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Maximum Power Point Trackers (MPPT) for PV Panels

This section covers the theory and operation of "Maximum power point tracking".


What the heck is a MPPT?

A MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels or cells), and the battery bank, DC motor, or DC pump. (These are usually called power trackers for short - not to be confused with PANEL trackers, which are a top-of-pole 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 are designed to put out from 15 to 20 volts, with most being in the 16.0 to 19 volt range. The catch is, is that a nominal 12 volt battery is pretty close to an actual 12 volts - 10.5 to 13.2 volts, depending on state of charge. Under charge, most batteries want from around 13.4 to 14.5 volts - quite a bit different than what most panels are designed to put out.

OK, so now we have this neat 75 watt panel. Catch #1 is that it is rated at 75 watts at a particular voltage and current. The Siemens SP-75 is rated at 4.4 amps at 17 volts - 4.4 times 17 = 75 watts. Other brands and types have very similar ratings.

Now the Catch 22

Why 75 Watts does NOT equal 75 watts

Where did my Watts go?

So what happens when you hook up this 75 watt panel to your battery? Unfortunately, what happens is not 75 watts.


 

Your panel puts out 4.4 amps. Your battery is setting at 13 volts under charge: 4.4 amps times 13 volts = 57.2 watts. You lost 18 watts, or almost 24% of your power.

 

That 18 watts is not going anywhere, it's just not being produced because there is a poor match between the panel and the battery. With a very low battery, say 11 volts, it's even worse - you could be losing as much as 35% (11 volts x 4.4 amps = 48 watts.

One solution that pops to mind is - why not just make panels so that they put out 13 volts or so to match the battery?

Catch #22a is that the panel is rated at 75 watts at full sunlight at a particular temperature (25C). If you have a cloudy day, or the temperature is high, you don't get 17 volts. (actually, temperature is the most important factor, as lowered light reduces the AMPS quite a bit, but the voltage only a little, up to a point). At the temperatures seen in many hot climate areas, you might get 15 volts. If you started with 15 volts (as in 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. The panel will just sit there looking dumb, and your batteries will get even stupider than usual. Nobody likes a stupid battery.

What is a power point tracking gizmo?

First off, there is always some confusion about the term "tracking".

Panel tracking - this is where the panels are on a mount that follows the sun. The most common is the Zomeworks. These optimize output by keeping the panels very close to 90 degrees angle for maximum sunlight. These typically give you about a 15% increase in winter and up to a 40% increase in summer. Note that this is just the opposite of the MPPT, described below.
Maximum power point tracking - this is electronic tracking, and has nothing to do with moving the panels. Instead, the controller looks at the output of the panels, and compares it to the battery voltage. It then figures out what is the absolute best power that the panel can put out. 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 95-97% efficient in the conversion. You typically get a 30 to 45% power gain in winter and 10-20% in summer.

How Maximum Power Point Tracking works

Here is where the optimization, or maximum power point tracking comes in. A MPPT takes that 17 volts at 4.4 amps and converts it, so that what it puts out to the battery is no longer 4.4 amps at 17 volts, but 5.77 amps at 13 volts. Now you still have 75 watts, and everyone is happy. (Actually, you get about 72 watts, as PPT's are not quite 100% efficient). In an extreme case, such as a fully discharged battery at 10.5 volts, you would get nearly 7 amps at 10.5 volts out of the MPPT into the battery.

A MPPT 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 75 watt panel is actually capable of putting over 80 watts because the output goes up as 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.

MPPT's are most effective under these conditions:

  1. Winter, and/or cloudy or hazy days - when the extra power is needed the most.
  2. Cold weather - solar panels work better at cold temperatures, but without a max power tracker 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 the most.
  3. 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 those conditions at the same time.

Catch 22b: Most MPPT's on the market don't give you as much you would hope for. The reason is efficiency. So you gain 25% by adding a MPPT - that does not help you much if your MPPT is 85% efficient. You've only gained about 5 watts - not much to get excited about. It's an unfortunate fact of life that so far the only PPT we have seen that really approaches 98% under all conditions is the AERL Maximizer (made in Australia). The RV Power Products "Solar Boost" can approach 97% under some conditions, but typical is more like 90% to 95% - still a good gain, but not ideal. It is also much better than older designs, which averaged around 75% to 85% - almost a waste of time except for water pumping systems. Although nobody has been able to match the AERL performance in the 15 years that they we have been selling them, their market has always been pretty small because they are so expensive (2 to 4 times other brands).

Ok, so now back to the original question - What is a MPPT?

What a Maximum Power Point Tracker is:

The Power Tracker™ is a high frequency DC to DC converter. They take the DC input from the solar panels, change it to AC, and convert it to a different DC voltage and current to exactly match the panels to the batteries. MPPT's operate at very high audio frequencies, in the 20-50 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 frequencies circuits can be very tricky because the problems with portions of the circuit "broadcasting" just like a radio transmitter. Noise isolation and suppression becomes very important. The advantage of using high frequency is that high efficiency is easier to achieve, and component parts, especially transformers, can be made much smaller.

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 converters, this is strictly an electronic process - no real smarts are involved except for some regulation of the output voltage or current. This is the type of conversion used in the Solar Converters & RV Power Products brand MPPT's. You don't get quite the full efficiency as you do in the "smart" ones, but the price is 40-75% less - and you still gain 15 to 30% under most conditions. The AERL MPPT's are microprocessor controlled, but do not have a computer interface. They do, however, have some models that will match almost anything to anything, up to 185 volts, and have the best conversion efficiency of any MPPT.

Smart power trackers

There is another type of MPPT available that is microprocessor controlled. They know when to adjust the output that it is being sent to the battery, and they actually shut down for a few microseconds and "look" at the solar panel and battery and make any needed adjustments. Although not really new (the Australian company AERL had some as early as 1985), it has been only recently that electronic chips and microprocessors have become cheap enough to be cost effective in smaller systems (less than 2 KW of panel). A 1200 watt AERL unit can easily cost $1200, so though very effective, the costs are rather high. (One major advantage of the AERL units is that they are DOWN converters, so can match panel voltages of up to 180 volts down to as low as 12 volts - at 98% efficiency).

Fire Wind & Rain MPPT Controllers:

There have been quite a few reliability problems with these charge controllers. At this point no one is sure what the future status of the company is.

So, that's the basics. CP and Water Pumping controls operate in a very similar manner - they perfectly match the input to what is needed at the output load. They are based on the same type of high efficiency converter, and all have the effect of raising PV panel efficiency by 10-45%.