Abstract: The invention relates to a battery switch for use in a multi-battery vehicle to selectively connect or isolate batteries from each other. It includes a detector to monitor the voltage of at least a first battery to detect consistently present transient voltages and on detection of such voltages to connect or maintain connected the battery to at least a second battery. The detector monitors the continuance of transient voltages and additionally monitors the voltage of at least the first and second battery, and isolates the batteries from each other where the voltage of either battery falls below a specified voltage despite the detection of transient voltages at the first battery. The invention also relates to a method of initiating the interconnection or disconnection of at least two batteries on a single vehicle, and to a method of detecting the existence of a charging supply on a supply line to an open battery isolating switch.