Bottom Line
Power inverters
Do your homework

Truck sleepers are being described today as the functional equivalent of a bedroom, kitchen, office and den. There’s space for a microwave, TV, a coffeemaker – heck, even a coffee grinder – plus a sink and fridge. Manufacturers are ringing these bells and blowing these whistles as they market driver comfort to truck buyers.

Running these devices comes with a cost. Given the price of diesel, it’s expensive to use the engine as a source of power. That’s why interest in industrial-grade power inverters is running high. 

Inverters convert DC power from the truck’s batteries to AC current. They’ve been around for years, mostly as aftermarket items of varying quality and capacity. Today, they’re becoming part of trucks’ infrastructure. Freightliner, International, Mack and Volvo all offer robust inverters as a factory option, and other OEMs are expected to follow suit. 

That’s good news for maintenance managers who see the effects of low-end aftermarket inverters bought on the road and temporarily installed by drivers: premature wear on batteries, starters and alternators. 

“Electrical problems result from these jerry-rigged installations,” said Bob Jeffries, national fleet manager for Remy Inc., which makes starters and alternators.

The good news is you can find quality inverters or inverter/chargers in the aftermarket, but they’re not something you can shop for on a whim. Consumer-grade inverters simply can’t handle the loads and surge of a truck environment, nor will they have safeguards to protect the truck’s electrical system. Look for something more robust and designed for the rigors and electrical loads of a heavy-truck environment.

Inverter power ratings had the same problem as early engine horsepower ratings. No two manufacturers “rated” their continuous power the same way. So it was impossible to make comparisons. If a unit delivered 1,000 watts for five minutes and 500 watts for one hour, one manufacturer may have called it 1,000 watts and another may have called it 500, according to Brian Lawrence, OEM sales manager for Xantrex Technology of Vancouver, which supplies original and aftermarket inverters for trucks and other applications. 

“Now, according to Underwriters Laboratories, the independent agency that writes the safety standards, the true rating is what the inverter will deliver continuously at its rated ambient temperature,” Lawrence said. 

“Unfortunately, many of the light-duty, ‘plug-in’ inverters don’t use the standard.” 

The only way to be sure about power rating is to look for units listed with Underwriters Laboratories, he said.

Surge output
Many loads, like motors, require a jolt of power to get them started. Given some of the appliances you want to run – a microwave, for instance – look for surges of at least two to three times the inverter’s continuous power rating. Most units have built-in overcurrent or overheat protection; don’t buy one without both.

“Make sure the unit can deliver more than its rated power for many minutes before it shuts off,” Lawrence said. “If not, every time you add a load for a short time – for example, when the refrigerator cycles on when the microwave is running – the inverter may quit.”

Power curve
Make sure the surge rating and overload capability of the inverter is able to handle the total wattage of all the devices you plan to operate at any one time. Then confirm that the unit is able to deliver the required amount of overload power for an appropriate length of time.

What’s more, determine how you plan to power your inverter. DC power ports in trucks are rated for a maximum of 20 amps DC. This limits plug-in inverters to no more than 240 watts, or you risk blowing fuses. 

There have been cases where drivers, frustrated with blowing fuses in their cigarette lighter with higher watt inverters, try to move up to a 40-amp fuse. That’s dangerous. 

The wiring to those ports is not intended to handle the additional current. That’s why you need to match the inverter with the outlet. All other inverters should be hard-wired to the battery.

Low battery protection
Once you shut off the engine, all the loads run from the battery. Most new Class 8 trucks come with at least three if not four group 31 lead-acid engine start batteries – more than enough battery capacity to re-heat dinner in the microwave and watch a movie on the TV/VCR. For the largest power demands, consider a deep-cycle coach battery bank devoted exclusively to the inverter and isolated from the starting system.

Running down the battery can leave you in need of a jump-start, but deep discharge can lead to battery failure. An inverter with low-voltage DC cutout can shut down the AC power before the battery gets dangerously low. Some units let you select the level of protection you want; some inverters can also charge the battery when plugged into shore power. Better ones with three-stage charging can vary the voltage and current to meet different conditions and temperatures.

Other Considerations
Efficiency: The higher the efficiency of the inverter, the more run time from the battery. However, efficiency is like fuel economy – it’s not the same over the whole performance range.

“Good efficiency is better than 90 percent, but be careful of peak numbers,” Lawrence said. “Most of the time, you’ll run at less than full output, so be sure that the highest efficiency is in the lower power range.”

It’s no good to have a unit with great gas mileage at a speed you never run.

Power at Idle: Often an inverter is on but there are no AC loads to run. An idle power loss of just 30 watts can drain the battery in a weekend. Look for idle current as low as possible and a search mode or a sleep feature that drops the power to under 1 watt if no AC is needed for a while.

Heft: Weight is not usually a good feature on a truck, but inverters that use transformers tolerate tough electrical loads far better than consumer models that use small high-frequency switching designs.

An inverter shouldn’t be an impulse buy. Read the specs, match the inverter’s power curve to your requirements, and buy the right size for the load. Ask for help from manufacturers’ reps. And then go brew yourself a cup of coffee and see what’s on TV.