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First battery not as good as I'd hoped
I built a 4S3P battery from second hand cells. I measured all of them around 1800mAh and balanced each bank to with 1mAh of each other (not intentionally, I just got lucky). But when running down the cell over many hours with a fairly light load of 0.7 amps, 1 bank was significantly down on voltage. Three of the banks where around 3.3V but 1 was down at 2.8V. When charging back up 3 of them got to 4.2V but the same 1 was lagging behind at 4.07V. I left it charging for quite a while and it didn't get back to 4.2V.

So I'm guessing there is something I should measure besides pure capacity. Do I need to look at the entire discharge profile of each cell? The tester discharges at 1A but reduces the amperage to keep the voltage above 3.2V. It cuts off when it reaches 0.1 amp. Could it be that some batteries reach the point of reduce current much quicker? Or something else I need to look  at?
Take care, it is possible that some cells are not properly connected. Also, did you check your cells for self discharge? Some cells will test wit reasonable capacity but discharge on their own. The best way to check for this is to charge your batteries and leave them sit for a week or two, anything that has dropped below 4.15v should be set aside. I do this before capacity checking, as some cells will discharge so much, I don't bother using them at all.
As Geek said there may be a SD in the bunch.
There are two other things that might be causing your problem and that is IR and percentage of capacity.
Internal resistance of the battery will cause some imbalance of the pack especially with only 3P.
The more cells in P you have the less you will notice this but it still exists.
Lets take your battery as an example 4s3p:
First String
1 IR = 55mΩ
2 IR = 60mΩ
3 IR=  90mΩ
String  IR = 68.33mΩ
Second String
1 IR = 45mΩ
2 IR = 50mΩ
3 IR = 45mΩ
String IR = 46.66mΩ
Third Sting
1 IR = 50mΩ
2 IR = 49mΩ
3 IR = 45mΩ
String IR = 48mΩ
Fourth Sting
1 IR = 52mΩ
2 IR = 41mΩ
3 IR = 47mΩ
String IR = 46.66mΩ

So you see string 1 has a higher resistance than the rest and would cause your pack to be unbalanced. 
Cells of different manufacturers have a different IR thresholds. some will drop in capacity if they are over 60mΩ some won't exhibit any signs of loss at 90mΩ but at 100mΩ will fall flat on their face.
Also just because you got 1800mA out of a battery is not necessarily a good battery indication If the battery was rated at 2000mA and you got 1800mA out of it that's great. If the battery is rated at 3000mA and you got 1800mA then that battery is used up and should be recycled. rule of thumb is generally if you get 80% of capacity it's worth the time. So that is another item you want to take into consideration when you build your pack.

Again when you start to go 20p and up the differences in IR will slowly get massaged out, nevertheless when I build my packs
I will be looking at IR on each cell to match them as close as possible not just mAh capacity and percentage of capacity.

Church1182, Ibiza, BlueSwordM like this post
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 5/23/2019
Total Number of Cells                          5354
Cells  >80% of Capacity                      3801
Cells <80% of Capacity                       1553
Cells ≥2200mAh & ≥ 80% & ≤75mΩ    2645 (155) to go
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