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Large amperage gap between 2 identical batteries in parallel

kolek

kolek

Inventor of the Electron
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Two identical batteries in 2P16S setup.
Been sitting in parallel for 3 weeks.
During charging, 1 is charging at 16A, the other is charging at 25A. Voltage is identical between the 2.
This is according to the JK-BMS.
Verified the JK-BMS amperage difference with a clamp meter. The readings are accurate. Gap is real.
Checked Victron Smart Shunt, and the total amperage is 16+25=41A, so that checks out as well.
Everything is identical between these batteries. Same wire lengths, same wire. Same BMS settings.
How is this possible and if it's an indication of a problem, what would the problem be?
 
Last edited:
Opened the 2 batteries up, using Klein multimeter measured the difference in voltage between 2 cells.
Battery 1, cell 10, 3.345V (this battery is charging slower)
Battery 2, cell 10, 3.342V (this battery is charging faster)

So there's a 0.003V difference in voltage across cell 10 between the 2 batteries (I randomly picked cell 10 in both batteries as a point of measurement).
 
Measured the amperage difference again. This time, same thing, one is charging at 10A, the other at 2A. Seems 1 battery has a lower internal resistance.
 
Last edited:
Measured a bunch of individual cells, comparing between the 2 batteries. On average there is only a 0.001V difference. The BMS's show an identical voltage between the 2 batteries. The BMSs have had their voltage calibrated.
 
Ampster said:
What happens when they get to 3.45 to 3.50 volts per cell? That is where the charge curve is steeper and SOC should be closer together.
Click to expand...
I'll pay attention next time that happens and check.
 
They are fine imo, they have to balance out at the top and the bottom on the voltage curve legs. In the middle, they can wander around on each other but it's ok because from top to bottom you will always get full capacity out of both.

I pick one battery to be preferred and I use its SOC for my SOC based triggers, and the other battery I ignore and it's just along for the ride. If it wants to take more or less of the load / charge at any point in time that's fine.
 
kolek said:
Two identical batteries in 2P16S setup.
Been sitting in parallel for 3 weeks.
During charging, 1 is charging at 16A, the other is charging at 25A. Voltage is identical between the 2.
This is according to the JK-BMS.
Verified the JK-BMS amperage difference with a clamp meter. The readings are accurate. Gap is real.
Checked Victron Smart Shunt, and the total amperage is 16+25=41A, so that checks out as well.
Everything is identical between these batteries. Same wire lengths, same wire. Same BMS settings.
How is this possible and if it's an indication of a problem, what would the problem be?
Click to expand...
i had the same issues from day one, and in the end I just learned to live with it. basically no matter how close they are, there are internal resistance differences or even super minor differences in the resistance of cables joints etc. currently with three banks/packs, two of them 16s winstons, and then the 3p16s calbs the differences are generally not as far off as yours, but they can still be in a range of 20-30 amps between highest to lowest.

noticed the same issues when it was four packs of the Calbs in 16s. prior to buying the winstons. thats what made me combine them into 4p16s and I ended up with balancing issues. after testing each cell individually and then sorting them into a 3p16s pack (and removing the poor performers) i fixed the balance issue in the pack itself but still have the issue between the three individual banks.

I honestly do not see an issue at this point as they all stop charging roughly at the same time then the SCC's go into float.
 
kolek said:
Both are wired directly to 600A busbars with same length wire
Click to expand...

Is the wire that comes from the charger connected to opposite ends of the bus bar?

Pos
Charger -b1 - b2
Neg
B1 - b2 - charger

You can put it in the middle, but then loads would be uneven.
 
robbob2112 said:
Is the wire that comes from the charger connected to opposite ends of the bus bar?
Click to expand...
No, not opposite.


Charger +b1 +b2
Charger -b1 -b2

So technically if you include the distance between the terminals, there's maybe 1-2 inches difference in distance to the charger.
 
kolek said:
No, not opposite.


