I searched this thread, and found this post from someone who replaced the battery:
http://www.head-fi.org/t/782912/xduoo-x3-dsd-24bit-192khz-cs4398-chip-lossless-music-player/3975#post_12929950
Their link to the aliexpress page selling the battery doesn't work anymore. Regardless, I'd rather buy from a more reputable seller (and since I'm in the US, ideally a US seller). I found this, and while most of the specs look like a good match, the thickness (7mm) is probably too much:
https://chicagodist.com/products/lithium-ion-battery-3-7v-2000mah
I found it interesting that they say this:
"[COLOR=404040]Like most lipos, the batteries we sell do not have thermistors built in. This is why we suggest charging at 1 C or even less - 100 to 500mA is a good rate, and available from any USB port if using a USB-powered charger."[/COLOR]
[COLOR=404040]Does anyone know if the XDuoo X3 includes circuitry to limit the charge amperage? If not, maybe that's why so many people experienced battery issues (and/or maybe the batteries they include weren't reputable). I know I sometimes charged mine with a 2A charging source, and I wonder if my battery would have lasted longer from a 500mA only source.[/COLOR]