Battery Cartridge is covered with a sticky plastic plate glued on both sides.
Stickers are strong enough to keep the battery cartridge intact. One Green and one Red showing a direction of a connecting plug.
After taking out stickers:
We can see a holder that can be disconnected and two 12V batteries:
The battery used here was: LONG WP9-12SHR 12V 9Ah
Rechargeable Sealed Lead-Acid Battery
DC power, Constant voltage charge
Holder looks like his:
Assume it is simple to replace batteries with new ones that would fit specifications of APC battery. Possibly it is also a way to restore old batteries?