Big voltage drops aren’t good for marine electronics and manufacturers protect their product by installing an automatic shut off once the voltage going to the unit drops below a certain level. While this protection is good for your electronics, it sucks having them shut down on their own just because you cranked your motor. After doing some research, I learned that sporadic shut downs are a result of new electronics technology not interfacing well with old battery management systems. Isolating your house battery can fix this problem.
Every boat has some sort of battery switch. This is the one that was on my boat and is common for boats that have two batteries. Once you turn it on, your options are to select battery #1, #2 or both.
As you can see, while there are two battery inputs to the switch, there is only one output so you’re basically running everything off the same power source. Setting the switch to both batteries should give you enough cranking amps that you can start your motor without your electronics shutting off. The problem is that if you’ve been running a bait tank, stereo, VHF and lights you’ve probably drawn your batteries down enough that starting the motor will shut down your electronics.
I recently replaced my old battery switch with a new Blue Sea Systems Dual Circuit Plus battery switch. The switch is made to isolate two batteries, one as the cranking and the other the house. The options it gives you are off, on and combine. Off is obviously off, On means that both batteries are on and combine means the batteries are both running everything (like selecting both on the old version). Being able to combine batteries can save the day by letting you start your motor if your cranking battery dies or keep running your electronics if your house battery dies. Since your house battery is no longer connected to your alternator you’ll need a way to recharge that battery while the boat is running. My boat has a BEP Digital Voltage Sense Relay that takes care of that. What the relay does is sense when the cranking battery is fully charged and then transfer the alternator’s charge to the house battery. This one way circuit protects the house battery from being drawn down when the voltage on the cranking battery drops.
Here’s what it looked like installed on my boat. Simple enough. Any questions? Yeah, I only posted my mess to make you feel better about how yours looks once you do it on your rig.
I made this diagram to hopefully clear things up a bit. As you can see, when the switch is turned on, the house battery powers the positive bus bar and basically everything on the boat other than your motor. The starting battery powers only the motor.The yellow shows that everything is grounded. My motor is grounded on a different bus bar than the electronics but I didn’t want to over-complicate the diagram. The charging relay is connected to the positive lead of the starting battery and the positive lead of the house battery. Once the starting battery is full it diverts the extra voltage to the house battery.
A friend who’s a commercial squid fisherman turned me onto this lubricant and told me to spray it on every electrical connection on the boat. It’s totally safe and will protect everything from corrosion and saltwater intrusion so use it liberally.
I know that it probably feels weird to spray liquid onto your electrical connections but it works. The stuff is about three bucks a can so buy a few and keep them in your boat. Every couple months I’ll take a look behind the dash and give everything a good dousing.