Hello, I just hooked up 2 LIFEPO4 100ah packs, connected in parallel, 2 JK BMS's. Both packs were just top balanced and allowed to rest for 2 days, settling within .08v of each other. I decided to do a discharge test down to 50% of capacity. I started with a 10amp draw and noticed right away an imbalance. Through the BMS's one pack showed -5.8amps and the other -4.2amps. All connection are tight. I was wondering if there might be something in the BMS's that I am missing? Thx for any help.
Parallel discharging imbalance, BMS?
- Thread starter wntrhwk
- Start date
Is it possible some cells have different discharge rates..?
Did you calibrate the BMS current? They're not very accurate measuring current along the entire range. You could also verify with a clamp meter.
LakeHouse
Solar Enthusiast
I wouldn’t be worried about this. Two LFP packs in parallel shouldn’t necessarily be expected to follow the same SoC except at the very top and very bottom.
The packs will be slightly different, the connections will be slightly different (no matter how hard you might try to get the same, the mating surfaces might have different imperfections, for example), and all of this will cause them to behave slightly differently.
scrubjaysnest
Solar Enthusiast
With FLA's this would indicate a wiring imbalance or high resistance connection.
I've yet to hook up my 4 lithium in parallel yet but would expect the current to be much closer.
Are all parallel wires the same length and AWG? Are crimps identical and do you plus from 1 battery and minus from the other feeding the load?
I've yet to hook up my 4 lithium in parallel yet but would expect the current to be much closer.
Are all parallel wires the same length and AWG? Are crimps identical and do you plus from 1 battery and minus from the other feeding the load?
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WIres identical, crimps good, bms showing simialr resistance on balance wires. Afirm on feeding the load from both batteries.
I forgot that you can do that with the JK, I will throw a meter on the cables and see what I come up with. Thx for that.
Not sure how I would test for that.
In all though, those batteries will discharge just fine. They might not be 100% in sync all the time, but the nature of the chemistry (flat curve, internal resistance, etc.) will mean that it's not an issue. 5.8A Vs 4.2A isn't a big difference in any case - try increasing the load and see what happens (with a clamp meter preferably).
Thx for that, I am hitting up friends to see who has a clamp meter. Appreciate you help.
Probably more expensive than my cells, don't think I could get that past the CFO (my wife)
CFO.... she who must be obeyed... I feel your pain... could you spend $200 without scrutiny..?
I could, but it most likely wouldn't see much use beyond my current packs.
I just realized that I am not sure how to set up. Is the BMS that I am calibrating going to the positive cable that is going to my discharge device? I suppost there is no way to get current readings for both batteries without resetting the test on the other posts.
