over time indeed if it were connected to the vehicle directly. The Smartpass unit however is effectively a "relay" or diode between the main and leisure battery. Still, upon further reading I find that a lithium battery needs 14.6 volts to charge and I guess it is not like lead acid in that it does not charge with a lower voltage? It is a chart like this that I was looking for, how much charge at what voltage.

Normally I would take on the experiment myself but you guys are lucky down under in that you can find a reasonable quality lithium battery for say 1000 AUD, I only have the choice between "cheap" chinese for 1600 AUD at the very least or a victron or similar for a whopping 2100 AUD and I am not in a position to buy that for a test -g-

Anyway, if I can't get it to work in my current setup then I guess lithium is out...

Cheers,
-P

[PhilipA]

Still, upon further reading I find that a lithium battery needs 14.6 volts to charge

No a Lithium will charge at 14.4 or even lower and slower.

14.6 is the maximum safe voltage to charge and the BMS will usually cut off if more than 14.6 is present.
I have my DC/Dc charger set at 14.4V.
Regards PhilipA

[drivesafe]

Hi again prelude, and while I had not seen the Sterling video before, I did see the Victon video when the first released it.

Charles Sterling has simply copied the Victron “AD” video and both are based on the same stuff a bull drops on a barn floor.

Both videos are based on a deliberate “designed-to-Fail” principal and one you could not duplicate in the real world.

Both videos fail to point out the fact that for an alternator to be running at its full capacity, the alternator has to be spun at a given RPM, and that means the motor has to also be turning at a given RPM.

For any motor to be run at the alternators optimum RPM, you would probably be doing at least 50KPH and that would be pumping a HUGE quantity of air through and around the alternator, which stops the alternator from cooking the first place.

My Isolators have been around for more than 30 years and if the batteries are low, they will cause the alternator in any vehicle to run at full bore, with a vehicle towing a caravan with three large batteries, all in a low state, the alternator could be running at full power output for four or more hours and they never burn out.

Those two videos are nothing but a con to try to sell people something they never needed in the first place.

ALSO NOTE, if your P38’s alternator runs at 14.0v ( no higher than 14.7v ) then you can charge lithium batteries directly from your alternator. The only limiting factor, one you already know about, is the maximum CONTINUOUS charge tolerance of the lithium battery you choose.

Also note, you can full charge most lithium batteries with as little as 13.8v, it just takes longer to fully charge them.

[prelude]

hehehe, I DO have a large barn so there is enough space for those droppings... but I get what you are saying. Too bad that even fairly reputable vendors go along with such trickery. The only time I ever saw burned out alternators is on pleasure yachts, indeed insufficient cooling on very old alternators but that should not be a problem in a car.

Anyway thank you for the clarification. Lower voltage is lower current is what I was thinking. I'll just have to go and find some data on the lithium batteries that are available here to see what the average charge current is with a given voltage to see if reducing the cable diameter provides enough voltage drop to reduce the current AND keep things safe. You would not happen to have a voltage vs current table laying around for a 12v lithium battery would you? [wink11]

Cheers,
-P

[drivesafe]

Unfortunately, cable thickness alone, is not the only factor needed.

You require a stable constant current draw, set cable size and known cable length.

The first, stable current draw, is not a constant when charging any form of battery.

So your are better off looking for a lithium battery with a safe high charge current capability.

Have a look at SolarKing 100Ah lithium batteries!

[prelude]

So, some time has passed and many factors made this experiment take a while but! I have a bunch of results that may be of some use for others so here goes:

In the end we down in Europe can't buy all the brands that you have access to in oz and have been suggested to me so I had to make do with what was available. Victron is a brand I like but a single lithium battery from these guys will set you back 2k+ in euro's! which is a bit much. In the end I settled on 170Ah Renogy lithiums. The maximum charge current is 85A and the charge voltage is specified as 14.4V I have ordered two so in theory the maximum charge current could go as high as 170A which is far higher than I can provide (see the first post) and keeps me well within spec, which is what I like.

The other gear used:

2 x 50mm2 (1 AWG) cable to interconnect both batteries, 50cm length of cable.
2 x 50mm2 (1 AWG) cable from the batteries to the CTEK output and to ground. 5M in length each.
500A shunt, part of the BMV-700 by Victron
2 short lengths of cable to connect the input of the CTEK to the alternator and from the shunt to ground. Both 35mm2 (2 AWG) as far as I can tell.
800VA victron inverter, peak 1500watt power for very short periods.
900watt regulated switching mode power supply 0-60V 0-60A
750watt Meanwell switching mode power supply 15V adjustable between 13.5V to 16.5V


* All cables had PROPER lugs crimped on with the appropriate tool so losses in connections should be down to near zero and there was no chance of one failing and causing grief.

The first thing I was able to test, finally, was my "new"(had it for two years now:) ) high output alternator since I never had anything to load it up with decently. A few observations:

Output at idle was 75A which was the most I could pull, see further on in this post, the voltage remained stable at 14V at all times. Also, the starter battery and the car itself use some power when running, the shunt was only connected between the batteries and not the entire car so it probably provided even more but we can't be certain.
though I am sure the engine ECU is keeping the RPM steady where possible but I did not notice the RPM drop when the CTEK kicked in or the inverter was switched on.
if only I had a 200amp dummy-load to test the curve of the alternator :)


In any case, the alternator works as advertised and it should not be the bottleneck in further testing, apart from the output voltage being "only" 14v, see further on.

Now for the good part; the observations regarding this setup and most importantly: do cables work as current limiter? Well the answer is, of course, yes and a bit too well as it turns out. With two 5m runs of 50mm2 (1 AWG) the voltage drop as calculated would be around 1.75% which means the voltage would drop from 14V at the source (alternator) to around 13.75V at the battery terminals. This all with a current of around 50Amps which is what was observed. In reality the voltage dropped a bit further down to 13.7V since there is (minimal but still) loss in the CTEK which uses MOSFET switches, the shunt (how else to measure current) and the small connector leads between the CTEK and the source and the shunt and the ground as mentioned above.

The maximum charge current I observed dropped to around 50Amps once the system stabilised after starting up like I mentioned above which means I am just about making what the top model Redarc would provide. Mind you, it is still al lot more than my D250SE which tops out at around 20 Amps. So, in order to load things up a bit further I connected the inverter at the terminals of the batteries and put a small heater on the lowest setting. The current remained roughly the same but that is because the voltage drop does not bother the inverter, just less was going into the batteries. Switching the heater to full meant the warning/overload led on the inverter switched on and the current increased to around 75A (the figure I measured when testing the alternator at idle). This time it all went into the inverter and just about nothing went into the batteries, turns out that I actually used some of the battery capacity as well. In the test below I ran the inverter at full tilt and it climbed as far as 90A input.

