The MPPT Charge Controller | How it has Changed Everything!

July 23, 2016 · 6 comments

Would you do us a huge favor by sharing?

The maximum power point tracking… MPPT charge controller … has got to be one of the most innovative, game changing and misunderstood components in the solar industry.

solar mppt charge controllers

There is so much misinformation, it is time to clear up the myths of the MPPT charge controllers, explain what they do and explore what they cannot do.

Here is the MPPT solar controller in the shortest description…don’t worry…this will all be explained later.

#1   MPPT charge controllers attempt to connect to the solar module at its MPPV (maximum power point voltage or  “sweet spot” voltage (which is the voltage when the solar module’s voltage multiplied by the solar module’s current will produce the most electricity).

They constantly adjust the solar module’s voltage up and down as:

  • the position of the sun changes
  • the solar module gets shaded by clouds or other obstacles
  • the solar module’s temperature changes

#2   Maximum Power Point Tracking charge controllers then connect to the battery at the battery’s voltage (while increasing the amperage) to collect as much electricity as possible.

#3   MPPT controllers also act as charge controllers that make sure your battery bank is never overcharged by performing BULK, ABSORPTION, FLOAT and EQUALIZATION charges to make your batteries last as long as possible.

THIS CAN BE VERY DIFFICULT TO UNDERSTAND SO WE WILL DO OUR BEST TO EXPLAIN IT!


CLICK ON IMAGE BELOW TO ENLARGE…(or open PDF)

mppt charge controller diagram


The first things we must learn are:

canadian solar moduleSOLAR MODULES NEVER PRODUCE THEIR RATED POWER !!!!!!!

They just CAN’T and DON’T!

Not even with MPPT!

But MPPT (Maximum Power Point Tracking) will make a huge difference in some situations.

OTHER REALITIES ABOUT SOLAR MODULES AND MPPT

  • Solar modules are rated at standard test conditions (STC)
  • STC assume the available energy from the sun is 1000 watts per square meter (1000w/m2)
  • STC assume the solar module is held at 25 degrees C.
  • Solar modules are current limited
  • STC assume the solar module’s and battery’s voltage are perfectly matched

Morningstar's TS-45 MPPTSOLAR MODULES ARE RATED AT STANDARD TEST CONDITIONS (STC)

Let’s look at this. First we have to realize there has to be some way of comparing solar module’s that is not an apples to oranges comparison. If not, any manufacturer could say anything about their solar modules and there would be no way to prove or disprove their claims. That is why Standard Test Conditions (STC) were established.

STC assume the available energy from the sun is 1000 watts per square meter (1000w/m2)

All solar modules are tested and rated according to this hypothetical situation that the sun is beaming 1000 watts per square meter of ground in your location. That means that in a perfect world, 1000 watts of heat (energy) is being absorbed (for every square meter of earth) where and while the sun is shining. This rarely (if ever) is true. Even if it is true, the solar module would have to point exactly at the sun horizontally and vertically for STC to be accurate.

Interesting Fact: If solar modules were 100% efficient, a 1000 watt solar module would only be one square meter in size. However as solar modules are roughly 10-20% efficient, it requires 5-10 square meters of physical space to make a 1000 watt solar module (if 1000 watt solar panels actually existed).

STC assume the solar module is held at  constant 25 degrees Celsius

It doesn’t take a rocket science to see how rarely this would happen. Solar modules are in the sun. The sun heats the solar module. Unfortunately, the hotter the solar module is, the less power it produces because the voltage decreases. Just a fact.

Nothing we can change other than making sure your modules/solar array have ventilation (air gaps and spacing) to help keep them as cool as possible.

The upside of the temperature conundrum is:

“the colder the solar module is, the more power it will produce as the voltage increases”.

This can be a good thing for those in very cold locations. I have seen solar panels exceed their manufacturer’s voltage rating (and output) by as much as 15% in -30 degrees Celsius but your tongue will stick to the aluminum frame of the solar panel at that temperature. Try to resist.

ALL SOLAR MODULES ARE CURRENT LIMITED!

Solar modules are current limited. They can only make a certain amount of current no matter how bright the sun is. This is usually referred to as the short circuit current, Isc (I for current, sc for short circuit), Is.c., maximum current and a few other terms depending on the manufacturer.

Think about it: Let’s pretend we have a 200 watt solar module that is rated to produce 10 amps @ 20 volts (10A X 20V=200W).

