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Inverters (Author = DABurleigh)

An inverter is an electronic black box that converts electricity in one form to another, that is, 12V direct current from your vehicle or leisure lead-acid batteries to 230V alternating current or “mains”. It is therefore quite different from a generator (piston engine) or solar panel which generates electricity from other sources of energy such as hydrocarbon fuel (diesel, petrol or liquefied gas) or sunlight, respectively.

An inverter is a convenient and flexible piece of trickery, very popular with motorhomers when away from a site’s mains electric hook-up, for powering anything that requires a mains voltage, from low power devices such as TV, satellite receiver, satellite controller, a plethora of chargers for laptop, mobile phones, digital camera, GPS, MP3 player, portable radio, torch, etc., through to higher power devices such as hairdryer, microwave, vacuum cleaner and even limited air-conditioning applications.

There are only three principle issues to understand when considering an inverter, namely its maximum power, output electrical “waveform” and installation.

Maximum Power
Many motorhomers fall into the trap of buying a bigger inverter than they need, which wastes both money and the precious energy in their leisure batteries. An inverter’s maximum power, relative to the total consumption of the devices it will power, will dictate the efficiency of the inverter. Inverters are remarkably efficient, often at least 90%, when run from around half up to their full continuously rated maximum output. Conversely, although they only use as much power from the battery as they need, they can be quite inefficient ticking over at low loads, when much of the battery power they consume is wasted as heat. Sum the power consumptions of the devices you want the inverter to power simultaneously, but do not buy a bigger one than you need.

A small 150 Watt inverter can meet many needs. It is cheap, often under £20 (look at Maplins for offers). It does not generate that much heat, so no fan is required and is therefore silent. It is sufficient to power laptops, most TVs, receivers and small chargers for dedicated consumer electronic devices.

The limiting factor for the use of inverters for high-power devices is simply the energy capacity of your leisure batteries. For most motorhomers with no more than a couple of leisure batteries totalling 220Ah or less, a moderate hairdryer, say 600-1000 Watts, or a moderate microwave, represents the limit of what is practicable with disciplined use. Even if you do the consumption sums and convince yourself you have plenty of spare battery energy to cope with more than this, the electrickery (sic) detail will get you, and you will find the inverter will probably protest with a low voltage alarm before shutting down, even if you can still use the batteries for lighting, pump, etc.

Output waveform
The alternating voltage and current out of your mains sockets at home varies either side of zero in a smooth pattern called a sine-wave. The inverter, of course, has to create this smooth pattern from a constant 12 volts from your battery and it is not an easy or cheap thing to do. Fortunately, most mains-powered devices run from an inverter do not need such a smoothly varying voltage, so manufactures offer cheaper “modified sine-wave” inverters which provide only a crude square, stepped or triangular waveform as an approximation to the normal mains sine-wave. The saving in price can be dramatic; a modified sine-wave 150 Watt inverter is around £20, whereas a full sine-wave one is typically £90-£100.

Most of the time you will not have any problem whatsoever with these cheaper modified sine-wave inverters, but it is possible to predict where and how you might. Devices which present an “inductive load” can be troublesome. Indeed it has been known for inductive toothbrush chargers (no metal charging contacts) to fail. Electric motors, such as those in mains-powered desk fans, can hum noticeably; equally, many work just as if they were on a pure domestic mains outlet. The audio on some TVs can suffer with a low level mains hum. Some halogen lighting is inductive. Finally, whereas most laptops work quite happily off these cheap inverters, some do require the better quality output of a pure sine-wave inverter.

A small 150 Watt inverter is often provided with a simply cigarette lighter plug for connection to the battery. Be careful using the cigarette lighter in the dash, for this invariably will be connected to the vehicle battery, and you risk being stranded there with too much use of the inverter, especially if the socket is “ignition” switched. Many cigarette lighter sockets are only rated at 10 Amps (~120 Watts) or 15 Amps (~180 Watts) so bigger inverters should only be connected directly to the battery with an adequate clamp on the terminal post.

Higher powered inverters should be connected to the leisure batteries with short, thick leads. As these leads will carry high 12V currents you must prevent resistive losses dropping the voltage between battery and inverter which would then shut down. Conversely, the mains cable plugged in the inverter can be as long as you like; the currents at mains voltage are low so resistive losses are trivial.

Motorhomers are increasingly using an LCD TV and satellite receiver, often with a separate satellite controller. One convenient option is for these all to be powered from a dedicated 150 Watt inverter, with a single master switch by the TV. All devices are left switched on, so that with one switch near the TV the inverter, TV, receiver, satellite controller and dish LNB are simultaneously powered up and shut down.

Another convenient option is for the inverter to be connected and switched such that all the mains sockets in the motorhome are powered from the site mains, any generator, or the inverter as appropriate. There is a number of ways in which this can be done safely, varying in price and convenience. For example, you can simply arrange for the normal electric hookup cable to connect to the inverter output, but it requires much detailed planning if you want matters as automatic as possible - ensure the fridge, battery charger, water heater and any mains space heating are not powered from your batteries! A high-end DIY installation is shown here:
Another way to go is to use an integrated inverter/charger/uninterruptible mains power supply:
However, if you have any doubt over your electrical knowledge or abilities, employ a professional auto electrician. Do not take unnecessary risks.

Enjoy the convenience an inverter gives you!

Dave Burleigh
8 March 2009
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