This to me seemed to suggest that the lithium batteries would not charge fast enough with only 13.6-13.7 volts at the terminals. In order to do some more testing I put the entire setup on the test bench and used two ~60A power supplies in parallel to see if we could get the same results and if we could push more. Since the Meanwell does not switch on if it senses power at its terminals I first connected it to the input of the CTEK and turned it on. I ramped the adjustment screw up until the power supply could not provide any more and the load regulation kicked in. At that point I was pushing around 58Amps. Next I switched on the other supply and started regulating up until it picked up load after which the Meanwell would start pushing less so I had to turn that up and so it went. In the end the 900watt supply pushed 62.5Amps and the Meanwell topped at around 58 Amps. The BMV-700 showed I was pushing over 118Amps into the batteries including power losses in the cables, shunt and CTEK. The voltage over the output terminals of the power supplies showed around 14.5 volts at that time whilst at the battery terminals all the way at the end of the chain I would measure exactly 14.00 volts. Most of the losses were measured at the long lengths of cable, more than 350mV to be exact, the rest were losses in the short hookup leads shunt and Smartpass.

Of course, 14.5 volts is too much for the lithium batteries, borderline BMS cut-off so we can not maintain it since the voltage would climb as the batteries would reach a full charge but I made sure the batteries were quite empty when I did the test to have plenty of time to do some measuring and testing without causing any trouble.

I also did a number of other experiments that are less relevant for this story but I think we can come to some conclusions here for now:


If we could get the 14volt output of the alternator at the battery terminals I would be able to charge them at around the maximum rated capacity of the Smartpass and reduce recharge times significantly. The reason I want the batteries to charge faster by the way is two-fold, first of it is one of the perks of lithium and since I have them I want to utilize them because second; running around with 340Ah of batteries means that from empty to full I would need to drive for 7 hours straight just to recharge them. It's kinda the same problem lead/acid batteries were scolded for. Never mind if you need to do it using a DC/DC charger off a smaller size.
Even 50mm2 (1 AWG) cable has losses that are too high to reach a decent charge current. I can increase the diameter of the cables but it will add a lot of weight! I also own a fully programmable 50A DC-DC charger that I could couple in tandem with my CTEK for a total of 70A. That unit would probably weigh the same as the increase in cable diamater but I still need to check that out.
Going even bigger in cable diameter does become a bit unwieldy to install in a car. 50mm2 (1 AWG) is already fairly tough to get through small holes and bend around. Admittedly, this is not boom-box-car cable which has a lot of tiny strands that make them more flexible, this is cable used in industry for high voltage applications but still. I have two runs of 95mm2 (3/0 AWG) from the alternator to significantly reduce voltage drops to my start battery since the default cabling in the range rover is a bit small and those are quite the effort to get into shape!
The cable diameter is large enough that there is no danger of overheating or fire in my setup at least. Since 50mm2 (1 AWG) will not even get warm with just 50Amps moving through it, even bigger cables would not present a problem in the safety department, sure the current would rise due to less voltage drop but I reckon that going up to 70mm2 (2/0 AWG) would not even be enough to get the current up to 120A since that would still present a 1.2% voltage drop. 95mm2 (3/0 AWG) would take that down to 0.9% "only" and a cable like that is certainly rated for such currents.
Finally, raising the output voltage of the alternator would make a huge difference, though the starter battery would get a bit too much voltage I think to survive for very long and I am not even sure this is possible. The voltage regulator must be built in to this particular model of alternator and I am not sure it can be adjusted at all, perhaps with some extra circuitry?


It might turn out that large lithium installations simply can't be charged within their rated capacity using "normal" means at all. That would be a bit sad. In any case, it has been quite the journey so far and I intend to continue the experiments, not least because I have access to some left over lengths of thick industrial cabling that makes experimenting rather cheap.

Cheers,
-P

[sharmy]

So, it looks like my grand Idea to change my 2 100 amp agm batteries in my ute to one 300 a/h lithium and simply change my 25 amp dc to dc and separate 30 amp solar controller to lithium mode won't work--- or will it?

[drivesafe]

So, it looks like my grand Idea to change my 2 100 amp agm batteries in my ute to one 300 a/h lithium and simply change my 25 amp dc to dc and separate 30 amp solar controller to lithium mode won't work--- or will it?
Hi shady, that will work but be aware, to recharge that size battery with a 25 amp DC/DC device, if the battery is flat, you're be looking at at least 15 to 20 hours of drive time to get that battery up to 100% SoC.

[sharmy]

Hi shady, that will work but be aware, to recharge that size battery with a 25 amp DC/DC device, if the battery is flat, you're be looking at at least 15 to 20 hours of drive time to get that battery up to 100% SoC.
Who's shady? It's good to know that it will at least work, I travel from Bundaberg to WA each year so the hours are no bother and then I have solar to keep it topped up. Thanks diver.

[Blknight.aus]

FWIW.

dont us industrial high voltage cable thats hard to bend into good radius, its got heavy strands in it and they are prone to fatique hardening and fracturing internally from vibration. Not usually an issue in a building something of an issue in the automotive world. the high voltage rating comes soley from the insulation rating. They also have a larger airgap inside the wire strands which in the high voltage world ac world isnt such a bad thing but in the low voltage DC world where every mm2 of copper counts not so good, For example I've seen 85mm2 industrial cable with the heavy strand core carry current more like a 65mm2 welding grade cable.

The bestest cable is the finest strand cable in the dimensions you need, I run 65mm2 cable wich is is roughly 40000000 and counting strands of .3mm or finer copper. I mostly get mine from the sort of places that supply welding cable, and typically it has a good quality inner insulation and a lower quality but physically durable external sheath..

Dont rely on your cable as any form of current limiting in speccing up a system, in that direction melty insulation shorts and fires lie. Most things electrical have 3 power ratings which is their inrush or start up current which is nominally only ever for a second or 2 their surge which is a good whack higher than their normal running current and their running current. Manufacturers generally wire everthing to deal with only the run current as generally temps and conditions from the surge and inrush currents are only brief and the small build up easily dissipated once normall running loads develop, IMHO, build your cables to sustain the surge current and voltage requirements.

Always account for the drop on your cables.

My current pick of the bunch for value point and capability. is the kings 120 AH lithium with their 45A DC/DC. its about the right feed to get your battery charged from 20-80% within 3-5 hours of driving (depending on your load while driving) AND it has an inbuilt solar regulator that will take in the order of 250w of solar panel directly and the battery has enough give in it to run their 1500w inverter for most things (just dont expect it to deliver full surge or start power if the lithium is below about 60%)

they come up on their website as a combo occasionally. but that said. Redarc are now making a DC/DC that is somewhat better AND features back charging from the Solar side, which means if the aux battery is charged, it will then float charge the start battery from the solar with whatevers left over for whats going to the load. Nice, but with a nice price.

And yes I'm still using my cheap aldi stuff to charge things, the blanket connects to a Y cable, one goes to the DC/DC for the lithium the other into the cheapy regulator for topping off the standard battery.

[prelude]

Thanks for your input.

You are right of course to not advice using cable as current limiter since not everyone knows how and what. Hence, I do not advice it but I am game for the experiment myself and I am sharing my story here for others to enjoy/learn from perhaps.

