In general, one or more loads on a vehicle can be connected to both a first voltage source on the vehicle and a backup vehicle power system on the vehicle. If the voltage provided by the first voltage source to the one or more loads satisfies a voltage threshold, the backup vehicle power system does not provide power to the one or more loads. However, if the voltage provided by the first voltage source to the one or more loads falls below the voltage threshold, the backup vehicle power system provides power to the one or more loads.