Thread: current limiting lithium battery charging (by means of cable)

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%.

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
How to not blow up your Alternator when charging Lithium - YouTube

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

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

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.

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?

Cheers,
-P

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!

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

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?