Monday, 17 November 2014

Make a Simple Solar MPPT Charger Circuit at Home

How to Make a Solar MPPT Charger Circuit - Maximum Power Point Tracker Circuit

Todat we are going to to learn how to make a simple but effective and cheap solar MPPT Charger circuit at home. Here MPPT stands for maximum power point tracker, which is an electronic charging 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.
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 Power, and deteriorates as the angle shifts away from 90 degrees  The atmospheric temperature around the panel also affects the efficiency of the panel, whose power falls with increase in the atmospheric 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 maximum, Here 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.

Fore efficient Battery charging in such a conditions are, 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.

Here we are assuming that we have 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 strat charging cycle and starts filling the battery relatively fast and will attain it's voltage to around 13V within a couple of hours.

At this point of charging cycle the current should be automatically reduced to say C/5 rate (dur to earth's rotation.), this will again help to keep the fast charging pace without damaging our battery and increases its voltage to around 13.5V within the next 1 hour approximately.

Following the above steps, now the current may be further reduced to C/10 rate (due to earth's rotation) which makes sure the charging rate without damaging the battery.

Finally when the battery voltage reaches to around 14.3V, the process may be reduced to a C/50 rate which almost stops the charging process but maintains the charge to this level.

Here the entire process charges a very deeply discharged battery within day time of 6 hours without affecting the life of the battery.

Actually MPPT is used mainly for ensuring that the above procedure is extracted optimally from a particular solar panel during day time.

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 type device can be made by employing a LM338 IC and an opamps.

Circuit Diagram:

Here the IC 741 stage is the solar tracker section is the main component of the entire circuit.

The solar panel voltage is fed to the inverting pin2 of the IC, while the the same is applied to the non-inverting pin3 with a drop of around 2 V using three 1N4148 diodes in series.

The above situation consistently keeps the pin3 of the IC a shade lower than pin2 ensuring a zero voltage across the output pin6 of the IC.

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However in an event of an inefficient overload, such as a mismatched battery or a high current battery, the solar panel voltage tends to get lowered down by the load, when this happens pin2 voltage also begins dropping, however due to the presence of the 10uF capacitor at pin3, the potential stays fixed and does not respond to the above drop.

Here when the above situation is triggered, instantly causes pin3 to go high than pin2, which in turn toggles pin6 high, switching ON the BJT BC547.

Now BC547 immediately disables LM338 cutting off the voltage to the battery, the cycle keeps switching at a rapid pace depending upon the IC's rated speed.

The above operations make sure that the solar panel voltage never drops or gets pulled down by the load, maintaining an MPPT like condition throughout the charging process.

Since a linear IC LM338 is used, the circuit could be yet again a bit inefficient....the remedy is to replace the LM338 stage with a buck converter...that would make the design extremely versatile and comparable to a true MPPT.

Below is an MPPT charger circuit using a buck converter topology, now the design makes a lot of sense and looks much closer to a true MPPT battery charger circuit.

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