This is the Directed 520T battery backup system.

Cutting the battery is the standard way to silence a car alarm. Open the hood, disconnect a terminal, and the siren stops. The 520T removes that option.

It keeps 12 volt power going to your security system from its own battery pack, so the alarm carries on working even with the vehicle battery disconnected or flat. Better still, the module sends a negative trigger to your alarm the moment the main battery is disconnected — so cutting the power sets the alarm off rather than stopping it.

The charging circuitry keeps the pack topped up automatically, so there's nothing to maintain.

Also worth considering: a battery backup siren, which builds the same idea into the siren itself.