A MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (panel), and the battery . To put it simply, they convert a higher voltage DC output from solar panels down to the lower voltage needed to charge batteries.
(These are sometimes called “power point trackers” for short – not to be confused with PANEL trackers, which are a solar panel mount that follows, or tracks, the sun).
So what do you mean by optimize?
Solar cells are neat things. Unfortunately, they are not very smart. Neither are batteries – in fact batteries are downright stupid. Most panels are built to put out a nominal 12 volts but in actual fact, almost all “12 volt” solar panels are designed to put out between 16 to 18 volts. The problem is that a nominal 12 volt battery is pretty close to an actual 12 volts – 10.5 to 12.7 volts, depending on state of charge. Under charge, most batteries want from around 13.2 to 14.4 volts to fully charge – quite a bit different than what most panels are designed to put out.
OK, so now we have this neat 130 watt solar panel that it is rated at 130 watts at a particular voltage and current. The Kyocera KC-130 is rated at 7.39 amps at 17.6 volts. (7.39 amps X 17.6 volts = 130 watts).
Now the Catch
Why 130 Watts does NOT equal 130 watts
Where did my Watts go?
So what happens when you hook up this 130 watt panel to your battery through a regular charge controller?
What happens is not 130 watts!
Your panel puts out 7.4 amps. Your battery is setting at 12 volts under charge: 7.4 amps X 12 volts = 88.8 watts. You lost over 40 watts – but you paid for 130. That 41 watts is not going anywhere, it just is not being produced because there is a poor match between the panel and the battery. With a very low battery, say 10.5 volts, it’s even worse – you could be losing as much as 35% (11 volts x 7.4 amps = 81.4 watts. You lost about 48 watts.
One solution you might think of – why not just make panels so that they put out 14 volts or so to match the battery?
Because the panel is rated at 130 watts at full sunlight at a particular temperature (STC – or standard test conditions). If temperature of the solar panel is high, you don’t get 17.4 volts. At the temperatures seen in many hot climate areas, you might get under 16 volts. If you started with a 15 volt panel (like some of the so-called “self regulating” panels), you are in trouble, as you won’t have enough voltage to put a charge into the battery. Solar panels have to have enough leeway built in to perform under the worst of conditions. The panel will just sit there looking dumb, and your batteries will get even stupider than usual.
Nobody likes a stupid battery.
WHAT IS MAXIMUM POWER POINT TRACKING?
There is some confusion about the term “tracking”:
Panel tracking is where the panels are on a mount that follows the sun. These optimize output by following the sun across the sky for maximum sunlight. These typically give you about a 15% increase in winter and up to a 35% increase in summer.
This is just the opposite of the seasonal variation for MPPT controllers. Since panel temperatures are much lower in winter, they put out more power. And winter is usually when you need the most power from your solar panels due to shorter days.
MPPT’s look at the output of the panels, and compares it to the battery voltage. It then figures out what is the best power that the panel can put out to charge the battery. It takes this and converts it to best voltage to get maximum AMPS into the battery. (Remember, it is Amps into the battery that counts). Most modern MPPT’s are around 93-97% efficient in the conversion. You typically get a 20 to 45% power gain in winter and 10-15% in summer. Actual gain can vary widely depending weather, temperature, battery state of charge, and other factors.
Here is where the optimization, or maximum power point tracking gets good: Assume your battery is low, at 12 volts. A MPPT 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 the 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.