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I have several lipos that are swelled/puff up, what makes these lipos swell and are they dangerous? can they be charged safely?
If badly puffed, best, safer option is to dispose of them. Puffing is like toothache - it does not get better, until ultimately you yank that tooth out!!
Several reasons why they puff up. Commonest is being used outside of their capabilities, for example a battery capable of supplying 20Amps being used in a 40A setup. Battery will get hot and swell. Also any setup where the battery is discharged too far (safe voltage generally accepted to be 3.2V per cell) can cause the same.
Charging at too high C rating (say 10A charge of a 2A battery) and poor storage can cause problems.
They could also be rubbish batteries.