How MPPT Solar Controller Work?
Here is where the optimization, or maximum power point tracking comes in. Assume your battery is low, at 12 volts. A MPPT Solar Controller takes that 17.6 volts at 7.4 amps and converts it down, so that what the battery gets is now 10.8 amps at 12 volts. Now you still have almost 130 watts, and everyone is happy.
Ideally, for 100% power conversion you would get around 11.3 amps at 11.5 volts, but you have to feed the battery a higher voltage to force the amps in. And this is a simplified explanation – in actual fact the output of the MPPT charge controller might vary continually to adjust for getting the maximum amps into the battery.
On the left is a screen shot from the Maui Solar Software “PV-Design Pro” computer program (click on picture for full size image). If you look at the green line, you will see that it has a sharp peak at the upper right – that represents the maximum power point. What an MPPT controller does is “look” for that exact point, then does the voltage/current conversion to change it to exactly what the battery needs. In real life, that peak moves around continuously with changes in light conditions and weather.
A MPPT Solar Controller tracks the maximum power point, which is going to be different from the STC (Standard Test Conditions) rating under almost all situations. Under very cold conditions a 120 watt panel is actually capable of putting over 130+ watts because the power output goes up as panel temperature goes down – but if you don’t have some way of tracking that power point, you are going to lose it. On the other hand under very hot conditions, the power drops – you lose power as the temperature goes up. That is why you get less gain in summer.
MPPT’s are most effective under these conditions:
Winter, and/or cloudy or hazy days – when the extra power is needed the most.
Cold weather – solar panels work better at cold temperatures, but without a MPPT you are losing most of that. Cold weather is most likely in winter – the time when sun hours are low and you need the power to recharge batteries the most.
Low battery charge – the lower the state of charge in your battery, the more current a MPPT puts into them – another time when the extra power is needed the most. You can have both of these conditions at the same time.
Long wire runs – If you are charging a 12 volt battery, and your panels are 100 feet away, the voltage drop and power loss can be considerable unless you use very large wire. That can be very expensive. But if you have four 12 volt panels wired in series for 48 volts, the power loss is much less, and the controller will convert that high voltage to 12 volts at the battery. That also means that if you have a high voltage panel setup feeding the controller, you can use much smaller wire.
The Power point tracker is a high frequency DC to DC converter. They take the DC input from the solar panels, change it to high frequency AC, and convert it back down to a different DC voltage and current to exactly match the panels to the batteries. MPPT’s operate at very high audio frequencies, usually in the 20-80 kHz range. The advantage of high frequency circuits is that they can be designed with very high efficiency transformers and small components. The design of high frequency circuits can be very tricky because the problems with portions of the circuit “broadcasting” just like a radio transmitter and causing radio and TV interference. Noise isolation and suppression becomes very important.
There are a few non-digital (that is, linear) MPPT charge controllers around. These are much easier and cheaper to build and design than the digital ones. They do improve efficiency somewhat, but overall the efficiency can vary a lot – and we have seen a few lose their “tracking point” and actually get worse. That can happen occasionally if a cloud passed over the panel – the linear circuit searches for the next best point, but then gets too far out on the deep end to find it again when the sun comes out. Thankfully, not many of these around any more.
The power point tracker (and all DC to DC converters) operates by taking the DC input current, changing it to AC, running through a transformer (usually a toroid, a doughnut looking transformer), and then rectifying it back to DC, followed by the output regulator. In most DC to DC converters, this is strictly an electronic process – no real smarts are involved except for some regulation of the output voltage. Charge controllers for solar panels need a lot more smarts as light and temperature conditions vary continuously all day long, and battery voltage changes.