Types Of Inverter:

There are 2 main types of inverter that can be purchased today. The first is called modified sine wave (msw) and the other is called a pure sine wave (psw). They both have benefits and drawbacks. Lets discuss these briefly as the impact of them will be seen later....

Modified Sine Wave Inverter (msw): Benefits and Drawbacks

One further point about psw inverters: they are categorised into 2 sub-types: namely high frequency inverters and low frequency inverters. Here is yet another table of differences between the two:

Installing The MSW Inverters:

The supplied system comes with 2 inverters: a 600watt version and a 2Kw version, both modified sine wave versions. The pictures below show both, the 600watt has a single 3-pin plug and the 2kw has a 2 plugs. Both have fans to control temperature. To give you an example of how much 600watt is: this would run a hairdryer on a medium setting. It would also run a computer and monitor, or enough lighting for say 2-3 rooms. The 2kw version would run a kettle or toaster but equally could potentially light the whole house and more besides!

Firstly I connected the earth cables to the chassis of each inverter, which in turn is connected to the negative pole on one of the battery terminals. Then I connected each inverter to a separate battery, by snapping off the clamps, unscrewing the cable holder and adding the inverter cables. Although the inverter cables come with rounded terminators at both ends, I cut them at the battery end so that I could join them properly to the battery clamp. It is easier to add the inverters to separate batteries rather than try and fit them both to one battery, and doing this will also isolate the draw to the 2kw inverter from the 600watt inverter somewhat. Regardless of how they are connected, the power drain will be shared across all the batteries as they are connected together by the controller charge cables.

The cables that come with each inverter are specifically matched to the size of the inverter, to allow for the correct current to be drawn as required. You must use the correct size cables so that when the maximum power is required the cables are up to the job. The order of connections are important - the cables should be connected to the battery first and then connect the negative to the inverter and finally the positive. You can sometimes expect a small spark when making the final positive connection and this is normal.

Testing The MSW Inverters:

As mentioned above, the inverters are not identical to the main supply coming from the grid. So you will need to take care when connecting certain items. In advance of connecting to the house, i undertook some tests in order to qualify the power coming from the inverter. First I made up a simple extension cable and proceeded as follows:

• TEST1: I ran the extension cable from the GRID supply to a kettle rated at 1850watts. It worked perfectly. I did this only to prove that the extension cable itself was not faulty.
• TEST2: I ran the extension cable from my 2kw Honda generator to the kettle. It also worked perfectly and mimicked the grid supply as far as I could tell.
• TEST3: I ran the extension cable from the 2kw inverter to the kettle. I noticed an audible hum at the kettle's electrical contact. What could this be?

Now this may appear to be a very simple test but the results were significant. Test 1 tests a fully standardized mains power test. We would expect this to work. Test 2 is done using a generator. Because this uses coils rather than electronics to produce the power, it appears to more closely resemble the grid supply and thus produce a pure sine wave. Finally the third test, using the msw inverter, caused a hum in the kettle. On suspicion that this hum could be the result of a faulty earth, I went out and bought 2 more earth rods and hammered these into the ground in a different place, ran another earth cable from the inverter direct to this new ground point and ..... same result! the humming in the kettle remained. So the issue is not "earth" related.

I then tried out a coffee machine with the inverter. This particular model was rated at 900watts, but also had a timer on it. And about a minute after the machine was plugged in I detected a burning smell coming from the machine. A few moments later the machine gave up and died! The assumption here is that the timer mechanism on the coffee machine was not deemed 'suitable' for use the the inverter. Further testing proved that the inverter could quite happily run all the lighting I threw at it, but it could not handle being connected to the TV, again producing a buzzing noise on the TV electrical input.

Installing The PSW Inverter:

Following the tests done with the msw inverters above, I decided it was time to do some testing with a pure sine wave inverter. I chose a 3kw version, as shown in the picture below. This particular version allows for a surge current of up to 9Kw, and it considerably larger and more bulky than the 2kw version. Again it comes with integrated cooling fan and a standard 3-pin plug output. Also, the cabling coming from the back of the inverter, although only partially visible in the picture, is over 3 times the size of the 2Kw cable! This is to allow for the maximum surge current of 9Kw to be drawn if required.

The same connections need to be made for this inverter; i.e. ensuring that the battery cables are corrected to the right inverter terminals and connection of the earth grounding cable to the chassis. The inverter has a green power setting which draws less current from the battery when in idle mode, at the expense of needing a higher current to draw power from it.

Testing The PSW Inverter:

As before, I undertook some basic testing to prove the quality of current from the inverter. This time TEST3 was done again:

• TEST1: <as before> 
• TEST2: <as before>
• TEST3: I ran the extension cable from the 3kw inverter to the kettle. There was no hum from the kettle's electrics !

Emboldened by this result, I then hooked up the TV to the psw inverter. No buzzing or at all, in fact it was running exactly as if being powered by the GRID. I even hooked up the newly purchased coffee maker, happy in the knowledge that there would be no problems with timers this time. The machine again worked without any problem whatsoever.

In summary, I have have decided to use the 600watt modified sine version, where practical, for lighting requirements only. That way I can ensure that the sine waves from this inverter are only present on the lighting circuits. Then I will use the 3kw pure sine version for the appropriate electrical sockets within the house, without needing to worry about what appliance I am plugging in.

Connecting Inverter To Battery Bank:

No matter which inverter you chose, is is important to try and balance the load that the inverter is drawing from the batteries, in order to maximise the longevity of the batteries and also to allow each battery to supply an equivalent load to the inverter.

If you look at the diagram on the left, this is commonly how people set up their connections, taking the inverter positive and negative connections from one of the batteries in the bank. But studies have shown that because of cable resistance and length, that the last battery will be delivering more amps than the others, regardless of quality and thickness of cable which joins the batteries together. This would continue until the battery becomes further drained, and only then do the remaining batteries help out. Over time you risk reducing the lifespan of the last battery in the chain, due to the additional load being placed on it. You can check this yourself by placing a voltmeter across both terminals of the final battery when a reasonable load is being drawn, and comparing this to the voltage on the other batteries at the same time.

On the right is the alternative method, and although it does not appear sensible it is actually a more balanced system. This is because the load being drawn is now equalized across the battery bank. So all batteries are discharging at the same volume, because we have split the positive and negative draw to the inverter. In practical terms, bearing in mind that inverter cables will be the same length, you will need to re-arrange the batteries in a concentric shape, rather than the side-by-side layout shown above. If you do this properly you can have the first and last batteries placed next to each other, so that the inverter cables can still be connected easily. Also, make sure that all battery interconnect cables are the same length, to balance both the load and charging.