Blue Sea Systems 7718-BSS ML-Series 12V 500A Battery Switch

The Blue Sea Systems 7718-BSS is a heavy-duty 12-volt remote battery switch from the ML-Series, rated for 500 amps continuous. It's built to make and break big DC loads from a distance, so you can kill power to the whole boat, or combine two banks, without ever crawling back to the battery box. It ships with an SPDT ON-ON Contura remote control switch and runs silver alloy contacts inside. That's the short version. Here's what it actually means on your boat.

ML stands for magnetic latching. Once the solenoid throws, it holds its position on its own, without drawing coil current to stay there. So you're not bleeding amps out of your house bank just to keep the switch closed. You flip the Contura switch at the helm, the solenoid latches, and it stays put until you tell it otherwise. For anyone running a boat that sits for weeks between trips, that's a real advantage over a switch that has to stay energized.

The 500-amp rating is continuous, and this switch will handle real engine-start and windlass loads on top of that. But that current has to get there. Run heavy cable to it, 4/0 is typical on a 500-amp circuit, and torque the terminal lugs to spec. Loose lugs are the number one failure we see on any high-amp connection. They heat up, they corrode, and eventually they let go. Snug them down properly at install, then check them again at the start of every season.

Protect the circuit, too. Put a suitably rated fuse or breaker close to the positive battery post, within about 7 inches of the source, so the cable is protected before it ever reaches the switch. The 7718-BSS switches the load. It doesn't protect it. That part is on you and your fuse, and it's not a corner to cut on a 500-amp system.

So where does it shine? Dual-bank setups on trawlers, cruisers, and sportfishers where you want a clean remote disconnect plus the option to combine banks for a jump-start or a heavy draw. Wire up the ON-ON Contura switch and you can cross-connect two battery banks or isolate them, right from the dash. No more reaching for a big manual selector buried in a locker. Pair the 7718-BSS with a Blue Sea automatic charging relay if you want automatic charging between banks with a manual override in the same system. It's a common combination on well-rigged boats, and the two are designed to talk to each other natively.

A couple of honest notes before you buy. The black housing runs warm under sustained high load, so give it some air and keep it out of the hottest corner of the engine bay if you have the room. And it isn't a sealed unit. Mount it in a dry, protected spot, up out of the bilge. Moisture sitting on the coil terminals will corrode them over a season and rob you of reliability right when you're leaning on it. On a metal boat, run the remote switch leads in shielded cable to keep electrical noise off the signal so the switch doesn't false-trigger.

The silver alloy contacts are a big part of why this holds up. They resist welding under the heavy inrush you get from starting engines and spinning up a windlass far better than plain copper would. They'll still pit over years of hard high-amp cycling, that's just physics, but this is the contact set you want for the job. For a serious dual-battery boat that needs remote switching you can actually trust, the 7718-BSS is a solid pick. Wire it right, fuse it close to the post, keep it dry, and it'll run quietly for years.