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I was involved in ebikes 15 years ago.
at that time everybody bought LiFePO4 batteries from Ping on ebay. I am pretty sure he is still going.I asked on pedelecs forum a couple of years ago when I was thinking of rebuilding my kit and was told people just got them from aliexpress, and given a links but no recommended seller.
Any battery should come with a charger. It may cost a little bit more but I would get it rather than risking the ebay guys adapter. I would absolutely use it for riding but not for charging.
How did the adapter thing work out?