Van Power to Charge Boat Batteries

You may know that Big Duck sports a freezer on her aft deck. We use a 5 gallon brine brick to keep the temp in that freezer  28 - 34° for my beer and frozen food when the freezer is unplugged. Unplugged, the brine brick is good for at least 6 days even with much use.

The challenge is the refrigerator -- which I call Suzy's Lettuce Locker. When the original 12/120 unit bit the dust, I opted for a cheap dorm, all fridge (no ice maker) unit at 120 only.

Suzy's Lettuce Locker

We have a 3000 Watt inverter on board so getting 120 for the dorm fridge is no problem if the batteries are up.

Xantrex Freedom 30

On the water, we usually run the motor 2 - 3 hours a day and with the generator use at night for heat - a/c or just cooking, we have plenty of reserve for the 120 fridge. The challenge is on the road while we use Big Duck as a camper. Well, I guess if we stopped at campgrounds every night and hooked up to shore power we'd be OK, but we prefer to Wallydock = dry camping. So how do we keep the batteries up for the fridge without running the generator all night?

Most vehicle plugs have the ability to send charging current to the trailer break-away battery to keep it charged for emergency braking. My van manual says I should use a 30 amp fuse in the circuit to charge that break-away battery. I have wired that circuit to my house batteries so while on the road, my van alternator can charge my boat's inverter battery in addition to the break-away battery.

Van connector now feeds power to polarized connector

This is the boat connection that is wired to Big Duck's house batteries w/ #6 cable.

This doesn't complete the installation because now the polarized connector directly sees the full potential of the break-away battery...touching the hot pin to ground will be a direct short. I could just put in a switch but I know I'd forget to have it in the right position at the right time. Poking around in the electric parts locker I came up with a spare VSR (voltage sensing relay). I have these installed between all of Big Duck's batteries so if any battery is receiving a charge from any source, the relay closes and they all can receive a charge.

VSR or Battery Combiner

I wired the break-away battery to the VSR so he polarized pin is only hot then there is a charging voltage.

A test showed that a 10 amp battery charger will run fridge and keep the house batteries up but it will take some road miles to really prove the system.