Balancing without power supply.

[bostonbuzz]

I hooked up my Chargery 8T BMS with 1.2A of balancing to a 4s series of 280ah xuba batteries which were within 5 or 6 mV out of the box. I charged with a victron ip67 12/13 charger at 4a for a day and found over 300mV difference between cells! Obviously I either have 2 bad cells or I need to do initial balancing. Unplugging the charger slowly brings the cells back in balance overnight via the BMS.



I have placed them in parallel since I have no power supply and plan on waiting a few weeks with the cells in parallel. Do I:

1. Keep in parallel and just disconnect them from each other every day or so and check if they all stay at the same voltage after an hour (not sure this is the proper procedure)?
I have a Klein CL380 clamp meter for this, is that accurate enough?

2. Reconnect them in series and set my bms to balance in all states (charge/discharge/storage) down to 1mV (the highest accuracy of the chargery)?

3. Charge to 3.65v in parallel like Wills video suggests. I can’t do this without buying a power supply. I suspect a whole lot of lurkers and members don’t want another component lying around either.

Which method (1 or 2) would get me closer to the gold standard of top balancing to 3.65v in parallel (#3)? Would 1 or 2 achieve what #3 achieves just slower, or is it never as balanced?i

How long would the have to stay at the same voltage by chargery (or just sitting in parallel) before they were “balanced” (because voltage is not indicative of SoC).

 

[MisterSandals]

How do you have the BMS set that it allows a cell to get to 3.699v?

When Charged up at max volts, just a small amount of charging makes the voltage spike in the first cell on ~3.5v, while the other cells’ voltages change very little.

I would set your chargery to 13.8v max, 3.6v cell max and try again to see if this can be managed with settings.
Run your batteries down just below 13v then let them charge up and see if they behave well enough this way.

Note- these is general charging ideas. I have zero experience with chargery.

 

[DW SD]

Also, Xuba batteries are not LiPo. They are LiFe on Chargery setup. I see on your screen LiPo setting.
Might want to review the instructions.
Careful, seems like they might be overcharged as pointed out.

 

[bostonbuzz]

Also, Xuba batteries are not LiPo. They are LiFe on Chargery setup. I see on your screen LiPo setting.
Might want to review the instructions.
Careful, seems like they might be overcharged as pointed out.

You, sir, are a lifesaver. That explains why the Chargery minimum over-voltage protection settings were so high (min 3.9v).

I'm scared to charge up again without some sort of balancing beforehand. Shall I just leave them in parrallel for a few days then try charging to 14.4/3.6v? Will that even help? Now I'm back to my original question.

 

[Craig]

I would leave them parallel it will help but may take some time. You could also discharge a bit then charge with chargery using a cutoff voltage. Once one cell reaches the cutoff voltage and the others are lower. Remove it from pack and start again with remaining cells this will work down to 2 cells so you could have 1 last cell a little low. Let the chargery balance the last 2 it may not be perfect but will get them in balance if paralleling does not work.

 

[bostonbuzz]

Well to 1/2 answer my own question. I left them in parallel for a week around 3.25 volts. Then I put them back in series and it charged well up to 14.2V with about 30mv difference at the top - I thought I was good to go. I checked back a day later, and one of the cells was 100mv higher than the others and tripping the BMS when I tried to charge.

Chargery's passive balancing 1.2A only works, I THINK, by drawing 1.2A from the over-voltage imbalanced cells, so sitting at storage it will not appreciably balance. Charging and discharging WILL balance it. (note this is with balance set to "on" in charge, discharge, and storage, and set to 8mv difference).

Top balancing is possible without a power supply by charging them up with a normal charger and letting the BMS balance, but this isn't the tippy-top. My findings mean something is wonky which I still have to determine. In this case, where my balancer won't balance, I will have to apply a load to discharge and then charge again. Otherwise, the only solution is a power supply and paralleling the cells at 3.65V to "tippy-top balance". So, it looks like another 80$ to spend on this project for a cheap power supply ?.

 

[DW SD]

I did a quick voltage drop calculation a few weeks ago thinking about the balancing mechanism.
I believe the voltage differential is what would drive balancing.
In this example:
0.1V ~ (100mV) and with 22 gauge wire and 1 amp of current flow equates to 65% voltage drop across a 24" balancing lead!!

Wasn't totally sure what to think of this. But suspect it means that we wouldn't see much balancing action given limited flow of energy.

Maybe you could try leaving them in parallel @ 3.4V and see what happens. I think 3.25V isn't close enough to the knees to make a difference in terms of balancing.

I'm speculating and am not an expert, but think I understand the theory.

Doug