Load sharing between different size lifepo4 battery packs

[ericfx1984]

I am wanting a little help to predict the results of mixing the following battery packs



1.) 4s2p, 230ah Eve cells, 200 amp BMS (about 5.9 kWh)

2.) 4s4p, 304ah Eve cells, 200 amp BMS (about 15.6 kWh)

Total would be about 21.5 kWh (the smaller battery would be about 27% of the overall capacity). Does that mean then that the smaller battery would only pull about 27% with the remaining 73% coming from the larger battery?

Let's say I had a 250 amp load(I don't, but let's imagine)... Would the big battery get around 185 amps while the little battery gets only 68 amps???

Or will it be closer and less skewed than I am imagining?

 

[Supervstech]

If the bank is on a bussbar, and equally wired, the load would be pulled from both fairly equally, the larger battery should provide the bulk, and the smaller should eek it out to balance voltage as the total bank charge drops.
The draw on the total bank should drop evenly.

 

[stok]

big battery get around 185 amps while the little battery gets only 68 amps???

not exactly but almost , the 4p pack have half the resistance the 2p pack have(if the cells are the same,in this case the are not but still), so current will flow easier from the 4p pack, but the two packs will always equalize their voltages and remain at almost the same %soc through out the charge/discharge curve.

i have a pack of 100ah cells, that is paralleled with a 120ah pack and 50ah pack, the three packs have vastly different cells and manufacturers and they are almost always at the same SOC

 

[RandyP]

This is interesting and fun to follow

 

[DThames]

At the point where the two batteries are connected, looking back at the batteries, a lot of what happens will be what the effective resistance is of the wiring and the internals of the battery. There will be a tiny voltage drop across the internal resistance and the wiring. The amps from each battery branching circuit will flow based on that voltage drop and some other characteristics of the battery. Under a sudden load, you might see one actually discharge a bit more to the point that when the load is removed, one battery will then be charging the other one, a tiny bit. Some call this micro-cycling and some try to avoid it. It is not all that bad but should be understood as part of your planning and expected results.

If you are only lightly loaded compared to what the max amps that the batteries can deliver, this behavior will be, should be less. I have two older BYDs in parallel with one BMS (about 4.5kwh) and a set of 280ah Eve with its own BMS (about 6kwh), brought together on a large terminal block. I see the behavior above a little bit but it is not a real concern to me.

 

[ericfx1984]

Good stuff thank you everybody