Regarding the HV cable, I got some pieces for free and I am using them purely as test on the bench for now to get a feel for the setup and to test my hypothesis. It would be good to see if the fine stranded stuff has a lower internal resistance though so thanks for that tip!

Unfortunately once you get into the higher capacity battery banks it is neigh impossible to decently charge with a DC-DC converter/charger since the highest current units that I know of top out at 50A. I am running almost 350Ah of battery bank and from 0 to 100 would take 7 hours to charge and let's not forget, whatever you are running on that aux set has to be fed as well. It's not like the alternator is providing power to the users and the charger just charges the batteries so in reality 8 or more hours would be needed to charge the bank. Running multiple DC-DC units in parallel should be possible I guess but that runs into some other problems.

For now I am personally not worried about hot cables since I am so very close to the optimum charge voltage with my alternator that any voltage drop already reduces the charge current into my batteries. It puts out a steady 14.0 volts whereas lithium wants to be charged with 14.4 optimally. So the bulk charging phase will be limited by the voltage drop over the cable and as I described above even a 50mm2 run of cable will already cause a voltage drop sufficient enough to not be able to put out more than 50-60A into the battery. In the end when the voltage of the alternator is reached the DC-DC takes over to top up. I recently tried with two parallel 50mm2 cables and I topped out at around 88A which is more than feasible (and safe) for the cabling (does not even get warm to the touch) and since the voltage of the alternator can't climb any higher anyway (unless it's broken) I feel like I, in my situation, will be fine. once I have the setup planned out and I know what my cable lengths will be I will perform another test to see if the current stays within limits and I'll vary the length / diameter of the cable to adjust. I will, naturally, install fuses on both ends so if the system by accident could/wants to push more current, the fuses will blow.

Nothing wrong with cheap stuff when and if it works :)

Cheers,
-P

[Blknight.aus]

this may be a little long winded and all over the place so please if something doesnt make sense ask and I'll clarify it,





Unfortunately once you get into the higher capacity battery banks it is neigh impossible to decently charge with a DC-DC converter/charger since the highest current units that I know of top out at 50A. I am running almost 350Ah of battery bank and from 0 to 100 would take 7 hours to charge and let's not forget, whatever you are running on that aux set has to be fed as well. It's not like the alternator is providing power to the users and the charger just charges the batteries so in reality 8 or more hours would be needed to charge the bank. Running multiple DC-DC units in parallel should be possible I guess but that runs into some other problems.


for openers, the largest DC/DC style charger I've had my hands on that runs at the 12v nominal was 200A, Im aware of the following nominal amperages in increments of 5 from 5-50 in 10s from 50 to 100 and then in steps of 50a upto 400a. 120A DC/DC is not uncomon, heres a page one google result
12v to 12v Sterling Power ProBattC IP68 BBW12120 waterproof DC input battery to battery charger (https://www.sterling-power-usa.com/BBW12120-120amp-sterling-power-probattc-batterytobatterycharger12vto12v.aspx)
IF your alternator had the berries to drive its potential 2000w demand that would charge a 12v 350A lithium bank from 20% to around 90% in say 3.5 hours if you had no load on the batteries. buy that stage you would be running something like 120mm2 cable

I know of, by third hand knowledge, DC/dc that supposedly do upto 800A at 12V. I suspect that to be a little odd as by the time you get to those kinds of power demands you're getting into the 24/48v realm if not higher.



for some clarity on the configuration Im running and to offer up some insight as to how I'm configured when Im running the whole box and dice. The "loads" that wind up on my system vary greatly, and can be a mix of
intverters (150w, 300w 600w 1500w 3000w)
3x fridges
12v oven, jaffle maker, kettle
compressors/inflators
lighting
upto 4x 12v electric blankets (around 55w each when on flat out)
a couple of small fans
a diesel heater (10A start ~750ma run)
pc and screen (about 6A for both)

Normally the lithiums will run a small camp each for 1 or 2 nights and then be remounted to the vehicle and charged on the way to the next site or if staying over 2 nights have their solar panels connected.


In total my system (when its all mounted) is 2x n110 (equivelent) in parallel for the winch, big inverter and the startermotor plus the piffling of vehicle electrics this has its own dedicated 90W permanently mounted panel for maintenance charging and keeping the vehicle mounted fridge happy.
1x n70 stock lead acid battery with no built in charge regulation. This is the back up battery box (about to be retired for a not insignificant ultra capacitor bank) whose primary use is for boost starting vehicles.
2x 120AH lithiums in 2x battery boxes with a 45A DC/DC each which have the ability to charge from solar direct (160 or 200w panel/blanket) or, any other 12V (and supposedly 24v) dc source (alternator or the battery bank if its over 13.2v)
My loads are split across the lithium batteries and the batteries can not back feed the vehicle system unless I manually jumper to the output side of the lithium battery boxes. (which BTW will after about an hour will at the cost of the lithum batteries capacity recharge the 2 n110s from ~9.5v sufficiently for a start)
I can (and often do) when the lithiums are both low, connect the loads directly to the vehicles electrical system (the 2x n110s) so the DC/DCs have nothing to do but charge the lithiums.

This is done because I sometimes have to remote supply power for a fridge/lights/phone charging and unless I need to remote the medium inverter the 2 lithium batteries dont get hooked in parallel and unless I need to run a couple of special bits of kit they dont normally get hooked up in series. IF hooked in series they are never connected to vehicle power but may have their own independent solar panels connected.



For charging, when the engines running when the alternators sitting on 13.4v both my DC/DC units pull around 55A for about 20 minutes whorfing down almost all of the alternators capacity and definately gobbling up the paltry amp or 2 the onboard solar puts out on average.Fortunately being a mechanical diesel, and a slow vehicle I dont really need bright lights.......




For now I am personally not worried about hot cables since I am so very close to the optimum charge voltage with my alternator that any voltage drop already reduces the charge current into my batteries. It puts out a steady 14.0 volts whereas lithium wants to be charged with 14.4 optimally. So the bulk charging phase will be limited by the voltage drop over the cable and as I described above even a 50mm2 run of cable will already cause a voltage drop sufficient enough to not be able to put out more than 50-60A into the battery. In the end when the voltage of the alternator is reached the DC-DC takes over to top up. I recently tried with two parallel 50mm2 cables and I topped out at around 88A which is more than feasible (and safe) for the cabling (does not even get warm to the touch) and since the voltage of the alternator can't climb any higher anyway (unless it's broken) I feel like I, in my situation, will be fine. once I have the setup planned out and I know what my cable lengths will be I will perform another test to see if the current stays within limits and I'll vary the length / diameter of the cable to adjust. I will, naturally, install fuses on both ends so if the system by accident could/wants to push more current, the fuses will blow.

Nothing wrong with cheap stuff when and if it works :)

Cheers,
-P

Now I may be reading what your writing incorrectly but it reads to me like you might not be accounting for the increase in current as the charge system voltage drops.

