Auxiliary Battery
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2007 GMC Yukon XL, 2007 GMC Yukon, 2007 Chevrolet Tahoe, 2007 Chevrolet Suburban, and 2007 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
Auxiliary batteries are an available option on vehicles where many accessories can be utilized such as TVs, radios, lights, computers, etc. The charging of these batteries is explained in charging system description and operation. Also, the auxiliary battery is only for accessory use and not part of the starting system.