A vehicle electrical system for a vehicle includes a power source, two low-voltage buses, two power-distribution boxes each electrically connected to the power source and to one of the low-voltage buses and positioned to control electrical flow from the power source to that low-voltage bus, and two low-voltage batteries each electrically connected to one of the power-distribution boxes and arranged to supply electricity to one of the low-voltage buses via that power-distribution box. Each power-distribution box is programmed to perform a test on its low-voltage battery, the tests being isolated from the other low-voltage battery.