If everythings working right, once your supply voltage gets to cut in voltage for the DC/DC the alternator should ramp up to its maximum required wattage or until it hits its maximum power rating

In my case when the alternator hits 13.4V the DC/DC cuts in and all 110 amps of the alternator get burnt up with no rise in system voltage, but checking the output of the DC/DC and I'm getting 14.0 and rising on the lithium batteries. This carries on until the lithiums have sponged up enough amps for the input demand on the DC/DC to drop off and then the system voltage begins to slowly rise. Its the whole W=VxA thing.

When Im winching or running the inverter hard and the main batteries (standard lead acids) have dropped down to around 10v the alternators only putting out 10V but its doing so at something like 150A until the main batteries pick up some charge and then the amps drop off as the voltage come up to the point where the DC/DC can kick in.



Im interested to see where your experimentation with the coarse strand industrial cable VS the finer stranded high flex goes.

[prelude]

Long winded is fine :) I can be a bit, shall we say, verbose as well, allow me to be verbose.

I knew about sterling power but not that they had a 120A unit. It might be worth looking into but it does look like a hefty device (weight and price though for the current it looks very decent). Mind you I am not against DC-DC chargers it's just that I have already invested in the gear I own and I was/am looking into a way to get the most bang for my buck, ie let it run at/near it's rated capacity.

It sounds to me like you have more than enough gear to empty your batteries. My most power hungry device will be an AC, the rest is a fridge, some lights and some computer gear. Its the AC that will pull the most and more importantly, for long durations of time. Mind you, we only intend to use it in extremes ie. 42c with high humidity just to make it through the night.
On to the meat of the subject, charging voltage/power etc. I agree bright lights are overrated. [bigrolf] but I do get your point. The alternator will put out the set voltage until you (over)load it is what you are saying? The way I understand an alternator regulator works is it adjusts the current through the rotor which in turn determines the output of the stator. Once you reach maximum output on the rotor, the stator voltage is going to drop when the load is further increased. I should think though that by that time you are over the rated capacity of the alternator and with that the diodes as well.

Although it IS true that with the same amount of power requested from a power source, the current goes up as the voltage drops, I do believe it does not work the other way around. Yes with a DC-DC charger, if it can deliver 50 amps at 12V (for this example) it will result in 600 watts of power put into the secondary battery at all times since the buck/boost circuit will maintain the output current untill it reaches a set maximum voltage. The same goes for an inverter for that matter. When your input voltage (ie starter batter / alternator) drops to 10 volts and the DC-DC has not cut out the same 600 watts going into the secondary battery will draw 60 amps from the starter battery/alternator. However, my system has only got a 20 amp DC-DC converter (and also MPPT solar input) so on the whole, the bulk of the charging current is sent through the smartpass (hence the name) which does not regulate the voltage in any way shape or form. This is all assuming no losses in the devices which is close to zero for the smartpass but somewhere around 10% for a DC-DC charger or inverter.

In other words, with the smartpass when the charge current to the batteries increases, more voltage drop over the cable occurs in turn reducing the maximum current. The equilibrium point will be reached instantaneously of course since electricity goes real fast but still :) In fact you you have observed this in your system since you have two 45A DC-DC chargers and that might even be an argument for me against such a setup. (note: for me. It works for you so no arguments there from me!)

As noted I am not a huge lithium fan for reasons but they do have a few upsides. Apart from weight their nominal voltage is higher than a lead-acid battery so the amount of time my starter battery/alternator would have to be drawn down to say 10volts as you used in your example would be minimal. The voltage of a lithium battery would very quickly be over 12volt and thus not impact my cars original electrical setup for long if at all, ie draining my starter battery temporarily. In fact, both the smartpass and the D250SE will prevent that from happening.

Consider this table, from the manual:



cut in voltage
cut out voltage


D250SE
>13,1V
<12,8V


smartpass
>13,1V
<12,8V or V alternator < aux batt


There is a 5 second hysteresis on those values. That means that even if I would over draw current it would only be for 5 seconds at a time since the input voltage would drop below what the system can deliver. I do not expect that to happen but I will test this at some point.

Also, if my engine is not running, the voltage over the lead-acid battery will very quickly drop below 12,8volts and everything shuts down. Also, 12,8volts in general will not be high enough to charge the lithium batteries unless they are really empty so the smartpass will not engage and pass any current and only the D250SE DC-DC charger will at most push 20 amps for a short duration. When the smartpass kicks in, which is basically a relais but not made of mechanical bits but solid state MOSFETS it will draw/pass through all the power that is requested on the other end. Since it is rated for 350A no longer than 10/30 seconds in/out it can do way more than both my battery and/or alternator can push through. Also that is longer than the BMS in the lithium batteries will allow an over current during charging for (max 85A per battery). Now, my alternator CAN put out 200A according to the manufacturer so if I want to avoid overloading the smartpass I need to regulate the current somehow and the only way to do that is to reduce the voltage, hence, use voltage drop over a cable!

This way the alternator voltage will never drop below it's preset 14 volts since it can handle the current with room to spare for the car itself and both the smartpass and lithium batteries never see too high of a charge current. if I wanted to be extremely safe I would use 120mm2 cable over a length of, let's say, 25meters or whatever lenght is needed. This way the cable will not possibly even get close to warm and still the voltage drop would be sufficient to limit the current into the batteries since the voltage at the terminals could be as low as 13.5 volts which is too low for a lithium to charge at it's full rate. I however feel like using a thinner cable over a shorter length will produce the same result and still keep things safe. In fact it's this limit I am trying to discover. Reason being of course that such think cable of unneeded lengths takes up space and GVM.

My early research indicated that the internal resistance of the batteries was extremely low at 10 milliohms or there abouts so calculating a resistor to put in series with that value which would limit the current to say 100amps at 14 volts already gave me quite a small value. I was worried at the time that I needed to go as thins as 35mm2 cable to get the required resistance and with that the current down to acceptable levels, which would certainly warm up at the desired 100amps, but it turns out that the resistance of all the components in the system, including shunt, already limit the current below 100amps at even 50mm2 as described in my previous post(s). That gives me hope since the current at 50mm2 was too low to my taste and the cable was certainly rated for the currents I measured (75A at most). It means that to get the current up further, I need more mm2 of cable which is better rated for the desired currents and keeps things safe(r)

If I get my hands on a piece of that special cabling I will follow up on it here. I reckon that a store for boom-box-cars will have ample choice in diameter of fine stranded cable in both red and black to keep things tidy. Though the neoprene of welding cable does have its advantages.

Cheers!
-P

PS when I get my behind over to OZ, this might be a perfect subject for a brewsky and a campfire to compare our setups and findings :)

[Blknight.aus]

cutting a heap out of it for brevity and going to a point...



