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On combining 1800 to 3500 mAh cells in a pack...
#11
If all Your cells are above 80% You should be fine.

You can safely mix capacities, but if You do so, Pay attention to the fact that You can not draw 100 Amps from a 100P pack without drawing more than 1A from the highest capacity cells.

However, I have found a way to figure out how much load You put on the highest capacity cell, and You can then do the math and keep maximum load below the wanted max.

Example: 100p mixed from 1800 to 2800 mAh. If in this example, capacities are evenly distributed, total capacity will be 230 Ah.

Now we try drawing 100 A. Total time to empty is 2,3 Hours.

(I know the discharge curves are not linear, but this will get us near enough.)

Highest capacity cell is 2800 mAh and to empty this cell in 2,3 Hours You need to draw 1,217 Amps.
Lowest capacity cell is 1800 mAh and to empty this cell in 2,3 Hours You need to draw 782 mA.

In order to limit the drain from the high capacity cell to 1 A The max current You can draw from this pack is 82,1 A.

Another example we have a lot of 1800 - 2300 mAh and 20 really good cells clocking in at 3300 mAh. Now what?

80 x 2150 (average from the low cap cells) 172 Ah plus 20 x 3300 mAh = 66 Ah.

Pack capacity 238 Ah. Almost equal to the first example, but this time the draw is much higher from the 3300 cells.

Time to empty @ 100 A is 2,38 Hours.

3300 mAh emptied in 2,38 Hours will result in 1,38 A.

In order to keep the drain from the high capacity cell to 1 A The max current You can draw from this pack is reduced to 72,46 A

Apart from this reduction in max current, I see no problems in mixing capacities in the banks.

As long as the banks You build have the same capacity, you should be good to go.
 
Hope this helps a bit, best of luck building Your packs Smile

ChrisD
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#12
IMHO unless you te expecting longish periods of high current draw IR is pretty irrelevant, also mixing a range of capacities is fine.

The thing I’m very picky about is getting rid of leakers (self discharging cells), it’s the major thing I believe that will bring a system undone, fortunately it’s the easiest test to do accurately and consistently.
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