Battery Pack

Four batteries is the whole capacity of the Battery Pack, and that single number defines how it plays. Jeff sells it in The Mines for 100 Gold, and the wiki notes it has a 100% chance of being on offer in that shop, so you will always see one there.

It is a Resource Holder and a Dependant Item: on its own it does nothing, it only stores batteries so you can drop them later in the run, one at a time, where you need a recharge. That makes it a natural partner for rechargeable lights such as the Flashlight, the Straplight and the Laser Pointer, which all refill from a single battery.

Other ways to get one

Louie is the second source. Using a Crucifix against Louie gives a 33.33% chance that he drops a Battery Pack, and handing him a Bandage Pack, a Flashlight, a Straplight, Vitamins or Lockpicks gives a 20% chance.

Compared with the Bandage Pack

It mirrors the Bandage Pack, but where that one holds up to ten bandages, this one only accepts four batteries. The wiki also records that it was not in the Pre-Run Shop before the update that introduced it, so Jeff's stall and Louie are where you will find it.