The iMate contains an undocumented button cell battery, of type CR1225. The purpose of this battery is not confirmed, as Griffin have never formally acknowledged its existence. The consensus is that it provides the power required to switch on certain Macintosh models via the keyboard, from keyboards with a power button.
The battery is not required (the iMate will operate without it, but without the power-on feature), but when the battery becomes depleted, the iMate may fail to function. The case is of the snap type, and it can be prised open to allow the battery to be replaced or removed.
2010 Rev 6 iMate, with the PCB version that uses "Mate" instead of "iMate".