MPPT charge controller
5.1 How does a MPPT solar charge controller work?
What is the meaning of MPPT?
MPPT is the acronym for Maximum Power Point Tracking, which is a type of electronic digital tracking.
MPPT is more sophisticated – and also the more expensive – of the two. MPPT has around 94% – 98% conversion efficiency. That is power in (from the solar panel) almost equals power out (to battery bank).
The MPPT charge controllers read the output of solar panels and the voltage of batteries to figure out the best power point to draw from solar panel; then, the MPPT turns the voltage down to meet the battery charging voltage while raising the current. By doing this, the MPPT can increase energy that we finally get from solar panels by almost 40%, compared with PWM, since PWM cannot increase current to track the maximum power point.
Unlike PWM, which requires the voltages match with both sides, MPPT can be applied to the PV system, which voltage of solar array is higher than that of the battery bank. This feature brings the MPPT many advantages, which we will discuss in next time.
let’s move on to the examples so that you can catch the point quickly.
5.2 How to size mppt solar charge controller?
Remember the nominal 20V panels with 60 pieces of cells?
In the PWM circuit, they are too large to match a 12V battery bank and too small for a 24V battery bank, but the MPPT can solve this embarrassing situation.
The 20V panel has 30Vmp and 9A Imp, and its rated power = 30 x 9 = 270W.
Assume the 20V panel applies to the 12V battery. The MPPT will convert 30V down to around 14V to charge the battery, and increases the current so that it can draw maximum power from the solar panel.
If we take 30V down to 14V, the decreased rate is
30/14 = 2.14.
Then the increased current is
9 x 2.14 = 19.28A.
30 x 9 = 14 x 19.28 = 270 watts (power in equals power out);
since the output current is 19.28A, we multiply by 1.25 safe factor.
19.28 x 1.25 = 24.1A.
So, it will be good that we choose an MPPT with a current capacity larger than 24.1A.
Another example with 2 strings in 2 parallels using the nominal 20V panel to charge the 12V battery: the total power in is
270 x 4 = 1080 W.
The current output would be
1080 / 14 = 77.14A.
Multiply by 1.25
77.14 x 1.25 = 96.43A.
So, we are going to choose a 100A MPPT.
5.3 Charge controller sizing: the voltage of the controller
One more thing we need to pay attention to when sizing a solar charge controller is the voltage. Make sure the controller is capable of carrying input voltage from the panels. A 150V charge controller can only carry three 20v nominal panels in series. You may wonder…3 x 20 = 60V? That is far away from 150V!
That is because the actual voltage that solar panels generate could be much higher than 20V; sometimes, higher than the Vmp 30V. So, we use Voc to do the calculation. Voc = 38V.
3 x 38 = 114V
Then three nominal 20V panel in series is 114V
Since panel voltage will increase in the cold weather, refer to the NEC Table 690.7. Then we pick up the safest factor, 1.25, multiply 114v by 1.25,
114 x 1.25 = 142.5V.
Now you can understand why a 150V controller can only support three 20V in series, especially in winter.
Nowadays, newly developed controllers could have much higher voltages; some models even support as much as 700V input. This is very important when your solar array is settled far away from your battery bank.