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

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.

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

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,

Originally Posted by prelude

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
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.......

Originally Posted by prelude

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.

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. 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

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

Originally Posted by prelude

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.

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

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

Originally Posted by prelude

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.