Also, if my engine is not running, the voltage over the lead-acid battery will very quickly drop below 12,8volts and everything shuts down. Also, 12,8volts in general will not be high enough to charge the lithium batteries unless they are really empty so the smartpass will not engage and pass any current and only the D250SE DC-DC charger will at most push 20 amps for a short duration. When the smartpass kicks in, which is basically a relais but not made of mechanical bits but solid state MOSFETS it will draw/pass through all the power that is requested on the other end. Since it is rated for 350A no longer than 10/30 seconds in/out it can do way more than both my battery and/or alternator can push through. Also that is longer than the BMS in the lithium batteries will allow an over current during charging for (max 85A per battery). Now, my alternator CAN put out 200A according to the manufacturer so if I want to avoid overloading the smartpass I need to regulate the current somehow and the only way to do that is to reduce the voltage, hence, use voltage drop over a cable!

This way the alternator voltage will never drop below it's preset 14 volts since it can handle the current with room to spare for the car itself and both the smartpass and lithium batteries never see too high of a charge current. if I wanted to be extremely safe I would use 120mm2 cable over a length of, let's say, 25meters or whatever lenght is needed. This way the cable will not possibly even get close to warm and still the voltage drop would be sufficient to limit the current into the batteries since the voltage at the terminals could be as low as 13.5 volts which is too low for a lithium to charge at it's full rate. I however feel like using a thinner cable over a shorter length will produce the same result and still keep things safe. In fact it's this limit I am trying to discover. Reason being of course that such think cable of unneeded lengths takes up space and GVM.



I think your covered.....

Alternators try to be a constant voltage item, and in your case if your max voltage is set at 14v and your litiums are in parallele AND they are each happy to soak up 85A at 20% you dont really need any other form of limiting in your system.

1. max charging voltage for your batteries is above what your alternator is delivering
2. the combined maximum amps your batteries will soak up is more than what your alternator can deliver.

in theory the way your system sounds like it should be working is..
when the battery voltage is low you're simply passing maximum current into the lithium batteries by bypassing the dc/dc charger as your power source (alternator) isnt able to punch a high (assuming its 14v) it will simply crank out whatever amps the lithiums will absorb up until their internal voltage is equal to the voltage coming out of the alternator at which point theres no diference in potential so current flow stops. The batteries wont be fully charged and so the bypass kicks out and the DC/DC takes over to float the batteries up to fully charged.

on your bench test setup your cables might well be a limiting factor if you're using constant current power supplies. But thats not how (most) alternators work on your vehicle your cables are unlikely to be a limiting factor (unless they're undersized) and dont need to be setup to provide a limiting factor.

lets say youve got 3 120A lithiums that can each handle a sustained 85A charge rate. lets assume that your battereis are all matched and wired in parallel with the BMS's all doing the same thing.

in order to overwhelm the batteries(as a bank) ability to soak up amps youd need an alternator that could punch over 250A at 14v, youve got a 200A alternator and thats your current limit. In those circumstances your alternator voltage is now going to drop but the amps will start to go up, eventually youd hit a point where the current flow would trip the BMS but.... as the alternator voltage is dropping the potential is going to be lower so the current flow is going to drop off and eventually the battery bank will hit equilbrium and the whole thing stays in balance as the batteries soak up amps. Additionally you'll have whatever vehicle loads are on drawing down on that current supply as well. ( I dont have this, as i run 2 batteries independently with their own dc/dc my alternator just gets abused into ever lower voltages and higher amps until I hit the limit of the alternators ability to shove electrons or the DC/DC cuts out BUT I also have no risk of the BMS cutting the battery out as the DC/DC cant charge that hard.)

Speaking vaguely, so long as the battery banks ability to absorb amps is higher than the ability of the charger to deliver amps you'll be ok and you dont need to worry too much about charge current regulation. IF you're in the reverse situation and your ability to dump amps into the battery exceeds the batteries ability to soak up amps then you need a limiter.

Without knowing your exact setup...

ID suggest thast what you should probably be looking at running is a load bypass and something like a 60a-80A DC/DC charger. With a load bypass when the alternator is running and the voltage is high enough to be charging the lithiums all the load is connected directly to the alternator and the DC/DC has nothing to do but charge your batteries. When the alternator cuts down the lithiums take up the slack.

[prelude]

In an ideal world, all would have been clear to me from day one and I would have indeed bought an exact matching system (if my purse was matching as well). I reckon it would be a heavy duty relay that could handle say 250A that would kick in when the engine is running to charge the batteries up to the point they match the alternator voltage. I would also have a beefy DC-DC charger for the part above 14 volts (the relay would switch off) and indeed a load relay that would switch between the battery bank and the consumers. In fact I am already making something like it for my electric AC. That thing will use as much as 100A at full tilt so I want it directly connected to my alternator when the engine is running.

As it stands I already have I guess around 1K AUD in the ctek setup and I see no reason to replace it other than the current limitations. Regarding current limints, he only current limiting I need is to keep the smartpass from blowing up essentially. so if I can limit the current to say 100A which is below the 120A max of the smartpass, I would still have 100A left for the thirsty AC and whatever else is running. As I mentioned, seeing as even a very capable cable will already drop voltages and thus charging currents I am more than happy to continue on this route until I find otherwise.

Yeah, I think I'll be fine :) Once I get a decent cable for testing I will let you know but upon closer inspection I think this is already a reasonably stranded cable to begin with, we'll se how it goes.

btw, it is true that the power supplies I am using both work as a constant current supply since they have an upper limit in terms of current and then start dropping the voltage.

Cheers,
-P

[prelude]

If it is worth doing, it is worth doing well... Another update!

With the existing cables I had an opportunity to test indoors (with an exhaust extraction fan even) which is quite nice since we had snow and cold and what not. The setup:

70mm2 cables directly from the alternator to the starter battery terminals. Cables? yes I run a cable directly from on of the mounting bolts on the alternator housing to the negative terminal of the car battery as well as the positive cable. From the negative terminal I ran a 50cm piece of 50mm2 to the shunt which was directly connected to a 20x3 copper strip that also connect both negative terminals of the lithium batteries. Another 50cm of 50mm2 cable ran directly to the bus-bar input of both the D205SE and the smartpass. From there I ran both 5m long 50mm2 cables from the ctek's to the positive terminals of the lithium batteries in parallel. On the consumer output of the smartpass I hooked up a 800VA inverter and ran a heatgun at around 650watt's for a couple of hours, draining the batteries with around 55A of current.

After the batteries had run down for a bit, say 30% I started the engine and let the inverter run. At idle I got around 77A of current through the shunt. I then proceeded to turn off the inverter and rev up the engine. The current into the lithium batteries momentarily peaked at 144A as the alternators voltage regulator had to adjust but settled down to around 130A. The voltage over the starter battery terminals never went below 13.9volt (when revving) I am not sure how the current is distributed. In theory 110A should go through the smartpass and 20A should go through the DC-DC charger which would keep the smartpass from having to pass too much current. I did not let it run long enough for all components to heat up to see if it would derate but that is an experiment for another time perhaps.

