Cheap used one, 36v 11Ah, came with a used front wheel geared hub motor kit I bought on ebay.
The used kit is the 26" version of the 700c kit I just got new (without a battery). I only intend to use the 26" bike and old battery for short trips to town or to "the point". That's why I didn't mind buying a used battery (it was essentially a free battery because the motor wheel and controller are as good as new).
I haven't built the kit onto the bike and tested it yet. Waiting for an injury to heal.
I can use the same battery in the nice 700c hybrid bike too if I need to before I finish saving up for a really good battery for it (and decide what I actually want).
I'm now looking into the VRUZEND no weld cell connectors for making my own 48v pack from 18650 cells for the good bike.
With no welds I can open up my battery to test individual cells internal resistance and replace the first cell that goes bad, thus extending the life of the pack, relative to a spot welded pack that would need to be sent away for much more difficult and expensive refurbishment..
But I need to know what BMS to get and how to wire it all up.
It will hopefully be a bit less expensive than having to pay someone else to spot weld and wire a custom pack for me with the same cells, and it will be user serviceable.
If they made one for the 21700 cell type I'd be happier.. Fewer cells, fewer parallel connections needed for the equivalent Wh / Ah.