Choosing the right inverter/charger

by Gary Wilson

Inverters, the devices that convert DC power to AC current, have been around for years. They can be as simple as a cigarette lighter plug-in to power your laptop computer, or advanced enough to operate microwaves, "good" TVs and VCRs, computers, space heaters, vacuums, coffee pots, and power tools . you name it. The benefits of inverters are great. They allow you to use household items in your rig, saving you money while providing many of the creature comforts of home.

Interest in industrial-grade inverters has skyrocketed. A year ago, OEMs began thinking about AC power. Today, they're becoming part of the trucks' infrastructure. In the new millennium, Volvo, Freightliner, Navistar and Western Star have all announced they'll begin offering inverters as a factory option. Other OEMs are expected to follow suit. On the aftermarket scene, truck dealerships and direct sales through the Internet mean quality inverters can be ordered - good news for those looking for high-end systems.

Plugging into "shore power"

While inverters convert DC power to AC, another benefit of industrial-grade inverters is they allow you to connect to shore power. This means you can power your rig at truckstops, at home, or at a terminal with an outdoor extension cord. All the loads are run from the local utility rather than the battery, and in fact, the best units on the market accurately recharge the vehicle batteries at the same time. Since you're dealing with 120 VAC, care should be taken to make sure that transfer switches and connections meet all National Electric Code requirements. It's best and easiest if you get an inverter with a built-in transfer switch so that this feature is totally transparent and safe.

What to look for in an inverter

When it comes to inverters, it's buyer beware and the old adage of "you get way you pay for" is true. Many truck operators have been burned, almost literally, by consumer-grade inverters that simply can't handle the loads of a truck environment. Be sure the inverter/charger you buy is industrial grade and can handle the loads and surge you'll require. And look for safeguards and features that will protect and enhance your investment. Here's what to keep in mind:

Measuring up real power

Inverter power ratings had the same problem as early engine horsepower ratings. No two manufacturers rated the same way and so it was impossible to make comparisons. If a unit delivered 1000 watts for five minutes and 500 watts for one hour, one manufacturer may call it 1,000 watts and another may call it 500. 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. Unfortunately many of the light-duty "plug-in" inverters don't use the standard so the only way to be sure about power rating is to look for units listed to UL 458.

After continuous power the second factor is surge output. Many loads, like motors, require a high initial surge of power to get them started. Often this can be many times their nominal power. The inverter must be designed to deliver a big start-up punch. Look for surge of at least two to three times the inverter's continuous power.

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. If not, every time you add a load for a short time (for example, the refrigerator cycles on when the microwave is running) the inverter may quit.

How to "size" an inverter/charger

When selecting an inverter/charger, one needs only to be concerned with the inverter/charger's power window. Make sure the surge rating and overload capability of the inverter/charger can handle the total wattage of all the devices you plan to operate at any one time. Then confirm that the unit can deliver the required amount of overload power for an appropriate length of time.

These ratings can be easily explained by the following example. Consider Trace Engineering's T1112SB inverter/charger. This unit can surge to 3,000 watts for several minutes and provide overload power above its rated continuous output of 1,100 watts for longer than 15 minutes. With such an industrial grade inverter/charger, one could simultaneously operate a 900-watt microwave oven, a 100-watt TV/VCR, an 800-watt toaster and a 900-watt coffee maker long enough to comfortably make breakfast. All this from a unit rated at 1,100 watts continuous.

So, when you're analyzing inverters, be careful and read the specs - don't buy a bigger unit than you require. Instead, match the inverter's power window to your requirements and buy the size of inverter/charger right for you.

Low battery protection

As long as the engine is running there is no danger that a computer or microwave will run you out of power. However, once the truck is shut off, all the loads run from the battery. Over-discharge of the battery is dangerous because it can leave you needing a jump start, but deep discharge is also a leading cause of battery failure. Good inverters protect you and your battery with low voltage DC cutout where the AC power is shut off before the battery gets dangerously low. Some units are adjustable to let you select the level of protection you want.

Battery charging too?

Some inverters have built-in battery chargers for use when plugged in to shore power. Good ones with three stage charging can vary the voltage and current to meet different conditions and temperatures. This is much better for your battery than the engine alternator and it's easy since the intelligence needed is already in a good inverter. Charging from shore power cannot only save the cost of a jump start someday it also means good battery maintenance, which can add up to big savings.

How many batteries?

Most new Class 8 trucks come with at least three if not four group 31 lead-acid engine start batteries. This will provide more than enough battery capacity to meet the requirements of most inverter owners. A typical scenario would be operating the microwave to reheat dinner, watching a movie on a TV/VCR, making some microwave popcorn, heating breakfast in the morning, operating a coffee maker, and watching the morning news on TV.

If power demands exceed typical demands, additional battery capacity may be necessary. This can easily be accomplished by adding additional batteries to the engine start system. For the largest power demands, the installation of a deep-cycle coach battery bank devoted exclusively to the inverter and isolated from the starting system is recommended.

This type of system is commonly used on recreational vehicles and pleasure boats. Keep in mind the more batteries you add to the tractor the less payload you can carry on the trailer. n

What else should you look for in an inverter/charger?

Efficiency The higher the efficiency of the inverter, the more run time from the battery. However, efficiency is like gas mileage, it's not the same over the entire performance range. Good efficiency is better than 90 percent but be careful of peak numbers. Most of the time you will run at less than full output so be sure that the highest efficiency is in the lower power range. It's not good to have a unit with great gas mileage at a speed you never run.

Idle Power This is a real hidden battery killer. Often an inverter is on but there are no AC loads to run. The power consumption in this idle condition is important. An idle power loss of just 30 watts could empty the battery in a weekend. Look for idle current as low as possible and search mode or a sleep feature that drops the power to under one watt if no AC is needed for a while.

Reliability There's no use depending on AC power if your inverter is going to break and leave you in the dark. Look for a brand with a proven track record. One that is offered as original equipment has usually been tested by the truck manufacturer and is a safer bet. Also, check out the warranty. Finally, look for units that are heavier. Weight is not usually a good feature; however, inverters that use transformers on the output tolerate tough electrical loads far better than the light-weight consumer models that use small high-frequency switching designs.

Gary Wilson is truck division manager for Trace Engineering.