Problem Li-po battery

I have a handheld handheld console but something strange happened to me twice, I kept the device turned off with the xiaomi BM20 battery inserted and both times after a period of non-use (maximum 2 months) when I went to use the device the battery was swollen swollen, what could be happening? if I store the BM20 outside the device it does not swell in the stove but it is very annoying to keep putting and removing the battery from the device

When it's in the console, some current is still drawn and the battery discharges below its safe minimum level.

That wrecks lithium cells, they should never be fully discharged (below around 3V).

the batteries were at 75% in the Chinese universal charger but the voltage they were on was 4.1v

I don't know if the device even off draws battery charge I just pressed poweroff in its firmware and turned it off and stored the device with the battery inserted


can you tell me a good universal charger for BM20 and BP-5L batteries?