All in all, it is clear that the alternator can put out quite a punch and you can feel it heat up immediately. The engine itself was quite cold after sitting around for hours with the bonnet open. The alternator almost immediately became hot to the touch but I guess I am not worried since the viscous fan cowling is formed such that a constant stream of air passes over the alternator. I think I'll do some more testing when the time comes but I feel this is already quite the achievement. My only concern and something that I will test is when the lithiums are drained to near 0. My guess is that at that point the voltage difference between the alternator voltage and the lithium terminals (not the voltage drop over the cable) would be significant enough that the charge current would go up even further. Since the alternator can deliver that much current it is yet to be determined what the "sweet spot" cable diameter and length is to keep it safe at all times and what that will do to the charge current once the lithium batteries cross the 20% SOC threshold (where the voltage and current remain stable al the way up to 90-95%)

Cheers,
-P

[Blknight.aus]

All in all, it is clear that the alternator can put out quite a punch and you can feel it heat up immediately. The engine itself was quite cold after sitting around for hours with the bonnet open. The alternator almost immediately became hot to the touch but I guess I am not worried since the viscous fan cowling is formed such that a constant stream of air passes over the alternator. I think I'll do some more testing when the time comes but I feel this is already quite the achievement. My only concern and something that I will test is when the lithiums are drained to near 0. My guess is that at that point the voltage difference between the alternator voltage and the lithium terminals (not the voltage drop over the cable) would be significant enough that the charge current would go up even further. Since the alternator can deliver that much current it is yet to be determined what the "sweet spot" cable diameter and length is to keep it safe at all times and what that will do to the charge current once the lithium batteries cross the 20% SOC threshold (where the voltage and current remain stable al the way up to 90-95%)

Cheers,
-P

Depending on a few things that relate to how your battery BMS, Smartpass, DC/DC work together and how your load is hoooked up

IF you drain the Lithiums to near 0% capacity (for my batteries its about 10%) you may find that the BMS will not let the battery charge.

As I understand the logic, with so little charge in the battery the BMS will lock out large charging currents and high (say .5v over the voltage in the battery) charging voltages untill a trickle charge has bought the battery up to what the BMS regards as a reasonable voltage.

This is and isnt an issue for me as the BMS is between the battery terminal and battery properly. My DC/DC (the usual charge method) always starts out with a low charging voltage and current IF after some time (I think its 30 seconds) it doesnt see a proper voltage on the battery (BMS is allowing the battery to connect) it pauses power transmission and then restarts the trickle charge process. So charging a battery that has disconected isnt an issue the DC/DC deals with trickling the battery into a workable state. It does become an issue If Ive let the battery go flat and forgotten to take the load off.

Now Im not sure why BUT...
If I've got solar power available to the DC/DC, the solar side will drive the load and then the charger does it bit to put what it can into the battery. MY guess is... With the load on the DC/DC puts everything it can out of the DC/DC charging side towards the battery, because the load (typically the fridge) is drawing the voltage down theres only enough left over to trickle the battery and the BMS lets this through. When the load cycles off the battery has already gotten to the point where its BMS is willing to let it take all the power.


I did a couple of experiments and coupling a 3w bulb in series with the charge source and the lithium was enough to juice up the lithium without upsetting the BMS and then normal charging would also work.

Hope this helps if you wind up in the same place I was the first time i ran the batteries down to the BMS cutout.

[prelude]

Hmm that is something I have not tested tbf. It's not easy to drain a 340Ah battery bank in a test setup :) I mean with a maximum load on my test inverter it would still take 7 hours... Anyway, good point you make! I will have to look into that.
I know that the Smartpass' AUX output is limited to 80A and shuts down below 11,5V and kicks back in when the voltage gets over 12,0V for a lithium that's quite near empty. According to renogy the BMS will cut of the discharge when => 10V which I assume means anything close to 10V but certainly not below. I will however have the AC connected directly to the batteries unless I limit it's power usage to the 80A max including the other users. Something to think about :)

On the other hand, in most situations the DC-DC which has it's own builtin MPPT wil start early in the morning putting charge ever so slowly back in the batteries whilst most users would still be off (smartpass below cutoff voltage) so there is that. In any case, certainly a good tip! I'll have to test this and write it up in my large book of knowledge for the car.

thanks and cheers!

-P

PS I wrote to CTEK asking about how the smartpass handles overcurrent so here's hoping...

[Blknight.aus]

Today I havec learnt......

the DC/DC im using which has the solar in it...


will not put out enough to trickle up the battery AND the usb port thats built into the battery box...

if you turn the battery isolator off, the charge plug is also disconected from the battery......

manually hooking the DC/DC to the battery with a meter... it takes 8 minutes for the DC/DC to trickle the battery till the BMS cuts in, then the soloar kicks in properly.

[sharmy]

Hi shady, that will work but be aware, to recharge that size battery with a 25 amp DC/DC device, if the battery is flat, you're be looking at at least 15 to 20 hours of drive time to get that battery up to 100% SoC.

Well I bit the bullet and bought a 300 ah lithium battery and swapped the 25 amp dc to dc to a 40 amp one which is connected to a solenoid activated by a dash switch which only works when the ignition is on. I plan to put the old 25 amp dc charger in my camper trailer and connect it to an Anderson plug which is connected to the vehicle battery through a VSR to charge 2 x 100 ah AGMs. In both cases the solar is through separate controllers. Will running both these at the same time worry the alternator, the vehicle is a 2016 BT 50 (unfortunately).

[scarry]

Well I bit the bullet and bought a 300 ah lithium battery and swapped the 25 amp dc to dc to a 40 amp one which is connected to a solenoid activated by a dash switch which only works when the ignition is on.).

If the vehicle does not have a smart alternator,and you have the solenoid wired as per your post,do you need the 40A DC to DC?

[sharmy]

If the vehicle does not have a smart alternator,and you have the solenoid wired as per your post,do you need the 40A DC to DC?
I think so. I am pretty sure the DC to DC charges the battery to 100% but the alternator on it's own doesn't.

[scarry]

I think so. I am pretty sure the DC to DC charges the battery to 100% but the alternator on it's own doesn't.

That was one of Redarcs selling points,but i am pretty sure Tim has other thoughts

Anyway,hopefully he will chime in.

[drivesafe]

I think so. I am pretty sure the DC to DC charges the battery to 100% but the alternator on it's own doesn't.
Hi sharmy ( I got your name right this time, sorry ) If you use very little out of the Lithium battery, then yes a DC/DC devices will fully charge them to 100%

If you use a lot of lithium battery capacity, then a direct alternator charge may not fully charge the lithium in the drive time, BUT if your alternator can not fully charge your lithium battery, then the DC/DC device will have no hope of getting any real charge at all into the lithium, in the same drive time.

[prelude]

a few weeks have gone by and some more results from testing are available so here it goes:

I have been testing the charge currents for a longer period and used the body as negative this time. The charge currents are not noticeably lower when using the vehicles body, as one normally would, for the negative/ground of the system. One could argue that the amount of metal is certainly enough but then again, most of it is only touching through spot welds. Still, I could push 100A with easy without negative side effects. I am not sure that I will use the body but it was a nice test nonetheless.

As of yet I did not find a cable to my liking with more smaller strands so I have not tested that yet. Seems I get enough current for now so the question is if I still need to test that.

