Is the battery pack just multiple 18650 batteries (they do tend to be)? If so, replace them with some better ones, this is a reputable seller https://www.ebay.co.uk/sch/big_f_d_d/m.html?_nkw=&_armrs=1&_ipg=&_from=
Lots of the cheap knackered ones are taken from old laptop batteries.
Torchy has a good rep when it comes to batteries. His website has some interesting stuff on cell testing.
@Emyr started
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Is the battery pack just multiple 18650 batteries (they do tend to be)? If so, replace them with some better ones, this is a reputable seller https://www.ebay.co.uk/sch/big_f_d_d/m.html?_nkw=&_armrs=1&_ipg=&_from=
Lots of the cheap knackered ones are taken from old laptop batteries.