Automatic 125A Battery Charging Relay (12V) for Dual Batteries – Waterproof IP67

The 125A Automatic Charging Relay is an intelligent dual-battery manager for your boat or RV. Acting as a smart battery isolator, it automatically manages two battery banks without any manual switching. When your engine’s alternator or onboard charger raises system voltage above ~13.3V, this relay closes to combine the batteries so that the secondary (house/accessory) battery also charges. Once charging stops and voltage drops (~12.8V), the relay opens to isolate the batteries, ensuring that your starting battery won’t be drained by auxiliary loads. This means you’ll always have a reliable start, even after running electronics and accessories at anchor.

Built with a rugged, waterproof enclosure (IP67), this relay is made to withstand the harsh marine environment. Install it in the engine compartment or near the batteries – it’s sealed against moisture, salt, and dust. Its low self-consumption design allows it to stay connected full-time with virtually zero impact on your batteries. The internal VSR circuitry reacts quickly and safely, with no sparks and no intervention needed from the skipper. You get hands-free, trouble-free operation that extends battery life and ensures optimal charging of both banks.

Recommended Use: This automatic charging relay is ideal for any vessel with a dual-battery setup – from small powerboats and RIBs to mid-sized yachts that have separate engine start and house batteries. It’s especially valuable if you’ve upgraded your alternator or added charging sources (solar panels, shore charger), as it will direct charge current to both banks without risk of overcharge or feedback. By installing this relay, you enhance your boat’s electrical reliability: when the engine runs, all batteries charge efficiently; when off, your house loads cannot kill your start battery. This gives peace of mind and improved safety for every voyage.

• Small Boats (runabouts, RIBs, 10–20 ft): Ensures your starting battery stays charged for the next engine start, even while running fishfinders, lights or radios when the engine is off. Avoid getting stranded with a dead battery after a day of fishing or diving.
• Medium Boats & Sailboats (20–35 ft): A perfect addition when installing a second “house” battery for a fridge, lighting, or navigation equipment. The relay automatically shares charge between the engine and house batteries, so you arrive at the marina with both fully charged.
• Campervans & RVs: Beyond marine use, this VSR is great for dual-battery vehicle systems – keeping the starting battery and auxiliary battery isolated except when charging from the alternator or solar panels. Power your RV appliances from the secondary battery while parked without worry, and recharge both batteries automatically when you drive.