The battery can be pried out of the case with an iPod Nano opening tool, but unfortunately, it is soldered to the logic board.
The 3.7 V lithium-ion battery has a listed capacity of .39 Watt-hours that provides up to 24 hours of continuous music playback.
This iPod Nano's battery only has two wires, one red and one black. All the other iPod Nanos we've taken apart have included three battery wires. That third battery wire typically ties into a thermistor, a resistor whose value changes with temperature (a poor man's thermometer). Presumably the iPod Nano's battery is small enough and the charge rate is slow enough that overheating is not a concern.