Adding 120V AC Power To a Truck Bed

By Michael Zimmers
18 January 2005

When I was shopping for my current truck, I was told that a pickup with the crew cab option (four doors, full-size rear seating) was the fastest-growing segment of the new vehicle market in the US. It made sense, as crew cabs combine the convenience, ruggedness and practicality of a truck with a comfortable, roomy cab that seats four or more. There is, however, one indisputable drawback to a crew cab: it has painfully little room for installing accessories. The cab has almost no space, and the bed of the truck is unprotected unless one adds a "topper" or hard cover, both of which are expensive and compromise what you can carry.

One accessory I’ve craved to add to my truck, but couldn’t figure out how to house it, is a heavy-duty DC-AC power inverter and battery bank. For those unfamiliar, a power inverter takes battery-style electricity (known as 12V DC) and converts it to the kind of power (known as 120V AC) that you have in your home or office. There are a ton of uses for an inverter:

• camping and recreational off-roading
• construction and maintenance
• public safety/search and rescue
• AC-powered audio equipment
• AC-powered mobile communications

As appealing as an inverter is, though, installing one into a crew cab truck poses some real challenges:

• Space. As mentioned above, there’s very little room in the cab for aftermarket equipment. And even if I found an inverter of sufficient power that was still compact enough to fit somewhere in the cab, the auxiliary batteries would never fit there, nor should they (batteries don’t belong in the passenger area, for a variety of reasons). This means that the equipment must somehow go into the truck bed, leading to the next challenge:
• Weather. Inverters must be kept dry. Since I didn’t want to cover my truck bed, I had to provide a protective enclosure for the inverter. It had to be weather-resistant and provide easy access to its contents. This in turn, creates another challenge:
• Space (again). The beds in crew cabs are already shorter than full-size, to make room for the rear seating. I rarely need the full use of the bed, but I didn’t want to reduce my bed space any more than necessary. Whatever protective enclosure I was going to use had to have a minimal impact on the bed space.
• Security. This kind of equipment can be expensive, and it needs to be protected from theft.

The Solution

After many evenings of surfing the Internet for inspiration, I hit upon an idea that I felt would work: I could put the equipment into a tool box. The trick was to find a tool box that would steal only a little space from the truck bed, would be weather-resistant, and would be lockable to keep the bad guys out.

In order to preserve as much of my bed space as possible, I chose a style of box known as a "saddle box" that installs directly behind the cab and mounts on the bed rails. This allows me to slide long items (like full sheets of plywood) under the tool box when I need to. I found a saddle box with "gull-wing" lids that open from the sides of the truck, making the box accessible without having to climb into the bed. This box, with some creative modifications and enhancements, form the platform for this project.

This article will take you through the various steps involved in installing and configuring a "power box" for your vehicle. I’ll give you some ideas of where you can get the equipment, how to prepare your truck and the tool box, and specific instructions for the actual installation. I hope you find this informative and entertaining.

Components Of This Project

While there are many possible variations on this application, adding AC power to a vehicle entails four essential components:

power inverter. At the heart of this project is the power inverter. Inverters come in a huge range of sizes, measured by their output wattage, from 75 watts to 6000 watts or so. I wanted the same amount of power (around 15 amps) I get from a home receptacle, so I was looking for an inverter in the 1500 to 1800 watt range. I also needed the unit to be rugged and able to operate in semi-harsh environments. Some inverters (like the one I selected) include the AC receptacles (the slots where the plugs go) and a built-in battery charger. The latter is handy if you happen to drain your batteries a short drive from home and want to plug them into the wall to recharge.

tool box. The tool box was going to be my "work shed." The power inverter needs to be protected from the elements, but still be readily accessible. The value of any tool depends upon its handiness; if you have to climb into the truck bed to switch an inverter on or plug something into it, that’s a drawback. As mentioned above, I also needed a tool box that would preserve bed space, would be weather-resistant and would be very well built. No way was I going to put an expensive inverter into a crummy tool box that might not give it adequate protection. Oh yeah: it also had to look good. And this one does. I chose a box with a bright aluminum finish, which I found more attractive than the painted variety. I also expect the metal finish to outlast paint. That white stuff on the front of the box is protective tape applied for shipping that I hadn’t yet removed.

auxiliary batteries. Strictly speaking, auxiliary batteries aren’t essential, but without them, you can run down your starter battery pretty quickly. It’s also important to remember that you can drain a battery even with the vehicle running, since a large inverter can use up power faster than the alternator can provide it. Also, most starting batteries aren’t designed for being deeply discharged, and doing so repeatedly will greatly shorten their lifetime. I decided to install four deep-cycle batteries for my project. I calculated that this would be enough to keep my inverter going full-blast for a full hour, even without running the truck’s engine.

battery isolator. While my inverter’s built-in charger gives me the option to recharge the auxiliary batteries from garage power, I still wanted to be able to charge these batteries from the truck’s alternator. This is straightforward, but poses a bit of a challenge: if you simply connect your auxiliary batteries to the alternator, then the starter battery will effectively "join" the auxiliary batter bank, and discharge with the auxiliary batteries. In other words, I could run down my starter battery from the inverter, which could leave me stranded. Solving this problem is a battery isolator, which allows auxiliary batteries to share a charging source with the vehicle’s starter battery, but prevents the starter batter from being drained by current draws on the auxiliary batteries. (The electrical device that makes this possible is called a diode, which allows electricity to flow one way through a wire, but prevents it from flowing in the opposite direction.)

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