BMS question

[amberwolf]

Sorry, your busbars are too enormous for my print.

Funny...that almost scans to a famous tune by Right Said Fred....

 

[Geoffers]

Been doing a bit more testing on this,
Cell 12 sags by 100-120mV bellow the highest cell when pulling around 170A
Cell 12 is 80mV above the rest when charging at 80ish A

The battery is 14s2p made up of 4s modules, 380aH. Is this enough to be a problem? I have a spare module I could put in.

The only parallel connections are - , + and the BMS connections, my concern is some current must be flowing through the BMS wires between the 2 parallel cells of cell 12.

What's the best approach to swap in spare module? Charge everything right up first.

Or should I just leave it and not worry

 

[amberwolf]

The only parallel connections are - , + and the BMS connections, my concern is some current must be flowing through the BMS wires between the 2 parallel cells of cell 12.

How did you measure this current flow?

If it hasn't been measured, what leads you to think there is current flow?

If the series connections are correct and equal, the parallel connections are correct and sufficient, and the cells are equal, there won't be any current flow between parallel cells, as they will be a single cell to the system, electrically.

If there are no parallel connections between cells other than tiny BMS wires that have high resistance, then if the paralleled cells are not identical, current flow thru the system creates voltage differences between them, and current would flow, there will be voltage differences across the resistance of the paralleling wires.

The BMS then does not read the actual cell voltages, and cannot respond correctly to problems in the cells.


As for the differences between cells, whether it will be a problem depends on how long you can and need to leave the battery on the charger for every cycle, so that it can rebalance the pack, and how quickly the problem grows worse (since differences will nearly always get worse with time and usage; the harder they're used the faster that usually happens).

Note that BMS with a cell-delta protection limit function would normally shutdown a pack that has a tenth of a volt difference between cells, as that indicates a serious difference between the cells' characterisitcs, whcih usually indicates a problem with the cells that are that much different from the others. Some BMS with this function have a programmable value for it, or even the option to disable it...but it's a good function to have since it warns you that the pack is approaching a serious problem or even failure. Whether it matters to the BMS if this difference exists only during current flow or is there all the time, probably depends on the BMS design.

 

[Geoffers]

Thanks
I am assuming there must be current flowing in BMS wire as they are the only parallel link between the 2 cells of cell 12, I haven't tried to measure it.
I'll check my BMS settings for a cell delta function, but I'm seeing over 120mV difference when drawing 170A so I'm guessing one of the cell 12 is iffy.

It's pretty important this is right it's a system that will be going 24/7, downtime will be costly and a bit stressful.

If I've interpreted what you've said right it would pay to swap for my spare now and hopefully it will be ok?

I'm kind of wishing I'd waited for the lifepo4 batteries now but they were always a month away in the autumn!

Cheers Geoff

 

[Geoffers]

Been looking at this a bit more,
When I set it up I had to split one module to get 2x2s to got with the ( 3x4s)X2 to make my 14s2p battery, this split battery is above one series stack, so the series cable from the other is longer. Cell 12 BMS lead is fixed to - of cell 13.
I thought this might be giving a misleading reading so checked the voltage drop on the cables.
The longer cable has a 30mV drop, shorter less than 5mV.
But also one of the positive series cables (top of cell 14 had a 130mV drop. Cleaned it up and tightened it down.
I know have 70mV difference between cell 12 and the lowest cell when charging, 30mV of that is likely the longer cable, I'm not sure i can do much about that? Maybe replace with busbar?

Thanks Geoff

 

[eee291]

I wouldn't worry too much about a bit of unequal voltage sag under load, unless it causes unbalance when not under load.
Turn off the balance function and do like a dozen cycles, measure all the cells afterward if there isn't significant cell drift then it's fine.
Also see the same thing in all my batteries, regardless of cell chemistry.

 

[Geoffers]

Thanks, this is all new to me. There seems be quite a few ways to get it wrong!
It will be under load quite a bit, although hopefully the solar will run things alot in the summer

 

[Geoffers]

I wouldn't worry too much about a bit of unequal voltage sag under load, unless it causes unbalance when not under load.
Turn off the balance function and do like a dozen cycles, measure all the cells afterward if there isn't significant cell drift then it's fine.
Also see the same thing in all my batteries, regardless of cell chemistry.

Thanks for this, have done several cycles now with the balance turned off


They seem to have settled in now if that's a thing, only 0.07v diff charging at 90A