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I have a battery pack for 3S1P li-ion batteries, with a charger. The battery protection board inside the battery pack does not have cell voltage or temperature monitoring.

Can I replace the batteries and their configuration to a 3S2P configuration, and keep the charger? I plan on replacing all the cells with 6 identical, but higher capacity cells.

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The answer, sadly, is 'it depends'.

Adding a parallel set of cells will at least halve the impedance the charger sees. It probably would be able to handle it, but you should expect longer charge times and/or higher temperatures. And on the first couple runs, I would be ready with a IR thermometer and a fire extinguisher, but I'm kinda paranoid.

Note that you said "identical but higher capacity," which isn't really possible. Higher capacity cells in the same size typically have lower internal resistance, and that can also change the dynamics of the situation. It shouldn't be a problem, but it's something that should be noted.

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It is PROBABLY OK, but as the previous answer points out, it does change the dynamics of the charging somewhat and I would watch the first couple of charges closely and charge them outside on concrete to be safe.

I am also very suspicious of what the protection circuit actually does. You said that it does not monitor voltage, then what does it monitor - current only? Most protection circuits monitor voltage at a minimum. It doesn't actually matter, as adding more batteries in parallel doesn't affect voltage anyway.

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