Of more concern is my alternator. It is a 200A model and I have been in contact with the supplier and they rate it at 198c. During my last test run I had the engine running at 1750RPM for around 30 minutes so that everything heats up nicely and turned on everything I could find. 35A into the inverter, low and high beam, fans on full etc. During that time I would also see around 90A of charge current into the batteries. I reckon I would be running close to the rated output of the alternator at that point. The viscous fan would be in it's "low" mode ie slipping since it was simply not warm enough in the workshop but the engine did get up to temp very nicely.

I used a fluke FLIR camera to measure the temperatures and as expected the temperatures of the cabling and connectors were barely noticeable with regards to their surrounding. The air temperature being pushed from the radiator into the engine bay was around 80c and that is blown over the alternator for cooling. The alternator housing was fairly cold but that was to be expected; the emissivity of aluminium is quite a bit lower and the FLIR was not set for that. The parts of the windings that were visible through the cooling slits though measured really high. certainly above 198c though I could not find the emissivity value for enameld copper wire. I used copper but I might need to switch to enamel. Still, down bush or worse the simo that under bonnet temperature will certainly be a bit higher so I find myself living dangerously close to the temperature limit as advertised by the supplier.

Still, I did notice that after some time (and a worrying whiff) that the output voltage dropped so the regulator did notice the high temps and started limiting the output current accordingly.

The adventure continues!
Cheers,
-P

[drivesafe]

Hi Prelude, just a word of caution.

Be careful when producing high current outputs from the alternator while idling.

Even when moving slowly in traffic, massive amounts of air flows over and through the alternator.

When idling and stationary, the thematic fan is controlled by the heat of the radiator and gives very little cooling for the alternator.

The high currents you are producing while testing with the alternator. for such long periods of time and not getting a decent air flow over the alternators casing could easily lead to an alternator failure.

[sharmy]

Hi sharmy ( I got your name right this time, sorry ) If you use very little out of the Lithium battery, then yes a DC/DC devices will fully charge them to 100%

If you use a lot of lithium battery capacity, then a direct alternator charge may not fully charge the lithium in the drive time, BUT if your alternator can not fully charge your lithium battery, then the DC/DC device will have no hope of getting any real charge at all into the lithium, in the same drive time.

I should be right. The back of the ute with a 400 watt solar panel on the roof sits out in the sun most of the day and powers 2 engel fridges. The dc charger is for when I travel long distances.

[sharmy]

My main worry is whether the alternator can handle both the 40 amp and a 25 amp dc to dc chargers at once.

[prelude]

@drivesafe thanks for the tip. It feels like the belt driven fan does produce a lot of airflow but perhaps it is insufficient indeed. I will place a large fan on top during testing to simulate airflow as much as possible.

@sharmy you SHOULD be alright since by all accounts most alternators will drop the voltage when they are overloaded and thus the input voltage will drop below the DC/DC chargers cutoff point. This does mean your starter battery will suffer a bit though. In my case, my DC-DC is also my solar input so it takes from the panels what it can and then moves on to the alternator input. Besides, 65A is not THAT much for an alternator, depending on what else you run at the same time. Note that the engel fridge will run from the aux batteries which are powered by the DC-DC, in other words, your batteries charge slower but you are not pulling more from the alternator. Everything that is connected to the AUX remains within the 40/25A limits of the DC-DC's.

Cheers,
-P

[sharmy]

I spent most of the day sticking insulation inside the canopy, then took out the 2 agm batteries and fitted the new 300 amp lithium, hooked it all up and then discovered my solar controller, which I thought could do anything, doesn't have a lithium mode. Tomorrow I will swap it with the camper one that does. A real pain because it's a different size and shape and the power board is not going to look as good as it does now, but as long as it works.

[Blknight.aus]

Well I bit the bullet and bought a 300 ah lithium battery and swapped the 25 amp dc to dc to a 40 amp one which is connected to a solenoid activated by a dash switch which only works when the ignition is on. I plan to put the old 25 amp dc charger in my camper trailer and connect it to an Anderson plug which is connected to the vehicle battery through a VSR to charge 2 x 100 ah AGMs. In both cases the solar is through separate controllers. Will running both these at the same time worry the alternator, the vehicle is a 2016 BT 50 (unfortunately).

IT'll be fine if you're using a semi quality or better DC/DC


I think so. I am pretty sure the DC to DC charges the battery to 100% but the alternator on it's own doesn't.

In most Cases, Correct. (unless you have an alternator or smart alternator thats set for lithium charging)


Hi sharmy ( I got your name right this time, sorry ) If you use very little out of the Lithium battery, then yes a DC/DC devices will fully charge them to 100%

If you use a lot of lithium battery capacity, then a direct alternator charge may not fully charge the lithium in the drive time, BUT if your alternator can not fully charge your lithium battery, then the DC/DC device will have no hope of getting any real charge at all into the lithium, in the same drive time.

All valid, If your alternator holds 14.4 V you can get a lithium to nearly fully charged BUT, in most cases an alternator that produces a steady 14.4 V will often over current the lithium and the BMS should cut off charging.


My main worry is whether the alternator can handle both the 40 amp and a 25 amp dc to dc chargers at once.

easily, the alternator in my 6x6 is only around 80A and it handles 2x40a dc/dc chargers onto a pair of 120ah lithiums and a pair of n100 truck batteries, plus all the electrical load of the 6x6.


I spent most of the day sticking insulation inside the canopy, then took out the 2 agm batteries and fitted the new 300 amp lithium, hooked it all up and then discovered my solar controller, which I thought could do anything, doesn't have a lithium mode. Tomorrow I will swap it with the camper one that does. A real pain because it's a different size and shape and the power board is not going to look as good as it does now, but as long as it works.

you can hook the solar up in front of the DC/Dc and let it charge the main battery which will then once the voltage is over the DC/DC cut in voltage allow the DC/DC to charge the lithium. not efficcient but reasonably effective. you might also want to check the DC/DC if you're using anything of reasonable quality the DC/DC will have a solar input on it.

good luck.

[prelude]

Finally, another update!

So I got around to buying a metre of boom-car cable with finer strands. The current it can push through seems to indeed be higher than similar high-voltage course stranded wire. ie. the voltage drop is lower, something to keep in mind!

With that shiny new piece of cable I redid my test setup but in a way that I could test with the bonnet closed and driving. The results were... less impressive. I did not even get to the driving and testing part... Lets get into the setup first:

First I installed a Victron shunt between the alternator and the car battery. In order to do so I had to modify the mounting of the alternator in such a way that it was electrically isolated from the aluminium bracket holding it onto the engine. I realize that it would reduce cooling ever so slightly due to the loss of thermal mass but I figured that would be negligible. Victron shunts are setup such that they only work in the negative lead so I had no other option in this case.

The second thing I did was install a second earth strap between the battery terminal, where the shunt/alternator also terminates, to the chassis/body to increase the total diamater of the ground cable/straps and the contact area to the metal to reduce losses / increase current carrying capacity. The standard setup is barely up to the job with the default alternator so I simply doubled it.

