Blue Sea Systems Automatic Charging Relay | 120A, 12/24V DC

This Blue Sea relay watches your battery banks and automatically ties them together when it senses a charge source, then separates them again once charging stops — keeping your starting battery protected from being drained by house loads.

• 120-amp continuous current rating
• 12/24V DC auto-ranging input works across both voltage systems
• Automatic operation — no manual switch to remember
• Electromechanical contacts with silver contact material
• Quick-connect terminals for a clean installation
• Rated for 1,440 watts of continuous load

It's a solid choice for keeping dual battery banks charged together from a single alternator or shore charger, without the risk of accidentally draining your starting battery.