This Lithium-Polymer battery is made especially for us, and stays strong even at low temperature.
This battery is only needed when the first battery gets old and no longer fully charges. There is no point in buying a spare until you need it. It is not possible to change the battery quickly enough to use as a substitute for recharging a good battery that has drained from use.
The battery can be replaced by yourself. Simply remove the 4 small Phillips screws, extract battery and disconnect wire, and replace.