02/04/2014
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Inverters in your motorhome

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Inverters are devices that convert 12V DC (direct current) from your battery to AC (alternating current) mains to allow you to use your 240V devices when you’re not plugged into a mains hook-up.

There are two basic types: those that produce a pure sine wave smooth output that replicates the mains, and the cheaper types that produce rectangular AC waveforms.

The mains electricity that is generated by a power station is produced mechanically by rotating a magnet inside a coil of wire. The resultant voltage is, therefore, constantly but smoothly changing from positive to negative.

Solid-state inverters chop up the DC battery volts electronically and re-combine the bits to produce an AC voltage synthetically. The less costly types call their output voltage waveform by different names, ‘quasi sine wave’, ‘Q inverters’ and ‘modified sine wave’ being some of the most common. The pure sine wave units tend to be called simply that.

Many bits of mains equipment will work from the cheaper non-sinusoidal ‘quasi’ inverters, but all will operate from a pure sine wave inverter of adequate output capacity.

Factor in the start-up current

When choosing an inverter, always bear in mind the start-up current will be much greater than the labelled output power. For example, a microwave with an 800W output power may need a much higher supply during start up and a 1,500W inverter would be recommended for this particular duty. The trouble is, a powerful inverter will hammer your leisure battery.

The 12-volt battery is just one twentieth of the level of mains voltage (240V) so, for the same power, the amps drawn at 12 volts will be twenty times as much.

But it doesn’t end there as you need to allow an extra 10% for inverter inefficiencies so actually a ratio of 22:1 is about right.

Hence for every amp taken by the mains equipment you are powering, you will be taking about 22A from the battery. A 1,000W inverter at maximum power will take about 100A from the battery. This could drain a 100Ah battery flat in less than one hour (though you’d never do this in practice – it would be unlikely to recover from a full discharge).

What size batteries will you need?
 
Now for the maths bit: watts (W) = volts (V) x amps (A)...

So your 1,000W inverter will require some substantial batteries. As the battery voltage falls under load the inverter will need to consume more amps to maintain the output power level. So the effective battery ampere-hour capacity at a discharge rate of 100A will be much reduced and you’d never actually manage one hour of use from a single 100Ah battery.

Modern inverters are very good at compensating for falling battery voltage until they shut down automatically and your battery is urgently due for re-charge.

Inverters add flexibility!

Having an inverter does give you flexibility, especially if you have several bits of low-power mains’ equipment that you
 want to power such as a laptop, phone charger or a cordless drill charger (ideal for winding up and down rear steadies).

Some inverters come with DC leads terminating in a cigarette-lighter type plug. These may be okay for loads from about 100W to 150W, but are really dependent on the fuses fitted to your vehicle.

This article is an extract from a much longer series of articles detailing choosing, fitting and using inverters. To read the full article in the new-look MMM, click here.

Download the new May-14 issue of MMM now

Back to "Practical Advice" Category

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