I tried charging it. It charges to about 75% or so (hard to tell with the indicator) but seems to hold a charge well. Maybe it really isn't charging all the way. I tried letting it discharge completely and then charging it again to train the controller but it didn't make a difference. I bought one on eBay. This here is the listing for the one I bought.
https://www.ebay.com/itm/176108652412
Did you eventually find a better battery for your reader?