AC Power and Auxiliary Battery Install

An M3 is not for touring. The ride is stiff, the engine is high-strung, and the constant eyeing of the temperature gauge can make long hauls relatively uncomfortable.

But I love this fun car, and comfort won’t stop me from road tripping it.

I wanted an electric cooler so I wouldn’t need to buy ice every day. It came with a cigarette lighter power plug, but I also didn’t want the box crowding the cabin. An auxiliary deep-cycle battery in the trunk along with a 300W inverter was the best of both worlds, providing AC and USB power in the trunk.

33Ah of power was a nice addition for the car’s always-on router / Pi Zero / ESP-8266 collection. It’s been able to run all 3 for about 7 days continuously on a full charge. I put power onto one of the wires originally routed a year or so ago, so aux battery power should be accessible anywhere in the car now.

Finished pictures!

Unlike my usual hack jobs, this came out cleaner than I was expecting. It’s nowhere near perfect but the battery fits very nicely in the empty plastic to the left of the trunk and is held in securely with 3 metal brackets drilled into the chassis.

The gaping hole where the inverter and power cables come through was for BMW’s CD navigation unit, which has been pulled along with the dash. I used KeyLine’s battery isolator – a complete kit with wonderful instructions – and drilled it into the plushy tire inflator kit holster.

This isolator ensures that the aux battery is functionally separate from the starter. Any devices that drain the second battery won’t kill the starter, and it also ensures the aux battery is charged when the car is actually running. I’m not sure how long the battery takes to charge from empty, but it’s not something the alternator can’t handle!