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Little backstory: I built an electric skateboard about 4 or so years ago and like 6 months back I was riding it at my normal 35 mph and a solder joint came loose on a battery and opened the circuit. That stopped the board in its tracks. I got a nasty case of road rash and only 1 stich.

Anyway I decided instead of just resoldering that one spot I would pull the batteries apart and use a spot welder this time.

I'm glad I pulled all the tabs off because there was several that were not very secure and would likely cause the same thing to happen again - but most of them were crazy strong connections. There were 3 single batteries that were dead (0.5V.)

I got all the batteries cleaned up and they all have the same voltage, but now I can't remember the safest way to assemble the pack to avoid any sparking or anything.

I have them in a 6s4p and they are secured in those square plastic holders from Amazon and they are assembled 2 cells wide and 12 cells long so the top is in sets of 4: pos, neg, pos, and so on.

Do I just lay the nickel strips down over the top then weld them all up then repeat the same for the bottom, or do I weld all the 4p sets in parallel first then go and connect them in series?

I'm pretty sure the latter is the correct answer but that would involve welding some cells twice and that seems unnecessary. I feel like this is a really stupid question to have, but I'm having a brain fart right now and need some assistance.

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  • \$\begingroup\$ Does this answer your question? How to connect old 18650 different capacity cells as a 2S2P pack? \$\endgroup\$ Mar 26, 2021 at 18:50
  • \$\begingroup\$ naw, i know how to connect them to make the proper volts and such. I wanna know if i should put them in parallel first then series all those up, or if i can put them in series as i build the parallel packs? \$\endgroup\$
    – rasmukri
    Mar 26, 2021 at 20:34
  • \$\begingroup\$ An engineer might have to veto this suggestion, but I would think it wiser to connect parallel first. Don't forget to match charge voltage before connecting them as large currents will flow to balance out significantly imbalanced batteries. The series connection doesn't cause current to flow unless the ends of the arrangement are already attached to the load. Basically I think between these factors it's wise to get the more dangerous operation out of the way because whichever operation you do first is slightly simpler. \$\endgroup\$
    – K H
    Mar 26, 2021 at 23:26
  • \$\begingroup\$ Another thing probably worth mentioning is I hope you've read up on the complications of building your own S/P li-ion packs, as battery selection, balancing and matching are all important quality and safety factors that appear before you even start building a pack. Or rather they are necessary in order to build a good and safe pack. If it's not obvious that means you have to be careful about what you replace those 3 0V cells with. Not only should you replace a full set of 4 rather than just 3, but the replacements should be same in: chemistry, brand, mAh, Imax, ~age/use cycles. \$\endgroup\$
    – K H
    Mar 26, 2021 at 23:31
  • \$\begingroup\$ I am replacing 4 batteries for that exact reason, and the ones I'm replacing them with i bought at the same time as all the rest all those years ago, (it was cheaper to buy 60 than 50 from alibaba at the time) so they are quite literally the same 1 for 1 replacements. I know all about load balancing, as i mentioned this is a build I made 4 years ago. i have balance leads coming off every P cell and a balance port on my charger so when it charges they are all the same and even. \$\endgroup\$
    – rasmukri
    Mar 27, 2021 at 3:36

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