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I intend to put 4 X 3.7V 32650 Rechargeable Battery 8200mAh in parallel to keep 3.7V but increase the amperage so the battery last longer.

Looking around is very confusing if for paralel I need or no a BMS or 4 each battery has one.

Marcus Müller
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Teodor
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  • If these are Lithium Ion cells, and you care about your health and property, I would not do this kind of experiments. Just search youtube for Li-Ion fire, and you will see how flammable lithium is. Just mess up a little in your home lab, or make an accidental short circuit, and you will regret for ever. – Roland Feb 19 '24 at 20:05
  • Well thank you Roland, the project will be in the garden so no worries, check youtube for electric cars in fire and stay away from them on the street – Teodor Feb 19 '24 at 20:23
  • Just don't expect that any knowledgeable expert will help you with your dangerous experiments. Buy an off the shelf unit. You can get cheap battery packs for drills that include all charging and protection options, just add a cheap buck converter to get at the desired voltage, and you are done in a safe manner. – Roland Feb 19 '24 at 21:01

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Connecting battery cells with identical chemistry (Li-Ion in this case) in parallel is quite a common approach to get higher capacity. Electrically, you can treat the resulting parallel combination of cells as one single cell with larger capacity. As a result, you need only one BMS.

However, with cells as large as the ones you're using (8200mAh is no joke), you should add a fuse to each individual cell. This prevents the other cells from back-feeding current in case one of them becomes defective and shorts out. For 8200mAh cells, 10A slow-blow fuses should work okay (approximately 1C discharge rate). Make sure to charge every cell to exactly the same voltage before connecting them in parallel via the fuses.

Last but not least, use a proper spot welder to attach nickel strip to your battery cells. Do not, under any circumstances, solder directly to Li-Ion cells! Doing so will locally boil the solvent within the cells, which degrades them chemically and renders them unsafe to use. If you have already done this, please dispose of those cells properly.

It's also possible to buy cylindrical Li-Ion cells with pre-attached soldering tabs in case you can't spot-weld them yourself. That way you can solder to the tabs without risk of heating the cell itself.

Jonathan S.
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