Mains Power for Your Car!

This article was first published in 2004.

The idea of having mains power in your car initially sounds a bit crazy. I mean, waddya need a power point in your car for? Well, start thinking through the uses and you’ll soon come up with lots of reasons to have mains power. Perhaps that’s why it’s now a common option in new cars in the US– yep, you can specify an optional mains power outlet in current US market Avalons and GM Silverado / Sierra hybrid, to name just two.

Uses? Well, you can run power tools, a fluorescent lead light and a soldering iron. Imagine how useful that lot will be when you’re working on your car. Then you can have a TV and DVD – perfect if you’re at a car race or camping. And then there are the mobile uses, like running an on-board PC or laptop. Hey, you can even use a mains-powered amplified subwoofer – with the popularity of home theatre, some of these are getting so cheap that you can save heaps over a dedicated car system.

Mains power can also be cheaply added to your car - so how do you do it, then? The first step is to pick an inverter.

Inverters

An inverter is a device that turns the car battery’s nominal 12V DC into AC mains power – here in Australia, 240V at 50Hz.  Inverters have been around for years – they’re used in solar powered homes and many boats and motorhomes. Trouble is, they used to cost an arm and a leg. But the cost of inverters has now plummeted, with them available from as little as AUD$50.

There are two key specifications to watch when picking an inverter. The first is output power. A small inverter typically has a maximum output of 100 watts. As you’d expect, they’re then limited to driving a continuous load of 100W -  for example, two 50 watt lightbulbs or more usefully, a laptop power supply. The cost for an inverter of this capacity is currently about AUD$50.

From there the power capacity of inverters goes up and up – a big one is capable of supplying a mains power load of 1500W (1.5kW!). With this sort of power you can drive TVs, microwave ovens, sound systems, big power tools and so on. An inverter of this power will cost you about AUD$500.

So that’s it then? Just buy the biggest that you can afford?

Nope.

A key point to remember is that all the power is coming from your car battery. That 1500W inverter can draw over 125 amps – about the same as simultaneously having 30 car headlights on! So to avoid a flat battery, you’ll need at minimum to have the engine running all of the time.  When running big inverters, realistically the alternator will also need to be upgraded. So while big inverters are very useful in some situations (like those solar powered houses), for general car use they’re way too big.

The other important specification is the type of output that the inverter has. Mains power is AC (alternating current) which has a curving up/down movement in voltage called a sine wave. Most inverters don’t produce a sine wave output but instead something called a ‘modified square wave’. Modified square wave inverters will run most equipment without problems but electric motors can run hotter (and be noisier) and some laser printers and variable speed power tools won’t work properly. As you’d expect from the name, sine wave converters produce an output that better replicates what comes out of the wall socket. All mains-powered items will work fine with sine wave inverters.

A good compromise for a car is a 400 watt modified square wave inverter. The one shown here – Jaycar Electronics cat no MI5064 – has a maximum input current of 40 amps but at 400 watts still has enough power to be extremely useful. It’s also relatively compact and has automatic shutdowns if overloaded, overheated, or if the supply battery drops below 10 volts. (The latter saves a dead flat battery.) This inverter costs AUD$160. Another advantage of this particular design is that the input and outputs are electrically isolated, allowing both the AC and DC circuits to be earthed if required.

When selecting an inverter look at these specs:

• Continuous power rating
• Output type - modified square wave or sinewave
• Protection against overheating, output overload, low battery voltage
• Fan cooling
• Fuse protection
• Input/output electrical isolation

Installation

Smaller inverters have wiring connections allowing them to be either plugged into the cigarette lighter socket or connected straight to the battery via temporary alligator clips. But both approaches have problems – typically you shouldn’t draw more than 20 amps (and sometimes only 10) from a cigarette lighter socket, and in the case of the alligator clips, they’re obviously for temporary use only.

That’s why in the installation shown here, the inverter was mounted and then permanently wired into place.

The first step was to make some strong mounting brackets. This inverter doesn’t have any mounting holes (it normally just sits on four rubber feet) and rather than drill the heatsinks to take mounting bolts, brackets were made that gripped the inverter body.

The brackets were made from 50mm aluminium angle with the inverter sandwiched between the pieces of angle. Mounting tabs were formed at one end of the assembly by cutting away a section of the angle.

The inverter could then be mounted behind the trim panel in one side of the boot (trunk). You should try to mount the inverter in a cool, well ventilated spot as close to the battery and mains powered loads as possible. In this case that was straightforward because....

...in this car the battery (arrowed) is mounted in the boot. If the inverter is to be mounted a long way from the battery, heavy duty cable must be used to make the connections – eg thick car amplifier power supply cable.

The inverter shouldn’t be connected to the battery the whole time – not only will there be a small continuous current drain (even with nothing plugged in) but the inverter will always be live. Instead, a suitably-rated relay should be used to switch off power to the inverter when the ignition key is turned off. In this case, a 40 amp Ford relay had sufficient current capacity.

The relay was installed in a high-power base (cut from an old wiring loom) and then mounted on an aluminium bracket. The relay should be sited close to the battery and the positive voltage feed should be taken after a fusible link (or a new heavy duty fuse installed in-line). The negative lead from the inverter goes straight to the negative terminal of the battery.

Here’s the installation finished but for the boot trim being put back into place. The mains plug is easily inserted in the socket (arrowed).

Conclusion

As we said at the beginning, cheap, easy and effective!

Safety If undertaking this project you need to remember that you’re dealing with high voltages. We suggest that you don’t run multiple power outlets in the car – in fact, use only the single outlet provided on the inverter itself. The inverter should be securely mounted and the addition of high voltage warning stickers (as shown here) is a good idea. The on/off relay should be wired to the ‘accessory’ or ‘ignition’ settings of the ignition switch, so that when the key is turned off, so is the inverter. When no plug is inserted in the inverter’s socket, push in a childproof cap.