The BatPack is a piece of armor added by Industrial Craft 2. It's worn in the Torso slot of the inventory, and has the ability to recharge the currently held tool. When worn, the BatPack will recharge the currently held tool after one use of it. The BatPack can hold up to 60,000 EU.
Most tools added by Industrial Craft 2 can be recharged using the BatPack, including the Mining Drill, the Diamond Drill, the Chainsaw, the Electric Wrench, the Electric Treetap, and the Electric Hoe. All of the previous tools have a paltry 10,000 EU in their internal storage, aside from the Electric Wrench, which can hold 12,000 EU.
A charged BatPack can be placed in any machine or EU storage device to power it. Conversely, a BatPack can be placed in any EU storage device, Generator, or Charging Bench to recharge its internal storage. Finally, wearing the Solar Helmet, the LV Solar Hat, the MV Solar Hat, or the HV Solar Hat will recharge the BatPack passively, so long as the player is within view of the sky. The Static Boots will also charge a BatPack when worn.
The BatPack can be upgraded to the LapPack, which can hold the charge of five BatPacks and can charge more items.