Now let’s take the same module and connect it to a 12 volt battery that is full at 14.4 volts. (10A X 14.4V = 144W)

Now let’s take the same module and connect it to a 12 volt battery that is dead at 10.5 volts. (10A X 10.5V = 105W)

Now let’s take the same module and connect it to a 6 volt battery that is full at 7.2 volts. (10A X 7.2V = 72W)

EVEN THOUGH THE BATTERY VOLTAGE MAY BE SIGNIFICANTLY LOWER THAN THE SOLAR MODULE’S “MPP” or “SWEET SPOT” VOLTAGE, THE CURRENT COMING FROM THE MODULE WILL NOT INCREASE !!!

 

module label to explain mppt charge controller

STC ASSUME THE SOLAR MODULE’S AND BATTERY’S VOLTAGE ARE PERFECTLY MATCHED

This is where MPPT charge controllers change everything. In the SCHUCO solar module label shown we have the following specifications at standard test conditions:

  • Pmax (maximum power) 180.3 watts
  • Vmp (maximum power voltage) 24.0 volts DC
  • Imp (maximum power current) 7.50 amps

If we multiply the Vmp X Imp we get the following:

24.0 volts X 7.50 amps = 180 watts.

If, and only if, the voltage being produced by the solar module is 24.0 VDC, will the SCHUCO solar module produce 180 watts. If the output voltage is any less, so will be the output in watts.


schneider 60 amp mppt controllerUsing what we have learned, let’s look at a few different solar modules in a few different installation configurations to see how the MPPT solar charge controllers work…

Under STC, solar modules are rated at their maximum output level which only occurs when the electricity is being harvested at:

  1. a specific current called the “maximum power point current”
  2. a specific voltage called the “maximum power point voltage”.

As stated earlier…

“In the real world, this never happens!”

Let’s look at the specs of a common solar module…the Canadian Solar CS6P:

How MPPT solar controllers work

All solar modules have a specification sheet and a sticker on the rear of the module telling the maximum output (in watts) and how the company attained (and proved) that wattage output.

What we need to look at is the Vmp (maximum power point voltage) and the Imp (maximum power point current). Those numbers are essentially the “sweet spot” of the solar (or the voltage and current when the module produces the maximum or rated output).

We know that VOLTS X AMPS = WATTS, so when when we look at the Canadian Solar 260P we see its maximum (or rated) power is attained when the voltage is 30.4 and the current (amperage) is 8.56.

30.4 VOLTS X 8.56 AMPS = 260 WATTS (exactly the rated output)


BUT WHAT IF THE BATTERY VOLTAGE IS ONLY 24.6 VOLTS?

Now we need to change things up a bit and use the Isc or short circuit current because that is the maximum current (amperage) the solar panel will produce when the voltage is lower than the panel’s MPPV (maximum power point voltage) no matter how low the voltage is.

If we look at the specs for the same solar panel (Canadian Solar’s CS6P 260P) we can see the

Canadian Solar's CS6P short circuit current

Isc (maximum current/short circuit current) is 9.12 amps.

Now our output looks like this:
24.6 VOLTS X 9.12 AMPS = 224 WATTS


BUT WHAT IF THE BATTERY VOLTAGE IS ONLY 21.0 VOLTS?

First of all, that is one seriously dead battery bank. Don’t do this to your batteries!

If we look at the specs for the same solar panel (Canadian Solar’s CS6P 260P) we can see the

Canadian Solar's CS6P short circuit current

Isc (maximum current/short circuit current) is 9.12 amps.

Now our output looks like this:
21.0 VOLTS X 9.12 AMPS = 192 WATTS (NOT GOOD)


For a moment, let’s pretend we are incompetent and we used Canadian Solar’s CS6P 260P on a 12 volt battery bank with a standard charge controller.

BATTERY VOLTAGE = 13.4
SHORT CIRCUIT CURRENT = 9.12

13.4 VOLTS X 9.12 AMPS = 122 WATTS (OUCH!)


Those above numbers are not exactly what we hoped for when we paid good money for every watt a solar module is supposed to produce.

The problem is STILL this:

solar modules are current limited…

meaning they can only put out a certain amount of current (and no more)…

no matter what (or how low) the voltage is.

It would make sense for us to keep the voltage as high as possible to get the maximum amount of electricity out of each solar module as one of the factors in watts out is the voltage. (VOLTAGE x CURRENT = WATTAGE)

This is the where the MPPT solar charge controller rises above all the others!


