Outback power with inverters

Inverters are not a necessity when camping, but they sure are handy, especially in these days of high-tech, portable electronics with excellent battery systems to run them.

With so many devices needing to be charged, including laptops, phones, cameras and sometimes even vital medical equipment, inverters are becoming increasingly popular.

CURRENT ISSUES

An inverter is a device for converting direct current (DC), such as delivered by your 12V storage batteries and/or your vehicle’s alternator, into alternating current (AC), similar to a household mains electrical system.

There are several inverter types available, which produce different wave forms of electricity. The preferred inverter type produces a ‘pure sine wave’. This would be seen as a smooth sinusoidal wave on an oscilloscope screen. A less desirable ‘modified sine wave’ inverter uses stepped switching to roughly replicate the desired smooth pure sine wave.

One of the issues with modified sine wave inverters is they can be harmful to sensitive electrical circuitry and, reportedly, may shorten battery life, although evidence of the latter is sometimes contradictory. Laptops, tablets and similar devices should not be run on this power supply and plug-packs (those little black boxes that plug into the mains socket to convert 240V to voltage suitable to the device) can be damaged by modified sine wave power. Fluoro lights and many games platforms will not run on a modified sine wave inverter and they can introduce static to the television.

The most basic form – square wave inverter type – is rarely seen commercially and its sharp switching from peak to peak may seriously damage modern electrical circuits.

WATT YOU NEED TO KNOW

Inverters are rated by watts, reflecting the power available from the inverter and also somewhat reflecting the power needed from the supply battery system. In choosing an inverter capacity to suit your needs, add up the requirements in watts of the items likely to be plugged into it at any one time. As many appliances require a boosted level of current at start-up, allow for some margin to cover this surge demand if required. All inverters have a surge margin built into them.

Inverters come with a variety of equipment for connection to your power supply battery. Alligator-type clips are useful for temporary connections and are quite adequate. Cigarette lighter plugs and sockets have less current capacity, but will do for smaller loads. An Anderson-type plug (50A plug for up to a 300-400W inverter, larger as you increase inverter size) is a neat solution.

Use your inverter as close as possible to the battery to minimise cable length and thus voltage drop. This can be critical to inverter performance. Lower voltage, such as without the car connected, will lower the inverter’s output. Keep the inverter outside the battery compartment with ample ventilation around it though, as they do generate heat.

Making use of appropriately-sized wire will optimise efficiency. Where supplied, our inverters came with cable sized around 5mm² in area. This is minimal according to experts in circuit design, and performance would improve with inverters of this size by either shortening the supplied wire or replacing it with 8 B&S cable, which is close to 8mm².

Inverter weight should not be viewed as a negative, as units of greater weight are likely to have more windings internally and thus be of greater efficiency.

As a word of warning, 240V can be lethal, so avoid running an inverter while driving. An accident could potentially leave a device producing 240V shorting out, electrically charging the vehicle and electrocuting either you or the rescue personnel. Given the voltage output, they should be kept away from children, wet weather and liquids.

LOADING UP

There are three basic types of loads placed upon an inverter: resistive loads, inductive loads and capacitive loads. Each requires assessment before making an inverter choice.

Resistive loads require a steady delivery of a fixed current, for example, a 50W incandescent light bulb requires approximately 50W of power at all times. These are the easiest loads for an inverter to supply.

Inductive loads require a large rush of power to build a magnetic field around or within internal components (transformers, motors, ballasts, solenoids). This magnetic flux becomes a sort of stored energy. Inductive appliances include fridges, air compressors, pumps, power tools and fluorescent lights. These can draw up to 10 times the rated running power to start-up, thus a 100W fridge may require up to 1000W at start-up.

Appliances containing large capacitors, which store electricity, use capacitive loads. If used recently they may retain power in the capacitor and may not need much at start-up, but if the capacitor has run down through lack of use, a large surge current is required to fill the capacitor. Resetting the inverter several times if it trips out may gradually fill the capacitor and allow these appliances to start-up.

Some appliances, such as very large refrigerators, compressors, air-conditioners and other pump-driven equipment, have extremely high start-up currents because their inductive motors must start under compression stroke load and should only be run from an engine-driven generator.

Any appliance designed to produce heat, such as hairdryers, kettles, coffeemakers, irons, toasters, etc, require very high current (exceeding 1500W) and even though they are steady resistive loads, cannot be powered by smaller inverters and battery systems.

The full feature appeared in Caravan World #538 June 2015.