Thirdly I installed the lithium batteries, my CTEK setup and inverter in the back of the car in stead of on a test bench. I ran both 5m lengths of 50mm2 from the positive terminal into the car to the CTEK input terminals. From the CTEK output terminals I ran both short runs of 50mm2 and I hooked the bmv-700 shunt up to a large flat piece of copper strip that acted as busbar between the negative terminals of the batteries and the shunt. From the shunt I used my newly acquired test lead of 70mm2 and hooked that up to the body work of the car, to the passenger side mount point of the seatbelt to be exact.

I also used the rear mount point for the seatbelts to run a length of 25mm2 as ground for my inverter. The positive lead of the inverter was hooked up to the switched output of the Smartpass.

In order to load the alternator up even further I hooked my 600watt 12v boiler up to the car battery terminals running two lengths of 35mm2 that I had laying around, one short one long.

After checking everything over I discharged the lithium batteries to around 50% by running a 500 watt load on the inverter for a couple of hours. I kept the inverter loaded and running to push the load on the alternator as far as I could go. I ran high beams, the blowers at full and switched on anything I could.

My observations:


at idle the current was around 120A, makes sense most alternators can not do full output at idle.
increasing the engine RPM to 1500 I got to about 180-188A. This was as much as I could get, I had no more things to switch on to load up the alternator further :)
the alternator voltage dropped to only 13.89 volts at the terminals. I reckon the regulator was not yet pulling back nor was it overwhelmed (yet)
with the inverter off I would push close to 90A into the lithium batteries
the D250SE would be doing around 20A of that 90A total
It seems the DC-DC always running max output, see below
with the inverter on the charge current into the batteries would drop to 50A
I seem to "loose" 5Amps somewhere since the inverter on it's own pulls around 35A. Probably has to do with added cable losses and thus voltage drop.
The belt driven fan was fully engaged since it only disengages at higher RPM. ie full cooling was applied.
the alternator was getting way too hot


Most of the above requires no explanation. The most worrying thing is the alternator. I measured the temperature between the windings with my thermapen instant meat temperature thingy and it kept climbing up to the point I stopped the test at around 205c (the manufacturer stated that 198c was the design limit). Since the belt driven fan was running at around 1500RPM the fan that I placed on top of it barely made a difference. I think we can safely conclude that the alternator I purchased MAY be able to put out 200A but it is certainly not capable of doing so "full duty-cycle". I do not claim that no alternator can do that and that drivesafe and others are incorrect, I tend to believe that factory systems are most likely capable of doing full output without going into meltdown, but I do claim that this particular unit is a bit more sales pitch. As we tend to say; a lotta show and no go. Mind you I am aware that even in slow traffic there would be more airflow. Perhaps I am looking for trouble by creating a very specific failure case but... the under bonnet temperatures would certainly be very high when driving in a hot climate, in low range at low engine speed (say the 1500RPM mentioned above). My test was done with a cold engine, heating up as the test ran along in 20c ambient temps with the bonnet open AND an added fan to increase all the chances of cooling/airflow.

Imo the alternator has either: no temperature compensation in the voltage regulator, or the regulator and diodes are rated for the current and they just don't get hot enough and it's just that the stator windings can't handle the current, or: the cooling of the alternator is insufficient. I tried to provide as much airflow as I could possibly manage but these tests have made me a bit weary to test it whilst driving since I can't properly monitor the temps then. Since the housing is exactly the same as the standard 120A unit perhaps it is simply not capable of handling more thermal load or airflow for cooling.

In any case, for me this has meant contacting the seller and seeing what they have to say but also ahead of that searching for another alternative. I found one and shall link it here for posterity, though I should do that in the topic about high output range rover alternators but my search skills seem to fail me since I can't find it :) Alternators | 300 Series Alternators | Land Rover Defender | Discovery | Ranger Rover 320A Alternator (https://www.wosperformance.co.uk/alternators/300-series-alternators/land-rover-defender-discovery-ranger-rover-320a-alternator?fs=&c=2&t=350&m=16782&mo=56212) they have even higher output units for those of us that need it... but I'll stick with this one since it should be sufficient and fits in the standard location.

Regarding DC-DC charging. In setups with a smartrelay or smartpass in my case and a DC-DC charger connected in parallel: the relay will put the alternator voltage (and with that also the starter battery voltage) over the terminals of the aux batteries, minus cable losses and such. The voltage over the aux batteries will be X at that point and give Y current flowing into the batteries. Assuming that the alternator CAN provide more power, the DC-DC charger is not bound by the output voltage of the alternator and the buck-boost circuit will up the voltage over the aux battery terminals to the point that it reaches it maximum output. Ergo, a DC-DC charger will always do it's maximum output even if you have a direct starter to aux battery of some sort. Makes perfect sense but perhaps someone will find it written out handy :)

Over the months I have been in contact with CTEK and some nice little nuggets of info stuck in my brain that might also be of use to others:


the D250SE will prioritize solar over alternator
they can be paralleled (makes sense with my above text)


This basically means that if you have solar it will use all up all of the available solar watts before it seeks for power from the alternator and if you have more solar on the roof than the unit can use (or require more DC-DC power) you can simply add another unit to your existing system. Yes you could also add one from another brand I reckon, but having two units with different charging algorithms would not be my preferred choice.

To conclude:


limiting charge current with cables is certainly possible but a fairly extensive puzzle to solve
I have not seen any evidence that my setup would be unsafe in any form or way; the cable sizes vs losses etc. are well within safety specs
Once dialed in however there is no guarantee that it would work for a different battery setup (they don't last forever so would need changing at some point)
Oversized alternators require scrutiny, they are not all created equally and
The default wiring in the car is certainly not up to the task for such a device (ie alternator B+ and grounding cables).


AGAIN: IN MY SETUP! CHECK YOUR OWN SITUATION AND DON'T JUST COPY ME. You are free of course to use all my learned lessons and I would always be willing to help if you have questions :)

Whats left to do / ponder / consider? (last bullet list, I promise! :) )


I am considering increasing my solar capacity which would automatically increase my DC-DC capacity.
this would still not be enough but perhaps going full DC-DC and no smartpass/relay would be viable (remember weight is my biggest concern, cost my second)
mixing and matching is not preffered. This means I would need to get a sterling power DC-DC and a victron MPPT for instance and ditch the CTEK stuff.
I am not sure that the cable weight for the current limiting (extra length etc.) would weigh up to the above setup, in fact I think using equipment is heavier and certainly more expensive
running electric AC is one of the most challenging electrical puzzles one can give oneself!
since above; I have a free spot on the engine to run two alternators. Might be worth it so that I could run two factory standard units in stead of one expensive custom unit...
The second spot does not however have the same airflow as the first.
uneven loading would occur if you do not have two exacte same length cables.


Well, that's it for now. I'll give it a second go to find the topic regarding alternative alternators to update that as well.

Cheers,
-P