# How to connect old 18650 different capacity cells as a 2S2P pack?

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.

1. 1152 mAh
2. 1121 mAh
3. 931 mAh
4. 871 mAh

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?

• At the end of a long wire, in a wall of sandbags, with no flammables nearby? Commented Feb 25, 2016 at 20:35
• @Ecnerwal, where's the fun in that?
– RJR
Commented May 20, 2020 at 10:31

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.

• Your diagram doesn't show a Lithium cell protection circuit which @Ecnerwal was alluding to. Commented Feb 25, 2016 at 20:59
• @NickAlexeev If OP was designing a consumer electronics product it definitively should have one, but such a hobby project doesn't really need one. Op mentioned using an Imax B6 balancing charger, which is designed for LiPo and Li-ion packs used in radio controlled models. These RC packs never have battery management systems, protection circuits or balancing circuits of any kind, so I wrote my answer with that kind of application in mind. In any case I would never even try to salvage old, possibly unstable cells from old equipment.
– jms
Commented Feb 25, 2016 at 21:10
• @jms Safety Concerns with Li-ion. I wonder if you would be as cheery if the O.P. lived in the same building as you, and you knew about this?.. Commented Feb 25, 2016 at 21:25
• @NickAlexeev I already know the safety concerns with LiCoO2 cells, but I admit, you got a point there.
– jms
Commented Feb 25, 2016 at 21:31

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.

• Your English and spelling is much, much better than many of the English speakers on the site. Welcome to SE. Commented Feb 25, 2016 at 20:38

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!