Used 18650 construction of battery pack, IR + capacity question

shaglord .shaglord . Club Members Posts: 1
edited January 3 in Batteries

Hi fellow lithium battery enthusiasts :)

In a week or two I'll spotweld my "new" 84v unicycle battery together. It's a 20s3p configuration. Current draw is about 3A per cell on avg. Charging rate is around 0.8-1A per cell.

I'm building it from used LG MH1 18650 cells of slightly varying condition.

One way to keep it fairly balanced is to select cells so that each parallel group has similar capacity.

My question to you is how I can use the internal resistance measurement to my advantage when doing so?

Lets say I have a 30, a 40 and a 50 mR battery connected together in parallel. What's the resulting resistance for that group?

If you were me, would your first priority be to keep group IR balanced or the group capacity? I know both is the ideal answer, but if you could prioritize one, what would it be to keep the pack balanced?

Sounds complicated but I got many cells I can swap around. I'm sure there are some good combinations that result in a pack that discharges nice and evenly. This pack will after it's done never go over 4.15V or under 3.5V so it's fairly safe. It will also use the original BMS of the vehicle.

Thank you for reading and commenting. I really appreciate it!

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