12 Volt Battery Monitor
This product is another fine aftermarket example of what the Roadster might have been had there been more time to focus on Roadster improvements.
The 12 Volt system in the 2.x series Roadster depends on an afterthought small battery tucked into the passenger fender.
Initial Roadster design (1.5 versions) did not include this battery, with 12 volts fed directly from a DC-DC converter deriving input voltage from two of eleven sheets in the ESS battery pack.
This method resulted in uneven sheet usage and balancing problems, so a 12 volt battery was added which needed a home, and the inaccessible fender location was chosen. Gruber Motor Company has an aftermarket battery relocation kit that makes this battery far more accessible, for a DIY replacement.
Roadster 12 Volt Battery Monitor
The Tesla Roadster has a small 8 Amp Hour motorcycle AGM battery tucked under the passenger front fender.
The firmware will alert the owner when outright failure occurs, but often as it declines and voltage drops, the Roadster often begins to exhibit strange behavior with intermittent Key FOB functions, and other security malfunctions.
The APS system in the Roadster, or DC to DC converter, keeps a 13.6 Volt float charge on this battery, and as it declines, this voltage begins to drop as battery capacity degrades.
Along with monitoring the 12 volt system in the Roadster, a USB charge port allows charging of USB devices.
A cabin temperature monitor is also included.
Cell Phone Charging
An example of cell phone charging.
The 12 volt indicator will typically report around 13.2 – 13.6 Volts DC with a healthy battery dependent on 12 volt accessories in use in the Roadster. Devices fed by the 12 volt system include exterior lighting, the audio system, seat heaters, trunk lock, and certain electronics such as the Vehicle Management System module and certain PEM functions.
If you find the monitor begins reporting voltages below 13 Volts DC, especially after vehicle non-use, it indicates this battery is on its way out and due to be replaced.
Gruber Motor Company makes a retrofit 12 Volt battery relocation kit that eliminates Service Center visits for this battery swap, allowing a 3 minute DIY battery replacement, with an additional battery condition monitor mounted on the trunk mounted enclosure.