The basic principle of an air conditioner is the transfer of heat from one place to another – in this case from the inside of your vehicle to the outside. This is accomplished by absorbing heat from the inside air into a refrigerant gas flowing through an evaporator coil. The refrigerant is then pumped to a condenser, where the heat is released to the outside air. The refrigerant, a chemical with a low evaporation temperature, flows around a closed loop, driven and pressurized by a compressor. As a part of the cooling process, the air conditioner also removes moisture from the inside air, which makes the area feel more comfortable and keeps the compartment dry and mildew-free.
A belt-driven compressor on the truck’s engine supplies air conditioning whenever the engine is running. To keep the driver comfortable when the engine is shut down, it is necessary to have a separate auxiliary air conditioning system that uses an alternative source of power.
To that end, Dometic developed an auxiliary air conditioning system that runs on 12-volt power from an onboard bank of batteries using a device called an inverter, which converts the 12-volt DC battery output into 115-volt AC power. It can also run on 115-volt electricity from a shorepower hookup. No separate diesel genset or other internal combustion engine is involved. The batteries are automatically recharged by the alternator whenever the truck is running or from shorepower when used with Dometic’s recommended inverter/charger unit.
In specifying an all-electric battery-powered HVAC system, you should think of it as an integrated solution comprised of a number of different components, including:
- Heavy-duty deep-cycle batteries, designed for numerous discharge and recharge cycles.
- A high-performance alternator with external regulator to recharge the batteries quickly and keep them fully charged.
- A correctly sized and specified DC-AC inverter.
- The air conditioning components, with associated ducts, grills, electrical power supply, and controls.
- Shorepower connection.
1. 115 VAC flows directly to the air conditioner from a shorepower plug via a pass-through circuit in the inverter.
2. With an inverter/charger combination unit, batteries are charged when shorepower is connected.
Traditionally, truck batteries are designed to provide sufficient power to start the truck’s engines and to power lights, blower motors, electrical sensing circuits, and other “hotel” loads like a refrigerator or television set. It takes a lot more “juice” to power an air conditioning unit for eight to ten hours at a time without recharging. This is why Dometic specifies Group 31 absorbed glass mat (AGM) batteries. They are based on proven technology and are readily available from battery suppliers all across North America. The number of batteries depends on the capacity of the air conditioning units and the length of time between recharges.
You will replace your existing alternator with a higher-output unit rated at 185 amps minimum. The size of the new alternator will depend on the total number of batteries to be charged, and we provide some guidelines. Unlike traditional truck alternators, these units have an external regulator, which acts as a three-stage charger with a boost phase, a charging phase, and a float stage. This programmed regulator contains built-in sensors for temperature and voltage, automatically adjusting its output to best charge and protect the batteries.
All inverters are not created equal, and Dometic’s engineers have devoted many hours to testing and evaluating different units. We specify a specific Dometic/HDM 2,000 Watt inverter with a 115-volt pass-through circuit and integral charger to run the air conditioner and recharge the batteries when connected to shorepower. The inverter senses battery voltage and will shut down the air conditioning load when the battery voltage reaches a point where restarting the truck’s engine becomes an issue.The unit is also available as an inverter-only module.
Although there are relatively few shorepower hookups available at truck stops, rest stops, and terminals, these will become more readily available in the future. The Dometic all-electric system is designed so the auxiliary air conditioning system can run on an external 115-volt power source, which can also simultaneously recharge the truck’s batteries when the inverter-charger unit is installed.