Amp Draw - Parallel Packs

[alferz]

I have a 498Ah 8s2p setup where the two packs were recently installed in parallel. I top balanced both packs and charged both individually to 28.00v before connecting them together. As expected the voltages are identical as measured by my dual JBD BMS's and current flow between them at idle is 0. But under draw, or under charging, one of the packs draws or charges with slightly more amps, for instance at 18a total, 8.75a comes from pack A and 9.25a comes from pack B. Its not a huge difference but is throwing off my BMS SOC calcs somewhat and Im wondering if there will be other negative effects.

Using a clamp meter, I see that the amp difference only appears on the negative side, not on the positive cables where the amp readings are identical.

Pack B currently has longer cables to the distribution blocks where the two are tied together. The round trip is about 50cm/19.6" greater for Pack B. The thing is, Pack B has the higher measured amp draw. Due to the higher resistance of a slightly longer cable, I would expect the amps to be lower on that pack, not higher? Unless I'm missing something.

I'm planning to make the cable lengths identical to see if it helps. But I'm wondering about the science and why I might see different amp draw for Pack B which has a longer round-trip cable length. If not the cable length, what other factors could cause the current disparity?

 

[Ampster]

Have you measured the voltages of pack B at the battery and at the common bus bar where the packs are connected in parallel? If there is voltage drop along that length of cable then the amps will increase. It could also l be a connection with higher resistance. Let us know how it goes with the new cable. If it were me, I would shorten that cable and make sure the connections were clean.

 

[RCinFLA]

8.75A verses 9.25A (6%) is actually fairly good for current sharing delta between battery arrays. 20% variance is not uncommon.

Several things affect current sharing.

1) cabling resistance
2) terminal connection resistance
3) BMS series resistance
4) Battery overpotential voltage vs cell current (amount of cell terminal voltage slump under load and bump up during charging).
5) Cell impedance
6) Temperature variation between cells. (Effects overpotential voltage of cell under load current on cells)

 

[alferz]

8.75A verses 9.25A (6%) is actually fairly good for current sharing delta between battery arrays. 20% variance is not uncommon.

Thats great to know, thank you. Its definitely under 10%, most commonly 6-7% even at higher amperages under heavy load or 70a charging. I guess at the end of the day its never going to be perfect and they will constantly be evening out. I scotch-brited all the lugs, bus bars and battery terminals and used a thin layer of no-ox when making the connections. I'll try some longer cables for pack A and that will probably be as close as I can get them.

 

[alferz]

One thing still confuses me: why would i see the higher current in the longer round trip cable? It seems that with the higher resistance offered by the longer cable, the current on the longer cable would be lower compared to the shorter run pack.

The other thing mystifying me is why i would only see the amp draw difference on the negative cables, but not on the positive side with my digital clamp meter.

Not that I think either of these two negatively affect performance, just wondering about the technical reasons why this might be so.

 

[DougfromdaUP]

The current has to be the same, unless you have multiple cables, so we know that the ammeter is wrong. We just don’t know what the reading should be.