You can make a simple Current regulated Charger for your Solar Panel. It safely charges the 12 volt battery with a constant current of 540 Milli Amperes. It needs only a few components and can be hooked between the Solar panel and the battery. It also has an LED charge indicator and a revere polarity protection diode.
The circuit is designed for 12 volt 10 Watts Solar panel. In bright sunlight, the 12 Volt Solar panel gives an Open circuit voltage up to 20 volts but it drops to 14-16 volts when connected to the battery. For a 12 volt Lead Acid battery, we need more than 14 volts for charging. Usually this voltage will be available only during the bright sunlight hours between 11 am and 3 .pm. The current also varies proportional to the voltage from the solar panel. The 10 Watt Solar panel gives the following current at different voltages theoretically.
12 V – 833 mA
14 V – 714 mA
16 V – 625 mA
18 V – 555 mA
20 V – 500 mA
So any way, the current from the solar panel swings between 500 mA and 800 mA. So we need a constant current charger to give stable current for charging. This circuit gives a constant of 540 Milli Ampere current for charging irrespective of the voltage changes.
What the circuit doing?
The circuit uses a medium power NPN transistor BD 139 to maintain a constant current through the battery. The positive line from the battery goes directly to the Battery + Ve through Diode D1 which gives polarity protection and also prevents the discharge of battery to the solar panel at night. The Negative terminal of the battery goes to the Collector of the NPN transistor. An LED is provided at the base of T1 to make the Base- Emitter junction voltage around 1.08 volt. Two 1 Ohm resistors R2 and R3 are connected serially from the emitter of T1 to the negative rail and give a total 2 Ohms resistance. So the current across the battery allowed by T1, R2 and R3 is
1.08 / 2 = 0.54 Amps = 0.54 x 1000 = 540 Milli Amps.
Anyway it gives 500 mA constant current if the Solar panel voltage lies between 12 volts and 20 volts.C1 acts as a buffer for the voltage from solar panel.
LED lights when the current passes through the battery and turns off when the charging completes and at night. If the battery is not charging due to damage or clips not properly connected, the LED will not light. So it acts as a good indicator for the flow of current through the battery. Now this is the course of current flow.
Solar panel Positive – DI – Battery + Ve – