This relay charges two batteries at the same time with a single alternator. When the engine is switched on, the Voltage Sensitive Relay stays open until the starting battery reaches 13.7/27 V and then closes the contact and charges the 2 batteries at the same time. When the engine stops and the voltage drops below 12.8/25.2 V, the Voltage Sensitive Relay opens the contact and keeps the two batteries separated thus preventing the wrong battery from draining and avoiding current peaks that damage the onboard electronic equipment when starting the engine. It protects the starter battery from draining due to the use of onboard services. No voltage drop, zero consumption in stand by phase. Easy to install without modifying the alternator cabling. 12/24 V automatic selection.