Running slightly different AMP hour cells in series question

[Offroader]

I'm building an electric car battery pack and this will use 84 20 amp hour cells.

24 of the 84 cells will have 1 amp hour higher in capacity.

So 24 cells will be 21 amp hour and 60 cells will have 20 amp hour.

Assume the 24 cells move perfectly together in balance, and the 60 cells move perfectly together in balance. What will happen in the long run if I use all these cells without balancing them, but I stay within the safe voltage limits of the weaker cells?

Assume on the weaker cells I stay within 3.3 and 4.1 and on the stronger cells I stay within 3.3 and 4.1. Will I ever need to balance charge the pack if all the cells stay within balance in their 24 / 60 groups?

 

[fechter]

They should be close enough. The stronger ones will just never get fully discharged, so they might last longer. As long as you use some kind of BMS that watches the cell voltages the weaker ones will still be protected.

 

[Offroader]

I guess it makes sense to top them all off at full voltage, then lower them together?

Then monitor the strong ones to make sure they don't get overcharged?

The way I see it is I won't go anywhere near too low as I will stay above 20% capacity, but I like to charge sometimes close to full.

So I assume the cell voltages if topped off at 4.2 will just be like 3.3 weak cells, 3.4 stronger cells, at the bottom. Then when I charge all the cells they will all come back to 4.2 volts.

 

[amberwolf]

As long as you ensure the weakest cell is the lowest limit of the pack, and don't exceed it's capabilities, the pack will most likely be perfectly fine like that.


This is how I use my EIG packs on my bike and trike, although at present the pack I'm using is well-balanced and cells appear to be equal capabilities.

Just make sure you also test the discharge capabilities of the cells, because a cell that has less capacity may also have higher internal resistance, so it may perform differently. If you do have cells like that, then the worst performer is the limiter of the pack there, too. If you have one cell that sags a lot above 100A (but is ok below that), for instance, where the others don't, then 100A is the pack limit, and not pushing it to that is best to keep the worst cell from aging faster.

 

[dogman dan]

I would top them all off full, just before use. Then monitor voltage on the lowest capacity cell or cell group in the pack.

Bear in mind, no guarantee the lowest capacity cell is in the 20 ah bunch. Or if it is, that it stays that way forever. That's part of why you want to top off all each time. Balance at the top when you balance them. When not balance charging, its ok to slightly undercharge all of them to try for a longer lifespan.

 

[999zip999]

What size pack are you building ? With bms or without put a set of sense wires on the pack for ez monitoring.