It is hard to explain the maximum power point tracking charge controller (MPPT solar charge controller) without mentioning the first attempts at solar charge controllers or battery voltage regulators.

In the beginning there was the battery and the solar module. Solar modules were connected to a battery directly or with a non MPPT controller (with or without over-current protection) and the results were less than impressive.

The main problems were:

  • at night, the electricity would go back into the modules and slowly discharge the battery (without a controller)
  • the solar module was never able to operate at its maximum power point voltage
  • the solar module was never able to operate at its maximum power point current
  • the battery would overcharge if you didn’t constantly monitor the battery voltage (without a controller)
  • only a 12 volt module could be used to charge a 12 volt battery bank
  • only a 24 volt module could be used to charge a 24 volt battery bank (or two 12 volt modules in series)
  • large 12 and 24 volt arrays required huge wiring as the current could be huge

With these issues, we were lucky to get 50-60% of the solar module’s rated wattage into our battery without a charge controller and 60-70% of the solar module’s rated wattage with a standard solar charge controller.


outback power systems mppt fm60 controllerTHE MPPT CHARGE CONTROLLERS APPEAR IN THE SOLAR INDUSTRY

This was a VERY exciting time for solar energy professionals.

The first “REAL” high quality MPPT solar controller was designed by the “old TRACE ENGINEERING” boys when they left Trace to found OUTBACK POWER SYSTEMS.

They introduced the Outback M60 and it changed everything! More about that later…

The MAXIMUM POWER POINT TRACKING charge controllers can do the following:

  • increase a solar module’s or solar array’s output by 30-40%
  • connect to the solar module/array at its “sweet spot” and reduce the voltage to the system’s battery voltage
  • charge a 12, 24 or 48 volt battery bank from solar arrays with voltages as high as 600 volts
  • allow high voltage DC transmission from the array to the controller
  • allow much smaller wire sizes and breakers/fuses
  • provide bulk, absorption, float and equalization charges
  • prevent electricity from discharging batteries into the solar modules
  • protect batteries and battery banks from overcharge
  • include relay drivers for dump loads
  • prevent electricity from discharging batteries into the solar modules at night

INCREASE A SOLAR MODULE’S OUTPUT BY UP TO 30-40%

As discussed earlier, the higher the temperature of a solar module the less output (in watts) it will produce. Do your best to keep them cool by having an air space under the modules especially on a roof mounted solar array.

But imagine you have a 12VDC battery bank and a solar module with an output of 17 volts. The battery voltage is 13.8 volts and temperature is a mere 40 degrees C. The solar module’s voltage will drop significantly as the module is likely closer to 50 degrees C as it is dark colored and hot. Now you have a solar module that is nothing but a pretty ornament. The solution? Install two modules in series for 34 volts and use an MPPT controller to reduce the voltage to 12VDC.

Lower temperatures actually produce more wattage than standard test conditions. This is because the voltage actually increases as the temperature goes down. This may sound awesome for those of us who live in cold climates, but MPPT charge controller can…

BLOW UP IF THEIR MAXIMUM INPUT VOLTAGE IS EXCEEDED!

Let’s imagine we purchased a maximum power point charge controller with a maximum voltage input of 150VDC like the FM60, FM80, Apollo T80 or otherwise.

Let’s suppose you build a solar array that has a MPV of 145 volts at 25 degrees C (STC) and you live in an area that can go down to -40 degrees C.

The voltage

Either the battery voltage is too low, or the temperature is too high or there is fog, clouds or a number of other problems.

As we know that the watts out of a given solar module can be found by multiplying the volts x amps, we must try to keep those factors as high as possible to get the most out of the solar module.

raises and lowers voltage on the module side constantly to try and extract the most power available at any given time

Connect to the solar module/array at its “sweet spot” and reduce the voltage to the system’s battery voltage

makes the most difference when batteries are low

Charge a 12, 24 or 48 volt battery bank from solar arrays with voltages as high as 600 volts

ability to charge lower voltage battery banks from higher voltage solar arrays

large solar modules are 24 volt nominal or higher

suitable for using grid tie solar modules to charge 12, 24 or 48 volt battery banks

Allow high voltage DC transmission from the array to the controller

Allow much smaller wire sizes and breakers/fuses

Provide bulk, absorption, float and equalization charges

Prevent electricity from discharging batteries into the solar modules

Protect batteries and battery banks from overcharge

Include relay drivers for dump loads

some models can be used with hydro and even wind turbines using a dump load when the battery bank is full

Prevent electricity from discharging batteries into the solar modules at night

 

 


At what point does it make sense to spend the extra for an MPPT controller?

