power pack

In English, we might call them batteries.  They provide power for Federation devices.

Three sizes are used.  I measured them for this report.
The smallest is about 2 x 1.25 x 0.6 centimetres – smaller than the memory card of my camera.  It’s used in wrist units.
The next size is about 8.1 x 5 x 2.4 centimetres, about the size of a pack of playing cards.  It’s the most common size, used in stun-guns, torches, Carringtons, etc.
The biggest size, 32.4 x 20 x 9.5 centimetres, is about the size of a large packet of cereal on Earth.  It’s used in tractors, scooters and pegasuses.

Power packs look like black plastic, but each end has three metal holes – one in the middle, and one at each side of it.

Power packs have grooves in the sides, near each end.  When you push the power pack into a device, something clicks into these grooves to hold the power pack in place.