What is the battery under back seat of a buick lesabre for?

The battery under the back seat of a Buick LeSabre isn't a *main* battery like the one under the hood. It's likely a battery for the supplemental restraint system (SRS), commonly known as the airbag system. This smaller battery provides power to the airbag control module and other components of the airbag system, ensuring they have power even if the main battery is disconnected or fails.