Power Me Up
by Stephen Petit
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, plus a sink and fridge. Manufacturers are touting these features as they market driver comfort to truck buyers.
Running these devices comes at 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 the truck’s infrastructure. Most OEMs now offer robust inverters as a factory option.
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 or worse, fires.
“Electrical problems result from these jerry-rigged installations,” says Bob Jeffries, regional sales and service manager for Remy Inc., which makes starters and alternators. “The average fleet jumpstarts 10% of its trucks each month. An average of 15% of those jump starts are due to excessive electrical loads in the cab.”
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 it have safeguards to protect the truck’s electrical system.
Inverter power ratings had the same problem as early engine horsepower ratings. No two manufacturers “rated” their continuous power the same way and 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 call it 1,000 watts and another may call it 500, says 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 (UL), the independent agency that writes the safety standards, the true rating is what the inverter will deliver continuously at its rated ambient temperature,” Lawrence says. “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 to UL 458, he says.
Many loads, like motors, require a jolt of power to get them started. Given some of the appliances you want to run like a microwave, for instance, look for surge of at least two to three times the inverter’s continuous power rating. Most units have built-in over-current or over-heat 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 says. “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.”
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 hardwired 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 jumpstart, 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 in to shore power. Better ones with “three-stage” charging can vary the voltage and current to meet different conditions and temperatures.
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. While consumer-grade inverters available at the likes of Canadian Tire and Walmart may be suitable for powering a laptop or some similar low-voltage device, higher current draws from TVs, heaters, etc., are best left to the heavy-duty, professionally installed inverters. Do your research, and then go brew yourself a cup of coffee and see what’s on TV.