Hotel loads––those power-consuming convenience items that can help make a sleeper cab a home––place a tremendous load on a truck or tractor’s electrical system. Managing those loads successfully requires a thorough understanding of their electrical demands and having the capacity to power them.
Darry W. Stuart, president and CEO, DWS Fleet Management services, believes there’s not enough battery capacity in commercial vehicles now to supply regular electrical loads––even before adding the power drain of hotel amenities such as a coffee pot, refrigerator, microwave, laptop computer, DVD player, cell phone charger or electric blanket. These devices can draw anywhere from 10 watts for a cell phone charger up to 1,200 watts or more for a coffee maker (in brew mode) or a microwave oven (one rated at 600 W cooking power may draw up to 1,100 W). What we’re trying to do, Stuart says, is to supply electricity to a vehicle on the road for 14 hours, tap it for cab comfort the other 10, and make sure it still has enough power in reserve to crank the engine.
APUs and various battery-pack solutions are among the options. Another way to handle these electric loads is a power inverter, a device that converts direct current (DC) electricity to alternating current (AC) to supply the household appliances like those already listed that are appearing more frequently in the sleeper compartments of on-highway trucks and tractors.
Spec’ing an inverter
Inverters are a relatively inexpensive way to tap into a truck or tractor’s batteries to get AC power, and for fleets, the concern is to limit having inverters self-installed by drivers. If an inverter is the chosen solution, then it’s best to have your technicians do the installation to help avoid the problems that can arise with unsupervised electrical work.
Spec’ing right is not difficult, says Bruce Purkey, president, Purkey’s Electrical Consulting Inc. “Someone has to add up the loads and determine the amount of power needed–– and then make the decision on what can be supported and for how long.” When you do the math, don’t be surprised because the numbers can be staggering. It typically takes 12 times an amount of DC current to produce a given desired AC output. Simply put, that’s 12 amps of 12V DC to produce one amp of 120V AC power. (For more detail, see the Technology & Maintenance Council’s RP 163 Power Inverter Selection Recommendations.) However, inverters are not completely efficient at converting energy. Typically, their efficiency ranges from 75% to 90%, so even more DC amps are needed to produce AC amps. Purkey’s rule-of-thumb is to divide output wattage by 10 to get DC amperage. This calculation includes losses for inefficiency. For example, an 1,800 W inverter would draw about 180 amps DC (1800 W/10 = 180 amps DC) if the inverter is producing its full output. Therefore, it is essential to have a battery pack large enough to supply the load and still be able to crank the truck engine. Additionally, the alternator must be able to recharge the batteries, and the cabling must be sufficiently sized to handle the high current draw.
When sizing an inverter, it’s a good idea to add 20% additional wattage for extra margin. Don’t forget that some items, such as starting a motor or compressor, have an extra surge requirement that averages six times the rated operating power. “Buy bigger than you think necessary so you’re not putting it on overload,” says Purkey. Its rated capacity may not produce enough power. And be aware that even when the inverter is on, it still draws current. Even one amp of idle current can drain 24 amp-hours from the batteries if left on for just one day.
You have to make a choice: support the hotel loads or be able to start the truck. To help make sure there’s enough power to start the vehicle, select an inverter with a low-battery voltage shut-off no lower than 11.8 V DC. Any less risks not being able to crank the engine and could result in damage to the batteries if they are discharged too deeply. Also, be aware that an OEM-installed low-voltage disconnect may not have sufficient capacity to handle the inverter’s current requirements.
When determining the hours of need, be aware that users and manufacturers define duty-cycle differently, says Purkey. “Extended duty” and “continuous duty” mean different things. Continuous duty for inverter manufacturers might be four to five hours, says Purkey, whereas drivers would consider this to mean 24 hours.
There is no free energy, Purkey reminds us: a bigger inverter won’t necessarily solve the problem of greater electrical demand if the number of batteries stays the same. He believes batteries presently in use can only support hotel loads eight to 10 hours. Four batteries can’t support eight hours of hotel loads regularly. If the additional weight were not an issue, Stuart says he’d spec’ new tractors today with six or eight batteries instead of the usual three or four.
Do absorbed glass mat (AGM) batteries help? Technology costs money and sometimes comes with a weight penalty. “The more lead in the battery, the more capacity it has,” Stuart notes, but this can add weight that costs every time your rig heads down the road. AGM batteries have double the cycle life, Purkey explains, but you still have to put in more energy than you take out. “A flat AGM battery is no better than a flat flooded cell battery.”
As much as inverters may deplete a battery system, flat batteries are not the only possible reason for being unable to crank the engine the morning following a night of powering hotel loads. Starter motors are often blamed, but the real culprit may be the circuit. Check for corrosion or a bad ground. “A lot of good starters are removed because the actual problem is not being addressed,” says Stuart.
Alternators may put out less than their rated amperage at idle speed compared to full operating speed. This is true for both APU and big engine alternators, Purkey notes. If the loads are close to that amperage, there may not be many surplus amps available for fully charging the batteries. TMC’s RP 129 Heavy-Duty Vehicle System Wiring Checks; 12-Volt charging, 12-Volt Cranking offers more advice for troubleshooting voltage drops in batteries, starter and alternator.
Inverters tested to Underwriters Laboratories standard (UL 458) should meet the safety criteria for heat dissipation and thermal protection. Connections loosened over time by vibration, shock and temperature cycling can overheat the circuit, though, which can result in thermal damage or even fire. “Most fires are caused by bad maintenance or by exceeding the capacity of the wiring,” says Stuart. He adds that most fires occur around the battery cables. The solution? Keep battery terminals clean and the contact between battery cables and connectors as perfect as possible. Make it a practice to clean the connections at each PM service, Stuart advises. He does not advocate using sprays or grease that can attract dirt. Because copper wiring is a natural wick for moisture, the corrosion that can result will increase resistance, heat and current loss as the wiring ages.
You can’t always see corrosion, says Purkey, but it can be measured by the wire’s ability to transfer energy. His rule-of-thumb: For every volt drop in the cranking circuit, you’ll lose 30 RPM in cranking speed. A one-volt drop is not so bad, but three volts lost means a loss of 90 RPM. “Then the situation is, maybe it will crank and maybe it won’t.”
Inverters do offer an option for AC power, but perhaps at too great a cost in volts DC. There is another option ––shore power—simply connecting to the electric grid via a cable like you would do at a campground. It’s a solution, Stuart says, but one that has a high per-night cost––and he doesn’t think there are enough places to plug-in now at truck stops to make it a universal option.
Inverters can provide the comforts of home, but not all the time. “Maybe two hours out of 10,” says Purkey. If abused, an inverter can shorten battery life and limit the driver’s ability to start the truck. Efficient hotel-load power management requires some thought, some number crunching and maybe some give-and-take with your drivers. Staying within a reasonable limit may come down to charging a cell phone or iPod in the cab on 12 V DC while driving down the road, instead of at night from AC provided by an inverter. FE