Is paralleling batteries just wrong?

Posplayr

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May 27, 2021
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I think that having matched cells is less important than people think. In fact, if you do the math, if you have unmatched cells, it is better to parallel them at a cell level than to have multiple 4s packs with unmatched cells.

Adding resistance in parallel;
R= 1/( 1/R1 + 1/R2 + 1/Rn)

If you have a number of cells (say 12 cells) with a wide range of internal resistance, and you group them into 4 groups of 3, then add the resistances of the cells in each group, the spread of resistances will get much closer together. It is impossible for that to not happen. If you actually know the IR of each cell, you could group them so that all 4 groups had the same IR, even though the individual cells could vary significantly. So the 4 "cells" in your 3p4s pack would be balanced.

Having the cells in parallel assures that the voltage of the cells is kept the same. So, while the SOC of each cell might vary a bit (but not much)in the flat region, as soon as the battery gets into the knee (either charging or discharging) the cells will balance to the same SOC. All the paralleled cells will reach 2.5V or 3.65V at exactly the same time. This would work even with very different cells, even different capacities. You can parallel a 200Ah cell with a 280Ah, and you will get 480Ah.

If you built multiple 4s packs with unmatched cells, each pack could have radically different IR values, and might not share the load equally.

Now, real world, not theory: It doesn't matter. You probably don't have the capacity to measure the differences of what option may or may not work better. If you are pushing the cells *very* hard, charging/discharging at their max rate, then it might make a difference on whether or not you exceed a single cells charge rate based on the IR of the cell it is paralleled with. But at any "normal" rate, it won't matter, and everything will be good again when the cells get into the knee.

One possible answer to "bad things often happen" isn't really a matched cell issue, but a state of charge issue. Before you connect 2 cells in parallel, make sure they are at (nearly) the same voltage.

I went through this in pretty good detail documenting the capacity matching of parallel cells. It certainly appears possible to achieve better matching than factory matching especially if you are getting into the 8S2p and 16S2P configurations.

I don't think it is an internal impedance matching issue but rather capacity matching. Whether impedance matches capacity or not, the current sharing between parallel cells has to follow a SOC balance due to having the same bus-barred voltages.

 

Posplayr

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Watch this video to see how this man parallels LFP cells.
Yea Andy was really starting to go into the mad scientist category during that video. At the end, he was even contemplating putting two 16S strings in parallel (one fully charged the other discharged).

I don't think he did that video though LOL.
 

Honuz

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Oct 22, 2020
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I'm in a conversation on the Electrodacus forum in which I asked opinions on a configuration with an Electrodacus SBMS0 bms and a 2p4s battery pack versus two 4s batteries each with there own JBD 150a bms, both based on 8 280ah EVE cells. Electrodacus replied that paralleling lithium batteries is just wrong; only cells should be paralleled. Yet I see posts on this forum, particularly for RV set ups, where people seem to prefer the two batteries in parallel with there own bms compared to one 2p4s battery? I've seen tradeoffs discussed of both approaches but never a statement like this that it should not be considered at all. So is it OK or not?
You can find multiple discussions with electrodacus where the guy is completely wrong, there is one funny..on Peltier performance.
I would not follow this guy eyes closed.
 
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