In an age when almost everything seems to run on electricity of some kind, we need to have access to that necessary power. This is reflected in the oftentimes complex ‘electrical cupboards’ that now make up a corner of so many of our caravans and camper trailers.
Those cupboards are filled with cables, gauges, switches, fuses and gizmos that leave the non-technical in a complete quandary. What are all these things and how do they work?
One of the most misunderstood occupants of the electrical side of vans is the inverter. Most people understand that it makes 240V power out of 12V, but just how does it do that and what are the strengths and weaknesses? Let’s take a look.
How do inverters work?
More significantly than the voltage upgrade is the fact inverters actually convert the DC (direct current) from your vehicle or van’s batteries into AC (alternating current).
Old fashioned inverters used a mechanical switch. The flow of electrons would activate a magnet which flipped a spring-loaded metal switch to change the circuit and force the current to run in the opposite direction. The spring would pull it back and make it flow in the original direction until the magnet took over and flipped the current in the reverse direction again and so on. These inverters were renowned for the buzzing sound they made.
Modern inverters achieve the same result with oscillator circuits, using transistors or semi-conductors to reverse the current direction.
If we run a current through a coil, a magnetic field will form. If we place a secondary coil adjacent to the primary coil, that magnetic field will cause an electric current to flow in the secondary coil. If the secondary coil is twice the size (twice as many windings) as the primary coil, it will induce a current that is twice that of the voltage applied to the primary coil. Thus we can step up voltage, and with a transistor-mediated switch at the start of the circuit to the primary coil, we can create alternating current of a higher voltage from low voltage direct current. This is, in basic terms, how an inverter works.
However, this simple switching of direction produces sharp changes in current flow, and on an oscilloscope this shows up as a stepped ‘wave form’. This works okay for some appliances (drills, angle grinders, etc.), but is too harsh for sensitive electrical equipment (audio or visual equipment, GPS devices, scientific equipment, or anything running off a transformer.
AC power, as produced in your home power supply, is represented on an oscilloscope as a ‘sine wave’, and to achieve this ‘clean’ wave form from your inverter requires a series of filters, inductors and capacitors.
Of course, this filtering creates much more complex internal circuitry, and that costs money. Thus a basic inverter can be quite cheap, but a pure sine wave inverter is much dearer, though the electricity they produce is a more perfect sine wave than you will see from your home power supply.
An inverter can be a valuable fallback in the bush, if you need to use a tool such as an angle grinder. With smaller units, ensure that they can be removed to connect directly to the charging system for use near the front of the vehicle.
Choosing an inverter
The first step in choosing an inverter is to select something from the range available for the voltage of your source power. For the vast majority of cars, caravans and campers, that will be 12V.
‘Twelve volt battery’ is a generalised term – a fully charged 12V battery will operate at about 12.8V, or perhaps a little more if it’s a lithium battery. But they are designed to accept a charging current of up to 14.5V from your vehicle’s alternator, so that all inverters that operate from such a source will accept up to this voltage, and while they are nominally ‘12V’ appliances, they are actually rated on the basis that they are receiving 14.5V.
However, an inverter will only provide the power that the appliance requires to run. If, for example, an appliance requiring 100W is plugged into a 600W-rated inverter, the inverter will only output only 100W, regardless of battery voltage. The draw from the battery is most simply worked out by dividing the appliance wattage by 10. A factor of 10 allows for all the efficiency losses and gives a good working figure to calculate battery draw.
As you run a load, the battery voltage decreases and the required amps from the battery increases. But the wattage provided to the appliance stays the same. This is one of the reasons that inverters run more efficiently from lithium batteries than from deep-cycle or AGM batteries, as the lithium’s DC voltage stays fairly constant (about 13V), resulting in fewer amps being drawn from the battery.
Also, it should be noted that ‘square wave’ inverters can be harmful to many appliances and should be avoided. Modified sine wave inverters have some filtering, so that the ‘steps’ in the wave form are reduced, and this will help with some appliances, but a pure sine wave inverter will work with everything. Basic square wave inverters are not seen commercially these days and even modified sine wave inverters are becoming less common.
The interior of a 1000W Enerdrive pure sine wave inverter indicates the level of complexity in a modern inverter.
Understand that there are going to be limits to how much power that can be drawn from any inverter. Every appliance must by law have a label indicating its power requirements. If you check the labels on some of the appliances around your home, you’ll notice that anything which heats or cools consumes a lot of power (i.e., a high number of watts).
In addition, many devices require a boosted level of power at start-up. This is known as peak or surge, and should also be listed on the appliance data label. This can be as high as 10 times that of the operating power draw. Most inverters also have a peak rating, so make sure the two marry up.
