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The circuit discussed here handles up to 4 amps of current from a single solar panel, which equates to about 75 watts of power. A charging algorithm called “pulse time modulation” is introduced in this design. The current from the solar panel to the battery is controlled by an N-channel MOSFET, T1. This MOSFET does not require any heatsink to conduct heat away as its RD-S(on) rating is only 0.024 Ω. The D1 Schottky diode prevents the battery from discharging into the solar panel at night and also provides reverse polarity protection of the battery.
In the diagram, lines with red highlights represent more likely current paths. The charge controller never draws current from the battery. It is fully powered by solar panel. At night, the charge controller effectively goes to sleep mode. When using during the day, as soon as the solar panel generates enough current and voltage, it will start charging the battery. The potential across the battery terminals is divided by resistor R1 and potentiometer P1. The resulting voltage determines the charge state of the controller. The heart of the charge controller is IC1, a TL431ACZ type voltage reference device with an open-collector error amplifier.
Here, the sensed voltage of the battery is continuously compared with the internal reference voltage of the TL431. As long as the setting at P1 is lower than the internal reference voltage, IC1 will turn on the MOSFET. As the battery begins to charge, its terminal voltage increases. When the battery reaches the charge state setpoint, IC1's output drops below 2 volts and effectively shuts down the MOSFET, thereby stopping all current flowing through the battery. When T1 is off, LED D2 is also off. No hysteresis path is provided in the integrated circuit of the regulator. Therefore, as soon as the current to the battery stops, the output of IC1 remains low, making it impossible for the MOSFET to continue conducting even if the battery voltage drops.
The chemistry of a lead acid battery requires a floating charge, so a very simple oscillator is implemented here to solve that problem. Our oscillator exploits the negative resistance of transistors first discovered by Leo Esaki and part of his work on electron tunneling in solids, which won the 1973 Nobel Prize in Physics. In this implementation, a common NPN transistor of type 2SC1815 is used. With the LED off, R4 charges the 22 µF capacitor (C1) until the voltage is high enough to cause an avalanche at the emitter junction of T2.
At this point, the transistor turns on rapidly and discharges the capacitor through R5. The voltage drop across R5 is enough to trigger T3, thereby changing the reference voltage setting. Now the MOSFET tries to recharge the battery. As soon as the battery voltage reaches charge again, the process repeats. It is found that the 2SC1815 transistor works reliably in this circuit. Other transistors can be more feisty, we recommend studying the work of the Esaki laureate to find out why, but be aware that there is some heavy math ahead.
When the battery is fully charged, the oscillator's on time is reduced while the off time is still prolonged, which is determined by the timing components R4 and C1. This is because a pulse of current is sent to the battery and this pulse will decrease over time. This charging algorithm can be called Pulse Time Modulation. To regulate the circuit you will need a good digital voltmeter and a variable power supply. Adjust the supply to 14.9V, which is a battery setting of 14.3 volts plus about 0.6 volts across the Schottky diode.
Rotate the decorative light until the LED turns off, this is the switching point and the LED will start blinking. You may need to try this setting several times, as the closer the comparator is to exactly 14.3V, the more accurate the charger will be. Unplug the charge controller and you are ready to use the solar panel. The 14.3V setting mentioned here should apply to most sealed lead acid batteries and flooded batteries, but please check the value with the manufacturer. Choose a solar panel so that its amp capacity is within the safe charging limits of the type of battery you plan to use.