Charger +b1 +b2
Charger -b1 -b2

So technically if you include the distance between the terminals, there's maybe 1-2 inches difference in distance to the charger.
Click to expand...

Might be interesting if your cables reach to switch one of the charger cables to opposite ends and see the results. There are some interesting articles on bus bars and how power flows in them.
 
I have 4 "identical" batteries. One always carries a little more of the load between about 95% and 60% SOC, then it drops below the other three and tracks pretty even after they catch up. It's the same on charging, it pulls more than the other three. They're from different lots and were purchased a couple years apart. It plays havoc with my OCD so I just look at the overall SOC as reported by the main shunt. You'll never get them to match perfectly and it really doesn't matter.
 
robbob2112 said:
Might be interesting if your cables reach to switch one of the charger cables to opposite ends and see the results.
Click to expand...
Just a point of interest, but actually the battery which is 1 inch farther away from the charger/inverter on both the positive and negative busbars is showing lessor internal resistance (higher amperage).
 
I have been seeing this for a year...
I even bought a second victron shunt to graph it.
It certainly evens out after 3.45 / cell

At this very moment I have 2 280k packs wired in a Z and 8 50ah renogy batteries wired in 2p/ bus run. 6 bus connections for all batteries.

So the short of it is:
The 400 Ah battery does all the work
The 560 ah battery does the "hurry up" after 3.45 v /cell
 
It’s early and most of my PV is going to loads, but this is typical.
 

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kolek said:
Just a point of interest, but actually the battery which is 1 inch farther away from the charger/inverter on both the positive and negative busbars is showing lessor internal resistance (higher amperage).
Click to expand...

still an interesting test to try - I dunno if it will even things out or not.

and I am gonna ask - are you using a torque wrench and no-ox-id special or equivalent grease?

Clean lugs on the cable ends (both) with the grease

Do you have a yr1035 meter that you can use to measure resistance of the battery and of the cables in milliohms? They are pretty cheap off aliexpress or other sites - seem to all be the same meter.

The YR1035 would narrow down any high impedance connection resistance very quick as well as mis-matched battery impedance. An all but perfect crimp, to much plating on the bus bar, higher impedance in the battery, etc. Just start on the whole and move one lead closer each time and see where they are different.

The difference would be tiny, but enough to throw things off.
 
yes, I would think it is a good idea - to avoid dissimilar metal corrosion. Now - realize that 99.99% of it will squeeze out when you tighten the bolts, but that is ok as it protects the wick between the surface and the lug and the nut/bolt.

My rule of thumb is use it on ALL mating surfaces that carry electricity or any you want to protect from corrosion - very thin layer.
 
robbob2112 said:
still an interesting test to try - I dunno if it will even things out or not.

and I am gonna ask - are you using a torque wrench and no-ox-id special or equivalent grease?

Clean lugs on the cable ends (both) with the grease

Do you have a yr1035 meter that you can use to measure resistance of the battery and of the cables in milliohms? They are pretty cheap off aliexpress or other sites - seem to all be the same meter.

The YR1035 would narrow down any high impedance connection resistance very quick as well as mis-matched battery impedance. An all but perfect crimp, to much plating on the bus bar, higher impedance in the battery, etc. Just start on the whole and move one lead closer each time and see where they are different.

The difference would be tiny, but enough to throw things off.
Click to expand...
I’ve noticed a trend in recent YouTube videos where nobody cleans the battery terminals and bus bars or applies no-ox anymore. The just plop the bus bars on and torque them down. I believe this leads to a lot of problems.
 
Brett V said:
I’ve noticed a trend in recent YouTube videos where nobody cleans the battery terminals and bus bars or applies no-ox anymore. The just plop the bus bars on and torque them down. I believe this leads to a lot of problems.
Click to expand...

Oh I agree - and I never take videos as the gospel - there is always something left out, and in many there is something that makes me cringe.

Unfortunatly there are a lot of new folks that see the video and assume they can just freeze frame and copy it for optimal performance.

NOTE - I am not pointing fingers at anyone, just in general
 
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