Voltage Sensitive Relay (VSR): Redodo 12V/24V 150A Dual Battery Isolator is all you need to charge Two Battery Systems. Voltage Sensitive Relay (VSR) cuts in at 13.4 volts and cuts out at 12.8 volts to ensure your starting battery is always charged and ready to start your vehicle. It's like having two separate power sources for each battery while only having one alternator!