Pass the Juice


One of the most promising developments at heavy-truck manufacturers now involves power inverters. An inverter can convert the DC power coming from the truck’s batteries to ordinary 120-volt alternating current, just like at home.

Volvo Trucks, Western Star, and Freightliner Trucks now offer industrial-grade power inverters as factory options, with International expected to do the same this spring. Heavy-duty inverter/chargers are available for the aftermarket, as well, and one company, Xantrex Technology, is expected to introduce a portable inverter, charger, and battery next year.

Going AC is a decision that affects the entire electrical infrastructure of the truck, right down to where to place your plug-ins. The benefit to a factory install is that you can be confident your truck won’t burn to the rails because its wiring couldn’t handle the surge of current when you hit the “start” button on the microwave. And you can feel assured that your truck will turn over in the morning because the heavy-duty inverter you spec’d will keep your batteries charged and protect them from draining, unlike the $75 truckstop special.

But the emergence of AC onboard raises important maintenance considerations, especially if you’re using a do-it-yourself aftermarket kit or an inverter that’s not designed for heavy-duty use.

“Drivers are like any other business traveller-they want to feel comfortable when they’re away from home,” says Brian Lawrence, heavy-duty truck manager at Xantrex, which makes inverters for the heavy-truck, marine, and RV markets. “Put in an AC plug and the driver will fill it. The risk you run, if your electrical system isn’t up to the task, is that all these power draws will overwork the alternator and suck your batteries down to levels that can ruin their ability to accept a charge.”

Lots of drivers just idle their engine, turning the alternator to keep the batteries fully charged at the expense of fuel economy. Many electronically controlled engines have “smart idle” features that detect voltage drop and will automatically restart the engine when it gets too low. This makes more sense than idling constantly, and as well as recharging the batteries, it serves the dual purpose in cold weather of keeping fuel and lubes warm.

An auxiliary power unit is a better alternative; it consumes just a couple of pints of diesel per hour. The upfront expense of the APU is going to be returned over the course of an idle-free winter.

However, if an APU isn’t in the cards, you might at least consider an isolated battery system. In this set-up, one or two deep-cycle batteries are kept separate to power the accessories, leaving two or three regular batteries ready to power the cranking motor. Once the engine is running, the two systems are automatically connected in parallel through a magnetic switch, or charged individually through a diode-type isolator.

“Creature comfort” items like radios, interior lights, heaters, and communication devices are fine for the isolated batteries. But essential items such as electronic engine controls, required vehicle lighting, and antilock braking systems should not be isolated. Why? Because drawing down a battery takes a heavy toll. In normal weather, a fully charged isolated battery will operate under a 10-amp load for around nine hours before it goes flat. Higher electrical loads and lower temperatures will change the time significantly. At -20° F, the battery will only last 3.4 hours. And a 25-amp load will reduce the battery’s effective life at 80° F to only 2.6 hours.

As for those power inverters, the best of the bunch have over-discharge protection. In other words, they’re able to detect dangerously low discharge levels and shut down AC output, preventing further discharge. In addition to ending the need for dead-battery jump-starts, this feature is sophisticated enough to avoid nuisance shut downs when load conditions create temporary low voltage excursions.

The charger feature in an inverter/charger can improve battery life, too. Typical inverter/chargers provide 50 amps of DC output at 12 volts that can keep pace with virtually all DC demands. This eliminates DC loads-such as marker lights or refrigerators-as a source of battery over-discharge.

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