MPPT Solar Regulators for Motorhomes and Caravans
If you have read my article that discusses the function of a solar regulator, you will know that a standard solar regulator is a fairly simple device with a fairly simple job – to stop the batteries from being over charged.
A new type of regulator called MPPT (Maximum Power Point Tracking) is beginning to be offered to motorhomers and caravaners in Australia. I have seen claims of 40% improved power production – So just how do these controllers do that and do they work?
Firstly let me say that the ideal way of answering the question of ‘do they work’ (in the real world) is to simply have two identical sets of panels in the sun – one attached to a standard regulator and one attached to an MPPT controller. By simply measuring the power produced each day by each set of panels we could get a very good idea of how effective MPPT is. I have not done that.
The information presented here is based on my knowledge of the operation of the controller and my experience with the environmental conditions in Australia. I would be very interested to hear from anyone who has actually completed some well-designed tests with MPPT controllers.
How do MPPT controllers work?
The voltage across the terminals of a 12 volt battery varies from about 11 volts when discharged, to about 14.5 volts when fully changed. To charge a battery you need to apply more voltage than is presently across the terminals. To be safe, most solar panels output around 17 volts (when the panel cell temperature is at 25 degrees). For most panels this output voltage is lower when they are hot (about 15v on a very hot day) and higher when they are cold (about 18v on a very cold day).
When the panels are connected to the batteries via a standard controller, any surplus voltage that the panels are producing is discarded.
This means that your 100watt panel is really only able to produce 67 watts of charge when the battery is empty and the terminal voltage is down at 11.5volts …
100w @ 17 volts = 5.88 amps
5.88 amps X 11.5 volts = 67.64 watts
An MPPT controller can perform some magic on the power coming from the panels. It first looks at the voltage across the battery terminals. It then looks at the voltage coming from the solar panels and “adjusts” this voltage so that it is just enough to charge the battery. In the process of this adjustment it is able to take the additional (unused) voltage and turn this into useful current (amps) to charge this battery. This is called the maximum power point. The MPPT controller constantly monitors or tracks these voltages and that is why it is called a Maximum Power Point Tracking controller.
When is an MPPT controller most effective?
An MPPT controller will be most effective when the difference between the battery terminal voltage and the panel voltage is at its greatest. For example when the panels are very cold and producing 18 volts (at -7degC with a wind blowing will produce a cell temperature of approx. 0 deg C) AND the battery is fairly empty – say 12 volts. In this case we have a huge 6 volt difference and the controller can turn this into a 50% increase in charge current. This calculation does not account for losses in the controller, voltage drop in the cables etc. so the real world gain under these circumstances would be closer to 20 – 30%.
Based on this, it should be quite obvious that the worst possible circumstance to test the value of an MPPT controller is very hot conditions with the battery terminal voltage high (not deeply discharged). For Australia, this is a far more likely situation. I don’t know about you, but I have not seen any temperatures in Australia anything close to -7 deg C.
Panels are rated with a cell temperature of 25 deg C. Unless you are planning a trip to the top of Mount Mt Kosciuszko in the middle of winter, your panels are going to have a cell temperature far closer to 100 degrees C than zero degrees C. This will result in a panel voltage far lower than the 17 volts specification and greatly reduce the effectiveness of an MPPT controller.
If you plan to be kind to your batteries by NOT discharging them deeply, then the terminal voltage is going to be almost always higher than the 12 volts used in the example – further reducing the effectiveness of MPPT.
I would treat salesman claims of 20% – 50% increased output with extreme scepticism. These results may be obtainable under ideal conditions. Real world results are likely to be a long way from these figures.
If I were buying a new controller and I had the choice between conventional and MPPT and all other things were equally (cost, features, support warrantee etc) – I would probably buy the MPPT controller – but I would not pay a premium or sacrifice other features in response to unrealistic claims of boosted charge performance.
Why not share your experience? Do you have an MPPT controller? Tell us about it – we would love to hear from you. Use the “Leave a Reply” box below to tell us what you think.
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