External Battery Pack
If you want to use multiple batteries for extended playing time, then the external battery pack is what you need. When the external battery pack is installed, the battery sits externally to the machine and is connected via a small power/charging cable.
When one battery runs low, simply unplug it from the machine and swap it for the full battery.
The power cable also doubles as the charging cable. So if you want to charge one battery while using another you will need another power/charging cable, which can be purchased separately.
All new machines come with the first battery free. Additional batteries can be purchased separately.