Recently I've opened up a laptop battery pack and got 6 18650 cells and two of them were 0V and the other four had 3.75V.
I fully charged them with a Imax b6 charger and discharged them at 1.0A to 3V.
The capacity readings are:
I was thinking of building a 2S2P pack for my Arduino project.
Since these batteries have different capacities, is it a good idea to put them as a pack? If it's okay, how should I arrange these cells?
If you place multiple cells of different capacities in parallel, they act as a single cell with the combined capacity of the two. You should never put cells with significantly different capacities in series, since the lowest capacity cell would discharge before the others. This is also why you should (unlike what the picture you posted portrays) connect cells in parallel when they are not in series, as it helps even out the different cell capacities.
By connecting the 1152 mAh and 871 mAh cells in parallel you would get the equivalent of a 2023 mAh cell, while connecting the 1121 mAh and 931 mAh cells in parallel would yield the equivalent of a single 2052 mAh cell. You could then put these two parallel groups in series, to yield a slightly imbalanced 7.4V 2023 mAh pack.
You should also aquire a 2S balance plug extension wire, cut the male plug off and connect the wires to each parallel group of cells in the pack.
simulate this circuit – Schematic created using CircuitLab
That said, I wouldn't try to salvage old, end-of-life cells from some old laptop when 4 brand new 2000 mAh 18650s can be bought online for less than ten bucks. Just don't fall for the "6000 mAh UltraFire" scams, research online which cells are actually good.
Putting cells with different capacity in serial is not a good idea. The cell with the lower capacity will run out of energy much sooner than the other.
Because of different resistance the pack could explode if you build your pack as 2s2p. Imagine 1 serial string with the higher capacity as a charger for the other string with lower capacity. The cell with the higher capacity and thus lower resistance would get overcharged. If you use protected cells (with protection circuit) the pack or one serial string would shut down when over/undercharged or too much current would be drawn. But even using protected cells with different capacitance in a serial setup is not a good idea. Hope this answer helps and you can understand it. My English is not the best ;)
edit: Remember to use a balancer when charging Li-Ion batteries in serial setup.
There is no problem if you are charging them with a good balancing charger like imax b6 equipped with a temperature probe that can cut the charging process if the temperature of the pack is getting high!
