The secondary 12 Volt battery, which provides power to Vehicle Management VMS, Horn, Headlights, Audio, Door Locks, etc. is located in the passenger wheel well.

The addition of this battery was an afterthought in the Roadster, with early cars feeding 12 Volt devices directly from the main ESS Battery Pack.

In 2.x Roadsters, a motorcyle battery was selected providing 12 Volts at 8 Amp Hours.

Our two replacement batteries are actually designed for electronics applications like this.  One is a 12 Amp Hour battery, providing more power for peak demand like audio systems in the car, another replacement is a Lithium Ion battery.

This battery requires replacement every two years, usually alerting owners with the "SWP Aux Battery Error".  

If you want to avoid future costly replacements 

Our 12 Volt battery relocation kit makes future battery replacements, a quick 3 minute DIY process.