Rechargeable Lithium-Ion battery that fits the Line 6 G10 & Boss WT Wireless transmitters.
Model is 17280 3.7v 600mAh
Quite simple to replace if you can handle a soldering iron.
Some helpful Youtube Videos below
PS: I've only replaced the battery in my Line 6 G10 transmitter, but other guys have done the same with the Boss WT Transmitter