Instructor: Mike Hirata, Director of RV Sales, Heart Interface (6/96)

An inverter is an electrical device which takes energy from the storage source, typically a battery, and converts it to 110 AC. Inverters do not take the place of generators, since they are not suitable for larger appliances such as air conditioners and electric water heaters. With an inverter system, probably the only time you'd want to use a generator would be to run air conditioners, or to charge the battery system using an efficient charger system.

Industry trends: RVers have demonstrated an increasing demand for inverter products. This is due to an increasing trend towards full time and dry camping. The demand is also growing for "silent AC power". This is one of the great benefits of inverters. No need to listen to the generator, and it can be used during hours when generators are not permitted. New RV units offer more and more built in AC appliance options, and the inverter provides a way to use them when shore power is unavailable.

There are two basic categories of inverters. High frequency inverters are light weight, light duty, light use devices, and typically a plug-in to a cigarette lighter. The permanent installations are typically line frequency inverters, and are much heavier duty devices.

Different types of inverters produce different wave forms. Square wave inverters do not offer good voltage regulation or frequency stability. The modified sine wave inverter is the most popular type on the market today. 99% of all appliances run well on the modified sine wave type. But more loads are becoming sensitive to the wave form. The problem isn't potential damage, but rather degraded electrical appliance function. Microwaves typically won't run at full speed.

NOTE: The modified sine wave inverter may NOT be appropriate for charging notebook type computers! If the device heats up abnormally, unplug it -- you may damage the computer!

The "sine wave" inverter is still on the horizon, as the costs for it are still quite high. This will be the solution to the problems inherent in the modified sine wave inverters on the market today.

When talking about inverter "systems", it's important to know that a high quality inverter requires the "right" types and sizes of batteries. Where large inverters are linked to small battery banks, the results are always disappointing. The deep cycle battery is the type most suitable for inverter systems, because they can handle many more "cycles" -- meaning discharged and recharged repeatedly. As a rule of thumb, do not discharge the battery bank more than 50% of its rated capacity. This will greatly prolong the life of the batteries.

A fully charged battery when at rest (needs to be at rest for at least 48 hours) would be 12.6 to 12.7V for wet cells, and 13.0V for gel cell batteries.

Favorite battery: Trojan's T 105 6 volt golf cart battery. They need to be connected in series, meaning positive is connected to negative. This results in the voltage being doubled, while the amperage remains the same. Where two twelve volt batteries are connected in parallel, the voltage remains the same and amperage is doubled. Where batteries are in parallel, always use the same age, type, and case size (i.e., Group 27, etc.) of batteries. "Mixing" them will cause the first "bad" battery to shorten the life of the others.

Most RVers today have "standard converters". It's perhaps a 45 amp converter. If you look at the small print you'll probably find it's a 3 amp charger too. If you want to charge your battery bank, perhaps 200-400 amp hours, it will take a LONG time to recharge the batteries at this rate!

A more modern method is a three stage charger. The rule of thumb: A maximum charge rate is the total battery capacity divided by 4. For 400 amp hours, a 100 amp hour charge is the maximum charge a battery should have. [Note: Greg Holder notes that the "long life" charge is more like capacity over 20-30.]

The more modern three stages of charging are these: Bulk charge -- this is the first stage. It provides a constant current mode until the battery reaches 14.4V for wet cells; and 14.1V for gel cells. Next is the "acceptance charge". It holds the voltage at a constant voltage (14.4 for wet cell, 14.1 for gel cells) and the current will ramp down as the battery accepts less amperage. Once the batteries are accepting only 10 amps, or 2% of battery bank, the voltage is lowered to 13.5V for wet cells, or 13.8V for gel cells. It's a low enough charge to maintain the charge of the battery. At the same time, it can now act as a converter. The voltage is low enough so that it won't evaporate the battery water. The old type of charger finishes at around 14V, which will evaporate the battery bank. RVs which don't have the lower "finish charge" should put their converter/charger on a timer so that it doesn't stay on all the time.