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Nuts & Bolts: Dual Batteries for Fridge

Posted in How To: Tech Qa on August 31, 2018
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I’m in the process of modifying my 2009 Toyota Tacoma into a mild overlanding rig. I don’t plan on doing any extreme trails, but I’m really into hiking and plan on using the Tacoma as a base camp for some multiday hiking trips in the backcountry. A 12-volt fridge is among the plans for my rig, but I’m really concerned about killing the battery on the truck. There will be times when I might be away from the truck for a couple of days, and it would be awesome to come back to cold drinks, not to mention fresh food. Should I try and add a dual battery system to the truck? How long can a couple of batteries run a fridge and still have enough juice to start the truck?

John M.
Via nuts@4wor.com

A 12-volt fridge-freezer is easily among the top five all-time best purchases we’ve ever made. Not having to worry about ice or soggy food is life changing, whether you’re out for a day or a week. For all their benefits, however, the one drawback over a regular cooler is that they draw power in order to work. If you’re off-roading it’s not a big deal, as the engine is running all the time and the battery stays charged. But a running fridge in a vehicle that sits for a day or more can suck a battery dry.

The actual amp draw of the fridge is going to vary according to several factors, including ambient temperature, the location of the fridge (shade or sunlight, inside a vehicle or outside), ventilation, and how often its opened. That said, it’s safe to assume it will draw 1 to 1.5 amps whenever it’s plugged in. Our experience has been that in order to keep up with an average fridge, you want to run a vehicle equipped with a good battery for a minimum of 30-45 minutes a day, preferably driving it around (not just idling). An alternator only puts out a fraction of its output potential at idle, hence why we recommend driving the vehicle.

For people who plan on running the vehicle that their fridge is plugged into at least once a day, nothing in the way of modifications is really needed. But obviously, running the vehicle once a day is not going to work for your extended hiking trips. Many fridges have built-in voltage monitors and shut themselves off before the battery is drained to the point of not being able to start the vehicle, but that doesn’t help if you’re looking forward to a cold beverage after a two-day hike, not to mention relying on a function that could strand you.

A dual battery system is a very viable solution in your case, especially one in which the fridge battery is isolated from the engine battery. A good deep-cycle battery with a large reserve capacity should be capable of running a shaded fridge for a couple of days and you would still have a fresh engine battery to crank the truck. Most dual battery management systems allow you to cross-connect or isolate the batteries on demand while also allowing the vehicle’s alternator to charge both when the engine is running.

Another thing to consider is a solar panel. Even a small 25-watt solar panel should be able to keep up with a fridge and help keep the battery topped off on bright, sunny days. We recently ran across a very comprehensive battery management system with built-in solar functionality from Redarc (redarc.com.au). The company offers several different controllers that can manage two or even three batteries.

While you would certainly want to thoroughly test your setup before setting off into the backcountry, a dual-battery system with an isolated fridge battery and a solar panel should offer plenty of power as well as peace of mind when parking your vehicle for several days with the fridge running. As a safety, throw one of those lithium-ion jump packs in the glovebox and make sure it’s charged up before you head out.

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