Voltage Sensitive Relay (VSR).
The Voltage Sensitive Relay (VSR) allows two batteries to be charged at the same time.
When the engine is started and the start battery reaches 13.7 volts, the VSR engages, allowing two battery banks (start and auxiliary) to be charged simultaneously.
When the voltage drops below 12.8 volts (eg the engine is stopped), the VSR disengages, separating the batteries. This system eliminates the possibility of draining the start battery and protects sensitive electronic equipment powered from the auxiliary battery from harmful engine start up spikes.