Keep cells balanced

[sunrise]

My 105ah x 2 batteries are already installed in my TT. the problem is that they are no longer well balanced, I have one cell at 3.6v and others are at 3.4. the whole bank would not be able to get much above 14v as the 3.6v cell would quickly trigger overvoltage protection. I have top-balanced all cells prior to the final installation.

I have 2 questions
1) Will the cells drift further apart? right now it's still ok, the curve is very steep above 3.4v so I am not really losing much capacity, but worried this would become worse.
2) Is there any way to slowly rebalance them with a certain voltage/current? I know the BMS balancing is weak, but I am wondering if given it long enough time it will eventually do its job? I no longer have the luxury to put the cells in parallel and rebalance them using original method.

Thanks!

 

[Bob B]

You could manually bleed off the high cells with a resistor ... a search on here will find the how to do that.

What BMS are you using?

 

[sunrise]

I use JBD 120A BMS

 

[DerpsyDoodler]

I use JBD 120A BMS

Are the terminals clean? Did you use noalox or oxguard?

 

[sunrise]

They are pretty clean, I use neither. Note that for most of SOC they are very close, like 0.003 - 0.01v delta

 

[DerpsyDoodler]

when you open the BMS app do you see “balancing” in dark next to that cell (Not grayed out). If you’re doing it in your phone (iOS) you might have to landscape view for it to appear properly.

 

[time2roll]

Could get a small active balancer to speed up the slow BMS balancing.

eBay active balancer $12

 

[MisterSandals]

How do you have your batteries wired? Are you charging/discharging from the + and - on one battery (wrong) or + on one battery, - on the other battery (correct) or ?

 

[sunrise]

How do you have your batteries wired? Are you charging/discharging from the + and - on one battery (wrong) or + on one battery, - on the other battery (correct) or ?

2 batteries in parallel, - on one battery, + on the other battery. Is this important? they are paralleled with 8 inch 4awg wires, I did the correct way you said but more for better organization reason.

 

[sunrise]

I also set the balance threshold to 0.02v, which is default I think, should I lower it to 0.01v to get balancing kicked in earlier?
Thanks!

 

[sunrise]

when you open the BMS app do you see “balancing” in dark next to that cell (Not grayed out). If you’re doing it in your phone (iOS) you might have to landscape view for it to appear properly.

Yes, but at the 3.5v the BMS just can't keep up with the rapid voltage rising.

 

[MisterSandals]

the whole bank would not be able to get much above 14v

I am a conservative charger and would make this the top charge voltage. There is dang little capacity above 14v.

These charts (second one in particular) shows 99% full somewhere between 13.5v and 13.8v, but look for yourself.

General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

I've looked at several places for a simple, easy to read at a glance "chart document" for LFP. I could not find anything comprehensive. I discovered an XLS Worksheet in my stored file and took parts of that to create this chart, I did not...

diysolarforum.com

I charge LiFePO4 to 13.8v.

 

[sunrise]

I am a conservative charger and would make this the top charge voltage. There is dang little capacity above 14v.

These charts (second one in particular) shows 99% full somewhere between 13.5v and 13.8v, but look for yourself.

General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

I've looked at several places for a simple, easy to read at a glance "chart document" for LFP. I could not find anything comprehensive. I discovered an XLS Worksheet in my stored file and took parts of that to create this chart, I did not...

diysolarforum.com

I charge LiFePO4 to 13.8v.

My converter doesn't have programable voltage, it's 14.6 bulk or 13.6

 

[sunrise]

To clarify I wasn't chasing the last 1% capacity, the problem I have is that the BMS voltage cut off is driving the converter crazy, when cutoff happens, the 3.65v cell will slowly drop for a few minutes, then it opens the circuit again, converter fan start spinning, for a minute or even less than that, cut-off again, then repeat.

 

[sunrise]

I am a conservative charger and would make this the top charge voltage. There is dang little capacity above 14v.

These charts (second one in particular) shows 99% full somewhere between 13.5v and 13.8v, but look for yourself.

General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

I've looked at several places for a simple, easy to read at a glance "chart document" for LFP. I could not find anything comprehensive. I discovered an XLS Worksheet in my stored file and took parts of that to create this chart, I did not...

diysolarforum.com

I charge LiFePO4 to 13.8v.

I might try this - when cell reaches 3.45(13.8/4)v, and charge stops, maybe the voltage falling back will be slower and it won't trigger the converter so frequently.

 

[sunrise]

I am a conservative charger and would make this the top charge voltage. There is dang little capacity above 14v.

These charts (second one in particular) shows 99% full somewhere between 13.5v and 13.8v, but look for yourself.

General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

I've looked at several places for a simple, easy to read at a glance "chart document" for LFP. I could not find anything comprehensive. I discovered an XLS Worksheet in my stored file and took parts of that to create this chart, I did not...

diysolarforum.com

I charge LiFePO4 to 13.8v.

Could you please share your BMS settings like

balance delta threshold and minimal voltage.
high voltage protection voltage, as well as resume voltage. (both cell level and pack level)
low voltage protection voltage, as well as resume voltage. (both cell level and pack level)

Thanks a lot!

 

[time2roll]

To clarify I wasn't chasing the last 1% capacity, the problem I have is that the BMS voltage cut off is driving the converter crazy, when cutoff happens, the 3.65v cell will slowly drop for a few minutes, then it opens the circuit again, converter fan start spinning, for a minute or even less than that, cut-off again, then repeat.

Turn the converter off 4 hours minimum. Maybe on just once a day.

What converter is it BTW?

 

[DerpsyDoodler]

I am a conservative charger and would make this the top charge voltage. There is dang little capacity above 14v.

These charts (second one in particular) shows 99% full somewhere between 13.5v and 13.8v, but look for yourself.

General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

I've looked at several places for a simple, easy to read at a glance "chart document" for LFP. I could not find anything comprehensive. I discovered an XLS Worksheet in my stored file and took parts of that to create this chart, I did not...

diysolarforum.com

I charge LiFePO4 to 13.8v.

Ive seen several different lifepo4 charts. Is there a qualified authoritative reference for that data?

 

[sunrise]

Turn the converter off 4 hours minimum. Maybe on just once a day.

What converter is it BTW?

WFCO 8955 LIS

 

[MisterSandals]

Could you please share your BMS settings like

Sure. The rv is sitting idle with panel breaker off. I store at 13 - 13.2. Looks like i have charge amps at 8a (because i am the BMS and it charges on its own to get back up to 13.2, gently).