The BatPack, or battery backpack, is a backpack filled with batteries that can provide energy to certain electric tools. It stores 60,000 EU, which is essentially 6 times the storage of any individual tool.
The BatPack is equipped in the Chest Slot, and is then immediately put to use by any relevant tools:
Using a tool with BatPack energy available will charge the tool (up to twice per use) from the BatPack. In other words, if you take a halfway dead tool and begin using it with the BatPack, the tool's energy bar will eventually be full. When using a tool, the BatPack supplies the energy needed for that use, as well as charging the tool itself. It can also be used to charge a dead tool, by destroying a block when the tool has no charge, this, however, may be a glitch. ref The BatPack breaks from a moderate amount of damage, but only when empty. It provides one and a half armor points.
Alternatively it can be charged when worn by using the Static Boots or the Solar Helmet. They can be used in conjunction for greater charging, or replace each other when the charging requirements are not fulfilled.
Again just like a RE Battery, a batpack can be used to power machines.