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Large amperage gap between 2 identical batteries in parallel

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...
It would be interesting to see the results of a capacity test for both batteries separately. My thought is that one is less than the other. You could have both meet the minimum rated and have one exceed it. So it does not mean that one is bad.
 
I use 1000grit garnet paper - I prefer not to leave behind tiny bits of steel from wool or other tiny metal bits from the various oxide papers
 
Brett V said:
I’ve used 1000 grit sandpaper and 000 steel wool, both with equally good results. Then clean both surfaces with alcohol and apply a very thin coating of no-ox on the aluminum contact surface of the battery terminal.
Click to expand...

How about apply the coating, then sand, then wipe off?
In other words, sand off native oxide while wet with coating, attempt to keep oxygen away?
 
robbob2112 said:
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.
Click to expand...

While current is flowing, measured with clamp ammeter, use a mV scale DMM to check voltage drop of each contact, cable, etc. and compute resistance.
 
ScropusGobbleBottom said:
Yup, I am one :)
Probably should get busy.
How do you clean them? Sand paper?
Click to expand...
take a stainless steel wire brush and put the ox guard in the bristles then wire brush the terminal or joint with the oxguard paste. it will clean and coat as it cleans leaving a prepared surface. if you sand or wire brush then apply you will have a surface flash coating of corrosion no matter what you do.

edit: this is how Gardner Bender's Ox Guard instructions read. YMMV

FOLKS: RTFM
 
I use a small circular wire brush on my drill on both the busbar and the ring terminals. Makes a real shiny surface after removing the oxidation. Then apply a thin coat of the no-ox on the ring terminals and the busbar. But then I leave the nuts loose on the ring terminals, so the ring terminals are touching but kinda still flopping around. Leaves a bit of mystery not knowing if those electrons are gonna make the jump across the partial gap or not, which I find desirable. Added benefit is you don't need to use a heat gun on your heat shrink, the heat generated from connection does that on its own, plus you get a nice orange glow indicating current flow. (y)
 
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Daddy Tanuki said:
if you sand or wire brush then apply you will have a surface flash coating of corrosion no matter what you do.
Click to expand...
Welp, there goes my life, down the drain. I gotta surface flash coating of corrosion on all my contact surfaces. :fp2
 
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Hedges said:
While current is flowing, measured with clamp ammeter, use a mV scale DMM to check voltage drop of each contact, cable, etc. and compute resistance.
Click to expand...
Hey Hedges, how are you doing? Nice of you to check in.
I'm gonna try to figure that out and try it.
 
Hedges said:
How about apply the coating, then sand, then wipe off?
In other words, sand off native oxide while wet with coating, attempt to keep oxygen away?
Click to expand...
How about contract with Elon Musk to launch into orbit on Space-X, and in the zero oxygen upper atmosphere, wire brush and THEN apply no-ox?
 
kolek said:
How about contract with Elon Musk to launch into orbit on Space-X, and in the zero oxygen upper atmosphere, wire brush and THEN apply no-ox?
Click to expand...

That'll work too.
But somewhat more practical is to put the part in load-lock, pump down, transfer to wafer processing chamber, use ion etch to remove oxide. Then vapor or sputter deposit a non-corroding metal (with stick layers first, if necessary.)

We used aluminum, sometimes pure, sometimes with silicon or other elements, for wafer interconnect. The native oxide made a good stick layer for PECVD SiO2 and SiN. Bare aluminum bond pads of course oxidized, but ultrasonic wedge bonding broke through.

Copper interconnect was "better" but a huge development effort, needed adhesion layers, got its own oxides if not coated, etc.
The company spent years pushing its copper MCM interconnect, before realizing that the simple and high yielding aluminum one performed pretty well.

PG&E just crimps terminals onto stranded aluminum wire. Might put a corrosion inhibitor in there, but I don't think there is much prep. Certainly not for inner stands (which I have done for screw terminal splices.) Crimping breaks the oxide and mashes metal together.

Battery terminals? They're supposed to be laser welded like G*d (I mean BYD, CALB, and all the demi-g*ds of lithium) intended.

I'm supposed to have some packs on the way. The cells have laser welded busbars joining them, and the end terminals with studs appear to be plated.
 
Is this the first cycle or charge? If so give it at least three cycles to 60% and see if the condition persists.
 
robbob2112 said:
Did you rearrang the cables on the bus bar and did it make any difference?
Click to expand...
I haven't had time yet, but given that the one farther away from the inverter is apparently the "stronger" battery with less internal resistance, it makes me doubt that is the issue, and it seems like most people here say it's a non-issue and ignore it. Moving big cables around my 600A busbar is not a task I take lightly.
 
Does this difference in discharge narrow as the discharge amperage hoes up?

That was my experience.

IMG_2019.jpeg
 
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