This section covers the theory and operation of “Maximum Power Point Tracking” as used in solar electric charge controllers.
A MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank or utility grid. To put it simply, they convert a higher voltage DC output from solar panels (and a few wind generators) 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 PV panels are built to put out a nominal 12 volts. The catch is “nominal”. In actual fact, almost all “12 volt” solar panels are designed to put out from 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. Catch #1 is 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 times 17.6 volts = 130 watts).
NOW THE CATCH 22
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?
Unfortunately, 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 times 12 volts = 88.8 watts. You lost over 41 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?
Catch #22a is that 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.
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