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Installing an inverter

by Tom Jones

An inverter will allow you to run AC appliances for some period without being hooked up to shore power or continuously running a generator.

What is an inverter?

An inverter is an electrical device for converting 12-volt DC battery power into 120 volt AC power. Inverters are available in sizes ranging from as low as 30 watts up to 4,000 watts or more. For pratical purposes, an inverter size of 2,000-3,000 watts will be needed (or will already be installed) in an RV.

Dual functions of an inverter.

The primary use of an inverter is to be able to run 120 volt appliances while not hooked up to shore power. This is especially useful when bookdocking and you wish to avoid using a generator, or don't have a generator. But don't plan on running the air conditionning or electric heaters from the inverter because they would discharge the batteries in a relatively short time.

The secondary purpose of an inverter is to re-charge the inverter batteries. There are likely to be two, four or more batteries that provide 12 volts to run the inverter and, after a period of inverter use, the batteries will require recharging. In many cases, this is done automatically by a charger within the inverter itself. The charging function contained in dual-purpose inverters usually charges at a significantly higher rate than normal battery chargers. It is also likely to be a multi-stage charger, automatically reducing the charging conditions as the batteries become charged.

Does an inverter function automatically?

Operation of an inverter is, for the most part, automatic, although most require some settings to be programmed. When shore &/or generator power is lost, the inverter senses this and provides its own power to maintain an uninterupted power source. When shore power is restored or the generator started, the inverter will again sense this and, usually after a short delay, will automatically switch shore or generator power to the RV.

Battery charging can also occur automatically, although it's usually possible to override this manually.

Selecting an inverter.

There are two basic types of inverter available, known as modified sine-wave and true (or pure) sine wave, the former being the most common and least costly. Some appliances have been reported to not run on a modified sine-wave inverter, although the author has not experienced this with various inverters I've installed in RVs and boats.

Installing an inverter.

Installation of an inverter is best done by a qualified electrician, although the installation is relatively straightforward. The AC power line is cut and the wires from the incoming power are connected to the inverter's AC Input terminals, and the wires going to the power distribution panel are connected to the inverter's AC Output terminals. If your RV has an on-board generator, the AC power line should be cut after the shore/generator changeover switch or relay.

Depending on make and model, the inverter will likely have a remote panel that is used to control the functions of the inverter and monitor what's going on. The panel will likely come with its own signal wire(s) for connecting to the inverter. The panel should be mounted somewhere that's easily accessible for daily viewing. The inverter may also come with a temperature sensor for measuring the battery temperature; The sensor is typically attached to the outer case of one of the batteries and a wire runs to the inverter.

Next are the connections to the deep cycle batteries. Use heavy gauge wires such as size 1/0, 2/0 or 3/0 welding cable, depending on the size of the inverter, to connect the batteries to the 12V DC input of the inverter. (The actual wire gauge should be determined from the ratings of the inverter and length of wire). Install a heavy duty switch and fuse between the battery positive terminal and the inverter 12V DC + input terminal. The switch, fuse holder and fuse can be bought at a marine store or RV parts department. The fuse will protect the inverter in the case of an overload and the switch will allow you to isolate the batteries in the event that something catastrophic happens.

Selecting batteries.

Because of the repeated heavy discharge & charging cycles, deep cycle batteries will be required rather than chassis or engine starting batteries. Golf cart batteries make an excellent choice because they are designed for daily heavy discharge and charging. However, be advised that golf cart batteries are 6 volts and you'll need two wired in series to produce 12 volts; This simply means connecting the positive terminal of one battery to the negative of the second battery. The remaining negative and positive terminals provide you with 12 volts.

Alternatively, you could use 12 volt deep cycle batteries, but their capacity will be roughly half that of a pair of golf cart batteries and you'd need two 12 volt deep cycle batteries to provide the equivalent capacity. The net result is that you'd need the same total number of batteries.

If you have space, four or even six golf batteries would provide additional capacity, allowing the inverter to run for a longer time before battery recharging is necessary. If you use 6 volt golf cart batteries, connect them in series-connected pairs, then connect the "free" positive terminals together and the "free" negative terminals together to produce 12 volts. If you use 12 volt batteries, merely connect all the positive terminals together and connect all the negative terminals together.

One pair of 6 volt golf cart batteries wired as above will have a rated capacity of approximately 220 amp-hours. Since you shouldn't discharge the batteries by more than 50% before recharging, the usable capacity of this pair of batteries will be 110 amp-hours. Four golf cart batteries wired as above will give you a usable capacity of 2 x 110 = 220 amp-hours. We'll use this usable capacity in the next section to determine how long you can run the inverter before having to recharge the batteries.

Do not mix battery types. Because of different charging and discharging characterstics, it's important to use the same battery type for all batteries in the bank. Do not mix lead-acid, gel cell and AGM batteries. You should not mix deep cycle batteries with "combination" or "marine" batteries, nor should you mix 12 volt deep cycle batteries with pairs of 6 volt golf cart batteries.

How long will an inverter run before the batteries need recharging?

This really is a function of the amount of load (number and duration of appliances used) and the type and size of batteries. The greater the load and the longer the duration the load is applied, the sooner the batteries will discharge. The larger the battery capacity, the longer it will take before the batteries discharge. It would be helpful to make a list of the appliances that will run off the inverter, their power consumption (in watts) and the number of hours you expect each to be operating during a 24 hour period.

Calculate how many batteries you need.

Take your list of 120 volt appliances, power consumption in watts and estimate of operating hours and calculate the amp-hours of capacity drawn from the batteries in a 24 hour period from the formula:

Amp-hours = (watts x hours)/11 (allowing for efficiency)

The following example might help, but be sure to use the actual power consumption from the owners manuals of your own appliances and adjust for your own usage of each:

Appliance          Watts    Hours   Amp-hours
Microwave oven     1500       0.5      69
Coffee pot         1200       0.5      55   
TV                  180       3        49
DVD                  55       2        10
Computer            100       2        18
Stereo               60       3        16

Total                                 217

Number of pairs of batteries required = (total amp-hours)/110 
                                      = 217/110
                                      = 2 pairs of 6 volt batteries
                                        (or 4 12 volt batteries).

Programming the inverter

If your inverter has a control panel, you might need to check or modify its pre-programmed settings. Depending on make or model, you'll be able to adjust things such as battery type (important for the correct charging of your batteries) and various modes of operation and monitoring as well as alarm conditions. One of the settings will be a synchronization of your batteries with the monitor - so it knows the state of charge of the batteries to start with. It is important that you read the owners manual for both your inverter and your control panel to determine the correct settings and procedure for changing them.