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The two types of charge controllers / what is MPPT? 2010-05-13

Reviewer: HMMWV

If you've read the reviews, by now the acronym MPPT has shown up more than once. If you know what that means, read no further as this is not too entertaining. If you would like to know the difference in MPPT and non-MPPT solar regulators, and can tolerate some basic math or skim through it, read on and I can explain why this non-MPPT regulator will do the job, just not the BEST job possible.

MPPT stands for Maximum Power Point Tracking, which sounds like a good thing, and it definitly is a must for large house size systems, but what exactly is it? Simply put MPPT finds YOUR solar array's Maximum amount of power for a given condition (full sun/overcast/panel tilt). It is always searching for the best voltage and current conditions to maximize power from the panels.

Let's explore how MPPT works. Normal power regulators like this one interface between the solar panel array and the storage battery. When the battery is full, they quit charging to protect the battery from boiling the electrolyte away. Pretty simple. But what if you need more power from your solar panels than the simple case? It turns out that MPPT can, using more electronics, harness more power from your solar arrays than a simple on/off charge regulator. How? by using a small IC chip that searches for the highest POWER from the panel, hence the name maximum power point tracking. The tracking just means that as the sunlight changes, so does the maximum power location.

Unfortunately Solar Panels are not just a source of simple DC power - that is they don't behave like a battery does. The fewer amps you draw from the panel, the higher the panel's voltage across the wires becomes. This is where most MPPT discussions end but with a couple simple equations (ohm's law) we can dive deeper into what MPPT does and why its good (and costs more).

Ohms law is really simple: there are two parts: Voltage = Current times Resistance (V=I x R)
Of course we can re-arrange it other ways: R = I/V, I = V/R Where I is in amps, V in volts, R in ohms. If we connect a 1 ohm (R) resistor across a 12V battery (V) then solving for I must be 12 AMPs flowing through the resistor.

For POWER (one P in MPPT) we need to know another simple equation Power = Volts x Amps (DC version)
In the above example, the resistor is most likely a power resistor (i.e. big) because 12 volts x 12 amps = 144 watts which gets hot real fast. In this case, the 144 watts is a power point from a battery. They are simple since small currents do not make large swings on the battery voltage and we can assume it to be nearly constant when using large storage batteries.

Finally we need to know a little about DC to DC converters, as they are the logic controlled core of the MPPT solar battery charger. True they don't convert DC directly to another DC voltage, but they are pretty efficient (97%+) by taking an input DC POWER, making a AC voltage from it much like an inverter would, then converting that AC back to DC again for their output to the battery. And they may alter the battery voltage depending on temperature for optimal charging or even change the voltage depending on how many cells you choose to make a battery array (e.g. 12V, 24V, 48V etc). They are flexible and always seek to get as much power from your solar array as possible and put as much power in your batteries until they are full. Some models take the remaining power from your array and produce AC to be sold back to the utility company, but not all have this feature. Big Solar systems can use 6000 watt DC/DC converters that have a 240 Volt AC output as well. They are not small or cheap but they do work.

How does MPPT work? Shine a constant amount of light on a solar panel so that it's output can be characterized. A "12V" panel isn't really 12.000 Volts. It may be 16-19 Volts if you put a digital voltmeter across its output terminals. This is known as it's Voc or Voltage, open circuit meaning zero current flows because the circuit is open. The compliment to Voc is Isc which stands for Current (I) short circuit. Very few devices can have the Isc measured directly, but for small (60W give or take panels) it is safe to put an ampmeter across the same two wires from the panel and know how many amps flow when the voltage is zero (it's zero because you shorted the wires with the ampmeter - the definition of zero volts is a short circuit) If you were to plot on the X axis of a graph voltage, from the two endpoints, 0 and Voc (say it's 19V) and on the Y axis you plot the multiplication of amps times voltage, which we know to be power, you will find a curve with a peak. The MPPT seeks to operate your solar array at that peak. Without MPPT, the voltage is more a function of the battery's state of charge (from 11-13.8V) and the current that flows is on the graph of your panel's power function for a given voltage.

A 16V, 30 watt panel might give you 33 watts at a particular voltage, or an extra 10% using MPPT. In other installations, wiring your panels in series for higher voltages may be where the maximum power is found. The chip inside the MPPT tracker searches for this peak in your system and operates the solar panel (or array of panels) at it's absolute maximum power, which is sent to the DC/DC converter to charge your batteries.

We might get 10-30% more power doing this, but the voltage will be all wrong for charging our battery, or our battery array. And that's where the DC/DC converter steps in. As a simple example, say you find that 2 panels in parallel give you 14.5V at 1.9A going through your non-MPPT charge controller, or 27.5W delivered to your batteries. Wired in series using MPPT they give you 37V at 1.25A or ~48W. You would get more power by wiring in series for a higher solar voltage of 37V, which is too high to charge a 12V lead acid storage battery. Now you need a MPPT.

The MPPT takes the 37V, 1.25A from your panels and gets ~48W, looses 2W in conversion inefficiencies(give or take) for 46W out, and converts it to 13.6V to charge your battery array at 3.38A whereas had you wired the two panels in parallel you would get about 27.5 watts in this illustration.

MPPT systems are found in high voltage solar arrays (100V+) because the peak power points in these systems are up high in voltage and low in current.

To sum it all up -

For really small solar panels, (1w) and big batteries (100 AH) you really don't need a charge controller just a diode to prevent backflow when the sun is down. Car battery mainters are made this way.

As you increase power and run the risk of overcharge, some controller is needed. The cheapest is like the one sold here - the non-mppt which works well with solar panels that are close to the battery voltage.

And finally as you cover your house with solar panels, MPPT systems sqeeze every last drop of power so long as you operate them within their specified parameters (often limits on voltage and current comming in or watts converted) Outback is a major MPPT converter which can handle a wide range of input voltages and run a grid-tie system. As for 4 stars - you can buy MPPT trackers for $112 now.

Sunforce Digital Charge Controller 2010-02-07

Reviewer: John E. Miles

I've purchased the Sunforce 30 Amp controller, and the Sunforce 39810 80-watt solar panel. I completed the easy installation and both units are working as advertised. The panel provides approx 4.7 amperes on a sunny day to recharge my battery.

great controller at a fair price 2009-09-10

Reviewer: Sea Angel

I've lived with solar panels for 10 years now, mostly on a cruising yacht, and I decided to try one of these controllers with a couple solar panels I just installed on my recently acquired land yacht (a used 5th wheel trailer) and it works great! So good in fact that I just ordered a second one to bring back to my boat in the Caribbean this fall to regulate the 4 solar panels which I live with there. It's much more than just an on/off relay so it charges the batteries much better.

Sunforce 30 Amp charge controller 2009-08-31

Reviewer: matterhorm computers

Item works, I expected a little better performance. Shows amperage of charge accurately when it is higher than 2.5 amps, at less then 2.5amp display goes to 0.1 and does not display lower currents, then starts weirdly to "pulse".
The built in Voltmeter is not too exact. It is important to know if battery bank has 12.8V or 12.5V since this reflects the percentage of its charge - 12.8V = close to 100%; 12.5 = 70%. The Voltmeter in the device I got is about 3% off ; about 0.3V to 0.4V - I never know till I do an exact measurement).
So basically it is o.k. if one is not looking for 99% accuracy. Nevertheless, for the money I spent, I thought I'd get little more.

So far it works great for me! 2009-08-14

Reviewer: Viviannicol

I have not had any problems with this charger and Ive compared it to my volt meter and all seems to match up!