When you should not use an mppt controller

 

Outback MX60

Outback FM60

Outback FM80

Morningstar MPPT Tristar 45

Morningstar MPPT Tristar 60

TS-MPPT-60-600V-48
(Standard)

TS-MPPT-60-600V-48-DB
(With Disconnect Box)

TS-MPPT-60-600V-48-DB-TR
(With DC Transfer Switch)

TS-MPPT-60-600V-48-DB-TR-GFPD
(Pre-wired with Ground Fault Protection Device)

Morningstar SunSaver MPPT

Midnite Solar Classic 150

Midnite Solar Classic 200

Midnite Solar Classic 250

Schneider Conext XW 80-600 MPPT Charge Controller

 

Leave a Comment

{ 6 comments… read them below or add one }

Jerud Crandall August 12, 2016 at 2:22 am

We love having an MPPT charge controller, and it’s not because of the “boost”. You mention the advantage of charging a low voltage battery with a high voltage array, but just to spell out one of the practical upshots of that: Overcast days can yield more charge into your batteries.
With panels “matched” to battery voltage, an overcast day causes array voltage to drop, and before long it’s too low to push current into the battery bank, especially if you’re in absorb stage. Our array is “48V” but batteries are only 12V (this is in an RV). So even when the array is barely making power on overcast days, it’s still “only” putting out 20V or so, which is still enough that the charge controller can push current into the batteries at any stage of charging. This wouldn’t be possible with any other type of charge controller, as the full-sun output of the array could not be “stepped down” far enough in good conditions, and something would fry.
We’ve gone through 4- and 5-day grey spells this way. I have seen 300W coming out of our 1200W array /during/ a rainstorm. Also while being snowed on.

Reply

Jody Graham August 12, 2016 at 1:09 pm

Hi Jerud,

You make a great point. Just the fact you can charge a lower voltage battery bank from a high voltage array means you can reduce the wire size, breaker or fuse size. The extra voltage available on cloudy days is also a major benefit to MPPT. Most folks don’t realize that 12V nominal panels will not output 13-14 volts on overcast days leaving them with no power in spite of having a large solar array. You have already said it best…

“So even when the array is barely making power on overcast days, it’s still “only” putting out 20V or so, which is still enough that the charge controller can push current into the batteries at any stage of charging. This wouldn’t be possible with any other type of charge controller, as the full-sun output of the array could not be “stepped down” far enough in good conditions, and something would fry.
We’ve gone through 4- and 5-day grey spells this way. I have seen 300W coming out of our 1200W array /during/ a rainstorm. Also while being snowed on.”

Well done. Hopefully others can learn from your experience. Thanks so much…Jody

Reply

Russell July 31, 2016 at 5:06 am

I have a Apollo Turbo MMPT which takes the panels at 110v than converts to 24v to charge the bank,,have you heard of these and also what are your thoughts on Nickel Iron Batteries.

Reply

Jody Graham July 31, 2016 at 9:42 pm

Hi Russell,

I love the Apollo product line. We use an Apollo T80 high voltage model and they are second to none. The rest of our system is outback power but I sold Apollo at the time I needed one for display. They are great. I am a big fan of flooded lead acid batteries for solar systems or AGM. Anything else just doesn’t have long term test results. Might be great. Might be aweful. Wish I could be more help…Jody

Reply

J. David Cox July 23, 2016 at 1:48 pm

I have an MPPT charger feeding the front end of my battery bank and a PWM type feeding the other end. This combo of chargers has seemingly done the banks some good. They are pretty much always up. But I did not know that the PWM style allows power to flow backwards to the panels at night!! Is that right?

Reply

Jody Graham July 28, 2016 at 10:22 am

Hi David,
Thanks for commenting. Somehow this article was published before I finished it. The PWM does NOT allow electricity to flow backwards at night. Most PWM controllers actually open the circuit between the solar array and the battery bank when there is no usable energy available from the solar array. I will need to clarify that in the article but I will likely write an article about PWM controllers and how they work. MPPT controllers actually use PWM technology to limit the current from the solar module to the battery as the battery requires less amperage to hold the absorption voltage. The only time electricity is able to flow backward into a solar module is when the module is connected directly to the battery with no charge controller (shunt-type/relay-type/PWM/MPPT) or diodes (check valves for electricity). Thanks David…Jody

Reply