The maximum power point tracking…MPPT solar charge controller has got to be one of the most innovative, game changing and misunderstood components in the solar industry.
There is so much misinformation, it is time to clear up the myths of the MPPT charge controller, explain what they do and find out what they cannot do.
The first things we must learn are:
- Solar modules are rated at standard test conditions (STC) of 1000 watts per square meter at 25 degrees C.
- Solar modules are current limited
- Higher temperatures = lower watts out; lower temperatures equals higher watts out
Solar modules are rated at their maximum output level which only occurs when the electricity is being harvested at:
- a specific current called the “maximum power point current”
- a specific voltage called the “maximum power point voltage”.
In the real world, this never happens! EVER!
Let’s look at the specs of a common solar module…the Canadian Solar CS6P:
All solar modules have a specification sheet and a sticker on the rear of the module telling the maximum output 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 (the first column) 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?
24.6 VOLTS X 8.56 AMPS = 211 WATTS
WHAT IF THE BATTERY VOLTAGE IS ONLY 13.8 VOLTS?
13.8 VOLTS X 8.56 AMPS = 118 WATTS
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 this:
solar modules are current limited, meaning they can only put out a certain amount of current (and no more) no matter what 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.
This is the main difference between the MPPT solar charge controller and all the others.
It is hard to explain a 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 could be connected to a battery directly (with or without over-current protection) but 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
- 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
- only a 12 volt module could be used to charge a 12 volt battery bank
With these issues, we were lucky to get 50-60% of the solar module’s rated wattage into our battery (after factoring in nighttime losses).
higher temps less output, lower temperature higher output
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.
Enter…the MPPT solar charge controller.
protect battery from overcharge
higher voltage arrays
smaller wire sizes
ability to charge lower voltage battery banks from higher voltage solar arrays
increase solar output by up to 30%
include relay drivers for dump loads
prevent electricity from discharging batteries into the solar modules
provide bulk, absorption, float and equalization charges
finds the module’s sweet spot and reduces the voltage to the voltage of battery bank
makes the most difference when batteries are low
large solar modules are 24 volt nominal or higher
grid tie modules
VOLTS X AMPS = WATTS
several examples to show the benefit
at what point does it make sense to spend the extra for an MPPT controller
raises and lowers voltage on the module side constantly to try and extract the most power available at any given time
suitable for using grid tie solar modules to charge 12, 24 or 48 volt battery banks
some models can be used with hydro and even wind turbines using a dump load when the battery bank is full
when you should not use an mppt controller
Morningstar MPPT Tristar 45
Morningstar MPPT Tristar 60
(With Disconnect Box)
(With DC Transfer Switch)
(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