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MPPT stands for maximum power point tracker, which is an electronic system designed for optimizing the varying power output from a solar panel module such that the connected battery exploits the maximum available power from the solar panel.

We know that the output from a solar panel is directly proportional to the degree of the incident sunlight, and also the ambient temperature. When the sun rays are perpendicular to the solar panel, it generates the maximum amount of voltage, and deteriorates as the angle shifts away from 90 degrees The atmospheric temperature around the panel also affects the efficiency of the panel, which falls with increase in the temperature.

Therefore we may conclude that when the sun rays are near to 90 degrees over the panel and when the temperature is around 30 degrees, the efficiency of the panel is toward maximum, the rate decreases as the above two parameters drift away from their rated values.

The above voltage is generally used for charging a battery, a lead acid battery, which in turn is used for operating an inverter. However just as the solar panel has its own operating criteria, the battery too is no less and offers some strict conditions for getting optimally charged.

The conditions are, the battery must be charged at relatively higher current initially which must be gradually decreased to almost zero when the battery attains a voltage 15% higher than its normal rating.

Assuming a fully discharged 12V battery, with a voltage anywhere around 11.5V, may be charged at around C/2 rate initially (C=AH of the battery), this will stat filling the battery relatively quickly and will pull its voltage to may be around 13V within a couple of hours.

At this point the current should be automatically reduced to say C/5 rate, this will again help to keep the fast charging pace without damaging the battery and raise its voltage to around 13.5V within the next 1 hour.

Following the above steps, now the current may be further reduced to C/10 rate which makes sure the charging rate and the pace does not slow down.

Finally when the battery voltage reaches around 14.3V, the process may be reduced to a C/50 rate which almost stops the charging process yet restricts the charge from falling to lower levels.

The entire process charges a deep discharged battery within a span of 6 hours without affecting the life of the battery.

An MPPT is employed exactly for ensuring that the above procedure is extracted optimally from a particular solar panel.

A solar panel may be unable to provide high current outputs but it definitely is able to provide with higher voltages.

The trick would be to convert the higher voltage levels to higher current levels through appropriate optimization of the solar panel output.

A very simple yet effective MPPT device can be made by employing a LM338 IC and a few opamps.

Let's understand the proposed MPPT circuit with the help of the following points:

The figure shows an LM338 voltage regulator circuit which has a current control feature also in the form of the transistor BC547 connected across adjustment and ground pin of the IC.

The two opamps are configured as comparators. In fact many such stages may be incorporated for enhancing the effects.

In the present design A1's pin#3 preset is adjusted such that the output of A1 goes high when the sun shine intensity over the panel is about 20% less than the peak value.

Similarly, A2 stage is adjusted such that its output goes high when the sunshine is about 50% less than the peak value.

When A1 output goes high, RL#1 triggers connecting R2 in line with the circuit, disconnecting R1.

Initially at peak sun shine, R1 whose value is selected a lot lower, allows maximum current to reach the battery.

When sunshine drops, voltage of the panel also drops and now we cannot afford to draw heavy current from the panel because that would bring down the voltage below 12V which might entirely stop the charging process.

Therefore as explained above A1 comes into action and disconnects R1 and connects R2. R2 is selected at a higher value and allows only limited amount current to the battery such that the solar voltage does not crash below 15 vots, a level that's imperatively required at the input of LM338.

When the sunshine falls below the second set threshold, A2 activates RL#2 which in turn switches R3 to make the current to the battery even lower making sure that the voltage at the input of the LM338 never drops below 15V, yet the charging rate to the battery is always maintained to the nearest optimum levels.

If the opamp stages are increased with more number of relays and subsequent current control actions, the unit can be optimized with even better efficiency.

The above procedure charge the battery rapidly at high current during peak sunshines and lowers the current as the sun intensity over the panel drops, and correspondingly supplies the battery with the correct rated current such that the it gets fully charged at the end of the day.

What Happens with a Battery Which may not be Discharged?

Suppose in case the battery is not optimally discharged in order to go through the above process the next morning, the situation may be fatal to the battery, because the initial high current might have negative affects over the battery because it's yet to discharged to the specified ratings.

To check the above issue, a couple of more opamps are introduced, A3, A4, which monitor the voltage level of the battery and initiate the same actions as done by A1, A2, so that the current to the battery is optimized with respect to the voltage or the charge level present with the battery during that period of time.