Solar charge controller MPPT

Solar charge controller MPPT is a more sophisticated and efficient way of drawing power from a panel and charging a flat battery.

The older PWM controllers, connecting a higher voltage photovolatic panel, at say 17V, to the much lower value of the flat battery, have their voltage drawn down to that of the cell at say 11V. This means that the current delivered by the PV also drops.

On the other hand a MPPT controller effectively decouples the battery at 11V from the panel at say 17V, harvesting more power.

This is just as important early in the morning, and late afternoon, and during inclement weather when the radiation reaching the panel is low, and the voltage output drops and is unable to supply the margin needed to charge the battery.

One other factor of importance is that because the MPPT maintains a high voltage between the panels and the controller, the current is correspondingly lower according to the formula volts x amps = watts; if the stated power produced by the PV panel is constant, then I, the current, must decrease when V increases.

When the current is kept low by the MPPT, it means that the losses according to Ohm's law are reduced by the square of the amps.

Heat losses in the cabling is equal to the current squared times the resistance.

By keeping the voltage in the cabling high at that of the panels instead of that of the battery, the current and the associated heat losses are minimised.

All of this becomes even more important as more panels are added in series raising the voltage to perhaps 140V, far above that of the batteries; then the current and heat losses are much lower.

Solar charge controller MPPT

The solar charge controller MPPT harvests substantially more power from the panels.

In effect, the MPPT keeps the panel voltage at its most efficient value known as the Vmp rather than allowing it to drop to that of the flat battery. This enables the PV to deliver the maximum charge current, known as Imp which would otherwise drop substantially along with the voltage.

As a rule of thumb, the panel voltage should not be greater than that of the batteries by a factor of more than four.

For example, I have a 48V system with four 12V batteries in series; thus I should not have more panels in series than would make up more than about 200V.

In practice, it's not recommended that I use my MPPT at over 150V as the efficiency drops.

MPPT sizes

There is a range of MPPT sizes from 10A to 100A.

The twenty and sixty amp MPPTs seem to be the most popular. With electrical equipment I always purchase a device that is well within the specifications, rather than risk damaging it. However, MPPTs from a reputable company have protection built into them for occasional spikes when the solar lens effect of a cloud comes into play, magnifying the irradiation considerably.

However, panels rarely deliver the rated power except momentarily with a solar lens effect like this; it's not a concern for short periods.

In practice, on my west facing roof I have three 310W panels each with an open circuit voltage of around 46V, making 138V; I am well within the 150V limit even at extremes of temperature when Voc and Vmp can fluctuate.

930W /48V = maximum of 19.4A

But it only delivers about 90% of that because of various factors; internal resistance, the angle of the panels to the sun and moisture in the air; thus a 20A MPPT really is adequate. I might choose a 30A to be safe.

One can put two strings in parallel, thus I could put another identical row of three more panels; then I would need a 40A MPPT. Always plan for expansion; it's cheaper in the long run than having to upgrade and exchange your solar charge controller MPPT for another larger device.

This solar charge controller MPPT is clever; it always finds the optimal voltage of the solar panel to ensure maximum power from the PVs. They extract up to 30% more power than the old controllers.

Panels facing in different directions always need a separate MPPT as the voltage varies considerably according to the time of day. Connecting a string of three east facing solar panels, with a another facing west using the same MPPT would hopefully confuse the tracking; each needs its own controller.

Following the sun

Sophisticated systems turn either the panels, or a series of mirrors for following the sun. In practice, I suspect the cost for the whole home build doesn't warrant the expense; keep it simple, silly, is our motto. 

However, I'm planning on building a mobile string of panels, on the ground, that I can turn several times a day to track the sun. Since all the panels will be always facing the same direction, one MPPT is adequate; by fitting it on a small trailer it will be easy to move.

It will need its own controller as at times the voltage would be quite different to those existing panels.

Watch this space; however there are certain logistical considerations, the largest of which is a wife who doesn't want her garden ruined by yet another Bernard Preston experiment! But retirement sentiments are vital for those moving into the golden years; otherwise they will be seriously foreshortened.  She understands that and is reasonably tolerant of my idiosyncrasies.

Here's mark I. Three of these, on a 48V system, will provide 915 watts maximum.

915W / 48V = 19.06 amps.

A 20A solar charge controller MPPT will be adequate, since they rarely supply more than about 90% of the rated amount. I'm hoping it'll be around that for much of the day.

This is more of a fun project than a serious one for you. In the future when the three buggies are complete, I'll let you know the cost, how to build it, and how many kwh it actually supplies. What will the pay back time be?

Three 305 watt mobile panels will add another 25% to our arrays, taking us to over 5kW; more important they are already proving their worth by providing much needed early morning and late afternoon energy.

There remain concerns about shadows from that bar in front, but preliminary studies suggest the affect is minimal.

They cannnot be synched with existing panels so much have their own solar charge controller MPPT. Since I have a 48V system,

915W / 48 V = 19,06 amps.

A small 20 amp MPPT is adequate.

It's always been my policy to purchase a piece of equipment, learn to use it and build a much needed project. I taught myself to weld when we needed burglar guards in our new home; then it came in useful with this steel gate design, and now I'm using it for making small mobile solar panel trailers; the equipment has paid for itself many times over; and I've learned a new skill.

Maximum Power Point Tracker

So, your solar charge controller MPPT has the full name of maximum power point tracker. I think you now understand the meaning of the term.

It adjusts continually to the conditions, prevailing temperature, radiation arriving at the panel which is changing each moment with every cloud, extracting the maximum available power from the PV.

The controller in the picture above is the 20 amp version; how I wish I went straight to the 60A MPPT. Adding more panels in parallel meant getting another larger one; if you do it right from the start it hurts only once. You will expand your system if you build it yourself, believe you me.

Alternatively, if you have no interest in such things, get a professional company in to advise, design and install it for you.

Make sure you only deal with a reputable company; there are an awful lot of broken hearts around; mostly because they put in a Mickey Mouse solar generator. I recommend NEVER putting in something less than a 48V system, a 60A MPPT and a 2kW inverter. Never; you will not be satisfied with the output.

If you have the money, go straight to a 5kW or even 10kW inverter, because you WILL add more panels! Trust me, I've been there and have the T-shirt. Now we can use the electric oven on a sunny day, the breadmaker, the kettle as well as the computers and lights which was the original planning.

But all of this is dependent on correctly extracting the maximum power from your panels, and for that you definitely need a solar charge controller MPPT.

Good luck, it's been a terrific ride; if I was to do it again I'd either put the panels on the ground, but we have trees, or on a new dedicated building that could double up as an extra garage or workshop.

Bernard Preston

Bernard Preston is a physics major who taught high school science for seven years before turning to chiropractic care for the body. Coming to grips with a solar charge controller MPPT was no small challenge after forty years out of the field. 

Whilst our health remains his chief focus in life, his roots in physics and chemistry have never deserted him. After all, good health care is based on science. 

Building a solar farm on his roof, harvesting sunlight, in truth returning to his roots, has been one of the great challenges of his retirement. His solar generator provides for most of their needs; they use less than ten dollars of utility electricity per month.

On the escarpment where they live, dense mists often sweep over their home in the summer for five days; it makes sense to stay connected to the utility. Going off the grid is something of a pipe dream for them.

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