Massive difference in charge/discharge amps on 2 lith banks?

[Bossrox]

I'm wondering if anyone would know why 2 8s lith banks would have such a wide charge/discharge difference. I have a 280 & a 300 a/hr bank but the 300 amp bank charges & discharges at double the rate of the 280's. So in the morning after it's been running all nite, the 300 bank is more depleted than the 280 bank, then on top of that, with the inverter shut down & no solar charging, the 280 bank is charging the 300's at about 4 amps. Doesn't make sense to me.

 

[NVS]

Why do you have 2 banks of different size hooked together? The 300ah bank will never fully charge. The 280ah will control the charge cycle. Someone will chime in that can explain it better than me. But that to me is your problem.

 

[Bossrox]

That's the opposite of the way it charges. The 300 bank is fully charge well in advance of the 280's which are usually 100 amps short of full charge by then.

 

[Substrate]

These "lith" banks. What exactly are they? Since "lith" conveys no known li-ion chemistry differences (say between LiFePO4 and all else).

Lithionics, or something homebrew? See the problem?

 

[Bossrox]

These "lith" banks. What exactly are they? Since "lith" conveys no known li-ion chemistry differences (say between LiFePO4 and all else).

Sorry, I assumed most everyone is using LiFePO4

 

[Substrate]

Is your 8S configuration for an individual nominal 3.2v cell collection in serial, or the more common nominal 12v LFP batteries which would indicate a 102v specialized application?

 

[NVS]

That's the opposite of the way it charges. The 300 bank is fully charge well in advance of the 280's which are usually 100 amps short of full charge by then.

I would check cell voltages to see if they are really fully charged. Do you have Bluetooth bms to see that? You may also have a bad bms that's not fully charging the cells. You could have a bad cell.

 

[NVS]

Your 300ah cells could be way out of balance.

 

[Bossrox]

I would check cell voltages to see if they are really fully charged. Do you have Bluetooth bms to see that? You may also have a bad bms that's not fully charging the cells. You could have a bad cell.

Yeah, got the bluetooth, using jbd bms's on both. Both come up to full charge, it just takes the 280's much longer.

 

[Substrate]

Without knowing in more detail exactly what your battery *is* - if it is in fact a homebrew collection of cells, all we are doing is chasing our tails.

Please provide more information about your bank, so we can start analyzing on the right footing.

 

[Bossrox]

Your 300ah cells could be way out of balance.

The 300 amp cells are grade a & balance perfectly however the 280's I got from a crap seller & got 5 balance boards on it to keep it in line. Still the difference is remarkable.

 

[Bossrox]

Without knowing in more detail exactly what your battery *is* - if it is in fact a homebrew collection of cells, all we are doing is chasing our tails.

Please provide more information about your bank, so we can start analyzing on the right footing.

The 300's are eves, the 280's I think are catl

 

[joanna_lannie]

Don't quite understand... Are the two batteries a mixing battery? Are the chemical properties the same? If connected together, is it in series or parallel? There seems to be a lot of information missing.

 

[rmaddy]

Why do you have 2 banks of different size hooked together? The 300ah bank will never fully charge. The 280ah will control the charge cycle. Someone will chime in that can explain it better than me. But that to me is your problem.

There’s nothing wrong with having two batteries of the same chemistry but different capacity in parallel. If each is properly balanced and both start at the same SOC then both will stay at the same voltage and SOC. The higher capacity battery will provide more current and take more during charge. But with a 300Ah and a 280Ah it should be nearly the same.

 

[NVS]

From what I've read and learned is your cells or banks need to match capacity. I've built 840 lithium ion 18650 cells into a 7s120p battery and now I'm working with lifepo4 and this is the one constant between them. Yes it is fun capacity testing over 1000 little 18650 cells then putting them all into 7 matched capacity banks.

 

[timselectric]

I'm wondering if anyone would know why 2 8s lith banks would have such a wide charge/discharge difference. I have a 280 & a 300 a/hr bank but the 300 amp bank charges & discharges at double the rate of the 280's. So in the morning after it's been running all nite, the 300 bank is more depleted than the 280 bank, then on top of that, with the inverter shut down & no solar charging, the 280 bank is charging the 300's at about 4 amps. Doesn't make sense to me.

1. Check the balance between each cell in each battery.
2. Make sure that they're reaching full charge.
(No longer drawing current)
If they are balanced and evenly charged, they should charge and discharge at the same SOC.

 

[rmaddy]

From what I've read and learned is your cells or banks need to match capacity. I've built 840 lithium ion 18650 cells into a 7s120p battery and now I'm working with lifepo4 and this is the one constant between them. Yes it is fun capacity testing over 1000 little 18650 cells then putting them all into 7 matched capacity banks.

To be clear, I’m assuming the OP made one battery with 280Ah cells. Then the OP made a separate battery with 300Ah cells. Each of those two batteries has its own BMS. Those two discrete batteries are then wired in parallel.

I am not talking about making a single battery with one BMS using different capacity cells. That would likely not be a good plan.

 

[Substrate]

Have you seen the video? While it may be fun from a "one off" standpoint, how are we to know if *any* of our advice is applicable with all that other stuff bolted on?

This is what I mean about chasing our tails - we all WANT his system to work well, but there are so many variables left out of the original question, that it tends to make things just generic speculation.

 

[timselectric]

To be clear, I’m assuming the OP made one battery with 280Ah cells. Then the OP made a separate battery with 300Ah cells. Each of those two batteries has its own BMS. Those two discrete batteries are then wired in parallel.

I am assuming this, also.
Just going on the original post.

 

[NVS]

There’s nothing wrong with having two batteries of the same chemistry but different capacity in parallel. If each is properly balanced and both start at the same SOC then both will stay at the same voltage and SOC. The higher capacity battery will provide more current and take more during charge. But with a 300Ah and a 280Ah it should be nearly the same.

You can't keep two mismatched batteries any more balanced, than you can keep any 4 mismatched cells balanced it's that simple. If he was to combine the two banks into one 8s2p bank paring a 280ah cell with a 300ah cell in parallel (2p) then connect the 8 (2p) banks in series (8s) then he would have one balanced bank.