VSR

Voltage Sensitive Relay (VSR) allows two batteries to be charged at the same time. When the Voltage on the charged bank rises sufficiently the VSR engages, allowing the 2nd battery bank to charge. When charging stops and Voltage falls, the VSR automatically isolates the battery banks, ensuring that engine start batteries are kept fully charged.

This eliminates the chance of draining the wrong battery, while also protecting sensitive electronic equipment powered from the house battery from harmful engine start up spikes.

There is also Digital Voltage Sensing Relay (DVSR), which allows you to safely charge two or more independent battery banks from one charge source, such as an alternator or battery charger.