This small LM339 based solar battery charger setup is designed to optimize the charging of your battery using any solar panel while protecting it from accidental overloads. Considering the extended battery life it allows you to achieve, it is an investment that quickly pays for itself.
Our setup is nothing more than a dual comparator that connects the panel to the battery when the voltage at the terminals of the battery is too low and disconnects it as soon as it exceeds a certain threshold.
Since it operates solely based on battery voltage measurement, it is particularly suited for lead-acid batteries with liquid or gel electrolytes, which adapt well to this approach.
The battery voltage is divided by R3 and R4 before being applied to the inputs of the two comparators, IC2a and IC2b.
When it is below the threshold set by P2, the output of IC2b goes high, which also causes the output of IC2c to go high.
T1 is activated, and relay RL1 is energized, allowing the solar panel to power the battery and recharge it through D3.
When the voltage at the battery terminals exceeds the threshold set by P1, the output of IC1a goes low, which triggers the same response in IC1c and results in the relay being de-energized, preventing any overcharging of the battery.
To ensure that the thresholds set by P1 and P2 remain stable, they are powered through the integrated regulator IC1, carefully isolated from the voltage coming from the solar panel via D2 and C4.
This is because, during the relay switching, this voltage fluctuates significantly, which could affect the operation of the comparators.
A switch is also provided to allow manual control of the setup by forcing the voltage on the inputs of IC2a and IC2b to either high or low levels.
This allows for the interruption or, conversely, the forced charging when needed. For automatic operation, this switch obviously remains in the middle position.
The printed circuit board that we offer supports all the components of the setup, and the components themselves are easily available as they are standard.
With the specified element values, the maximum charging current that the setup can handle is 2.5 A, which is well beyond the capabilities of typical solar panels available today.
However, if you wanted to go beyond that, it would be sufficient to replace RL1 with a model capable of handling higher currents and make the necessary adjustments with D3.
For proper adjustments and considering that the setup will likely spend a significant amount of time outdoors, it is recommended to use CERMET potentiometers for P1 and P2 instead of carbon models.
The wiring presents no difficulties and should be done in the standard order of passive components followed by active components, ensuring the correct orientation of polarized components (diodes, transistors, capacitors).
How to Adjust the Presets
The operation is immediate but requires adjusting P1 and P2, which can be easily done with a simple stabilized power supply.
To do this, short-circuit the +BAT and +P points and power the setup between -P and +P using your stabilized power supply.
Set S to the middle position and adjust P1 and P2 so that the relay switches off when the voltage from your power supply is around 14.5V and switches on when it is around 13V.
If the setup is to be used outdoors, it should be placed in a protective enclosure to shield it from moisture.
Also, avoid exposing it to direct sunlight, as excessive heating could alter the switching thresholds determined by P1 and P2.
By following these two usage precautions, you can ensure many years of reliable and loyal service.