“We see many failures of inverters from customers doing two basic things wrong: typically undersized battery cables and/or battery cables run over long distances,” said Chris McClellan of Brisbane’s Enerdrive, a major manufacturer of inverters and other 12V accessories.
“The other major cause of failure is simply buying an inverter on price and thinking that it will do everything. If the demands from an inverter will be high, such as with air-conditioning, then a transformer-based inverter/charger is a must.”
Having taken all of this on board, you now need to consider the wattage of all the appliances you might want to run on your inverter at any one time, and then add 15 per cent to account for power losses, etc. Some appliances will run at lower wattage outputs but will do so at lowered efficiency. For example, a 500W angle grinder may run off a 350W inverter but will have a limited ability to cut or grind. A light may work, but do so with reduced light output.
Don’t try to run any high wattage appliances from an inverter that’s plugged into a cigarette-style plug as this is a system that simply won’t deliver.
If you want to run high draw appliances, such as an air-conditioner, microwave or electric kettle, you will require substantial battery back-up to provide the necessary power, perhaps 200 or 300 amp hours (Ah) of capacity, and use of these appliances can rapidly deplete the batteries’ state of charge.
Lower wattage inverters are simple ‘plug and play’ devices. Insert the plug into a 12V socket and plug your 240V device into the inverter, and away you go.
At outputs over 400W, your inverter is best connected via an appropriately rated Anderson plug, or, best of all, directly to the battery.
Locate your inverter where there is plenty of air flow as they do produce a lot of heat and have fans to blow cool air across their components. Ensure that any connecting cable is of adequate gauge to prevent voltage drop and is only as long as it needs to be.
Most modern inverters will have safety features built in, automatically shutting down in the event of a short circuit, overheating or current overload. Better units will have a low voltage alarm if your house batteries become depleted; a good feature if you want to run high-draw appliances.
Make sure you have a fuse in the battery cable close to the battery. Thermal circuit breakers are not advised.
A typical inverter installation in a modern camper trailer
What is AC and DC power?
At school, they taught us that electricity was electrons flowing along a conductor, like ants in a line walking along the wire carrying little packets of energy. That’s okay for understanding DC (direct current), where the current of electrons flows from the negative battery terminal around a circuit to the positive terminal, but AC (alternating current), in that analogy, would be the ants reversing their direction 50-60 times every second, so that they don’t go from point A to point B. They could be said to be running on the spot.
Watts, Amps, Volts
The three forms of measuring electricity should be understood when it comes to dealing with 12V systems.
We can look at electricity like the flow of water. If we drilled a small hole at the base of a dam wall (with the dam representing our stored electricity) then water (electricity) would squirt out a long way because of the pressure of the water behind it. If we placed a turbine wheel in this stream of water it would rotate, but not very fast because while the water is under high pressure there isn’t much of it.
That high pressure is the equivalent of volts. Therefore, high pressure (volts) without volume (amps) does not do much work.
If we drilled a very large hole high up in the dam wall, water would pour out at a great rate, but since there isn’t much pressure because it is close to the surface, again the turbine would not spin very quickly. Thus, because there is a lot of volume (amps) with little pressure (volts), little work is done.
If we went back to the base of the dam and drilled a similarly large hole, a huge amount of water would pour out at a great rate because of the pressure, and this time the turbine would spin rapidly.
Thus we see that high pressure (volts) combined with high volume (amps) produces a lot of work (watts).
Basic Dos and Don’ts
When using inverters, many people have different ideas on what can and cannot be run. Here are a few basic dos and don’ts for ‘what’s real and what’s dreaming with inverters’…
- Basic household AC appliances, such as televisions, microwaves, computers, etc., can all be run from the correctly sized and type of inverter.
- Air-conditioners are highly uneconomical to run off an inverter. Most large inverters can power air-cons without problem, but the battery bank will nearly always let the system down. If you must run an air-conditioner, look for an inverter that shares the load with a generator, as the generator will be the primary power source with the inverter assisting in start-up loads. Battery chargers will be more important here, so ensure the right sized charger is used.
- You cannot run a battery charger off a power inverter connected to the same battery bank. This generates a loop sequence that only discharges your batteries quicker than the charger can recover them. This is due to the losses and inefficiencies within the inverter and battery charger.
- Large heating elements such as frypans, toasters and fast boil kettles are very uneconomic to run from a power inverter. They require a large inverter system, quick battery charging capabilities and sufficient storage within the battery system. Generally to do this you would require around 400 amps of storage at 12V, along with an inverter rated to over 2000W. The charging would have to come from a large battery charger running from an AC generator (forget solar in this case). The easiest solution: just run your generator to begin with.