A good friend had a 10U that had a very short battery life. He was leaving in a week on a two-month cross country motorcycle ride and asked his local dealer about a replacement battery. The dealer told him none were available and that he would have to buy a complete new unit. He was very upset with that but had no other choice. I asked him to send the dead unit to me.
The old battery showed signs of overheating, melting and swollen plastic wrapping. I found the closest battery available in size and shape on eBay for $7.99 (an identical battery could not be found anywhere).
The OEM battery is a Li-po 300mAh, 3.7v, XK 511643. The last number is the outside dimensions of the unit; 5.1 mm X 16mm X 43mm.
The replacement battery is a Li-po 380mAh, 3.7v, EHAD 501646. (5.0mm X 16mm X 46mm) so it's 3mm longer and 27% more capacity)
The OEM battery leads have a VERY small 2mm plug that snaps into the "motherboard" connector. I removed the leads from the old battery and soldered them to the leads on the new battery, complete with the plug.
Unfortunately, the "bottom cover" to the battery-motherboard compartment was glued to the cover and destroyed when the compartment was opened. That requires the cover with the battery and motherboard to be attached directly to the SENA mount. Two small screws do that nicely.
All pics here: