Technical Tips #30
By Bob Lowe BOA #696
Inverters - What do they do?
An inverter, as popularly used today in automotive or motorhome applications, is an electronic device that converts 12 Volts DC from a car or motorhome battery source into 110 Volts AC. This conversion utilizes sophisticated electronic circuits that, in reality, mimic the alternating positive and negative swings of Alternating Current. The output is not a perfect sine wave, but a chopped or modified sine wave, which can operate all but the most sensitive electronic or motor-driven equipment. Generally speaking, the higher the price for the inverter, the better the power source supplied by it, will be. We can get a lot more technical, but the critical piece of information is that you can plug an electric device requiring household current into an inverter which, when connected to a good source of 12 volt DC, will allow the device to operate.
Inverter have become popular because of a rise in the use of laptop computer and cell phones, portable cooler boxes and the demands that motorhome owners have placed on coach manufacturers to supply a source of household current without requiring the generator to be running. Many Bounder Diesel motorhomes and most medium and high-end models are equipped with a 1000-2500 watt inverter that automatically provides power to some or all of the AC outlets in a motorhome. To provide satisfactory performance, the number of coach batteries is increased to four from two.
Can I power the TV and the VCR without running the generator? The answer is yes - but the critical question is for How long? We must understand the mathematics of power to properly answer that question. For those that want the detail, the following paragraph will assist you, for the remainder theanswer is unless you have four 6-volt deep cycle batteries (and the excess weight capacity to install them) you should limit yourself to an inverter rated at 300 watts or less. This size will power a typical TV, VCR and a satellite TV receiver, although you will safely only be able to operate two of the three at once.
There is a ratio between Power (P in Watts) and Voltage (V in Volts) and Current (I in Amps) that in mathematical terms states that Power is the product of Voltage times Current. (P=VI). Thus, if you want to operate a 120-watt laptop computer on 120V AC, it would draw one amp of power. To operate this computer from the inverter powered by 12 volts, we require 10 times as much current (or 10 Amps of 12V power.) Then, there is the efficiency of the conversion that is about 90%, meaning that of the 10 amps required, 1 is lost in the inverting process, so really you require 11 amps.
As you can appreciate the current draw from the coach batteries will rise rapidly if you use more power and the wiring that is connecting the inverter to the battery must be capable of handling this increased current. Many popular brands of inverters come equipped with a cigarette lighter style power plug. Becautious when using these, since the inverter usually has wire of a sufficient size but the car or motorhome power socket wiring may not have the capacity to handle this high current for extended lengths of time. A fire risk can exist if the insulation melts or the wire becomes too hot.
How do I install one?
My recommendation is to install an inverter of not more than 300 watts in the TV-VCR compartment. It should be installed as close as practical to the Coach Batteries or the Coach battery-disconnect solenoid. This unit is located under the hood on most Bounders, in the Battery Control Center on the driver's side of the coach. Remove the ground battery cables of both coach and chassis batteries before beginning to wire the inverter. You might want to record all the radio stations stored in the memory of the radio, since many radios lose their memory when the battery cable is disconnected. Connect the ground using 8 gauge wire to a good metal ground point on the chassis, or if distance permits, connect it to the ground cable connection at the battery. Remember the total length of both positive and negative wires should be less than 25 feet. Connect the Positive side to the load side of the battery disconnect with a proper ring connector and a heavy-duty inline fuse holder. Carefully run the positive wire using rubber grommets if the wire runs through any metal surfaces. Install a 25 or 30 amp fuse in the fuse holder.
When you want to use the inverter, turn it on, plug the TV or VCR into the AC outlets on the inverter, and turn on the TV or VCR. You should be able to watch it for a couple of hours if your battery is fully charged. Most inverters have a low battery warning and a low battery cut-out so that you will not be able to operate them to the point where your battery fully discharges. If you are without power for extended periods of time you will need to recharge the battery with a proper charger or by driving your coach. Many inverters are available from RV-Parts Plus at www.rv-partsplus.com