72v Adding new cells

[Jimsap3]

I have a 16s 105ah lifepo4 build that I have about 5 cycles on. I've since added to the pack and upgraded it to a 24s.

The new cells were sitting at 3.27v so I depleted the 16s pack down until all the cells were resting at 3.27v. Then built it to a 24s. Everything was charging up good in balance but now the new cells are far off. Any recommendations on how to remedy this situation besides bottom balance? Will the active balancer I'm the jk bms correct it over time? Thanks in advance

 

[amberwolf]

If the added cells are not identical in all characteristics (internal resistance, capacity, cell model and brand (and preferably batch), etc) to the original cells, including their age, they will always behave differently than the original cells, and the pack will always get out of balance over time (less and less time as it ages further).

In that event, there is no permanent remedy with the cells you have; they will need to be rebalanced most likely every cycle, depending on how different the cells are..

If every cell is identical to every other cell, then once they are all at the same state of charge (SoC) they will remain this way until their characteristics begin to vary from each other.


Regarding the voltage you started at....3.27v is around the middle of the capacity range, and with LiFePO4 that part of the curve is so close to flat that they might not all be at the same SoC at that voltage even if the cells were nearly identical. So they might not be in balance at that voltage...you might have to charge them all to full, or discharge them all to empty, to see where they are actually in balance.

But...remember that "balance" only means they are all the same voltage, at that moment.

It does not mean they are all the same capacity, Ri, etc., so their voltages may drift apart (become unbalanced) at other points in the discharge curve.

If checked at various points all along the discharge curve, voltage is an indicator of how closely matched they are to each other, but if only checked at one point in the curve, it only says they are the same voltage at that point.

 

[harrisonpatm]

I made a similar mistake last year. Amberwolf's analysis is spot on: 3.27 could be near the top or bottom of the LFP life cycle, no way to know for sure. Next time, especially with LFP, you'll want to either top or bottom balance your cells before assembly, rather than middle balance which is what you did. No worries though, you were paying attention and caught it before anything bad happened.

A suggested solution that worked for me is to top balance everything as it is. Individually charge each cell to 3.55v (or whatever you'd like your top voltage to be). You can do this without fully dismantling the pack, if you can access balance leads and use alligator clips (carefully!). After that's done, charge them all the way up to the same voltage, this time in series. 3.55 x 24 = 85.2v. Or 82v would be fine. Do this slowly, @ less than 4A current if you can. As they get to the top, you're likely to see them continue to be unbalanced, perhaps as much as 0.05-0.2V That's okay; keep charge current way low as they get near the top. Turn balance on with the JKBMS. If pack becomes fully charged, disconnect charger, leave balance turned on until it's less than .03v delta. Then discharge a bit.

Because of the nature of the LFP discharge curve, a well balanced pack will still deviate at the top and bottom ends of the curve. My 24s LFP pack has a delta cell V of less than 0.005 in the middle of its curve, but when I fully charge it, it creeps up to .05, .09V difference. That's normal.

Once your pack is balanced to your liking, turn balance off and leave it off. A properly balanced pack shouldn't need regular balancing, and if you see it deviate again, it's a sign of something amiss.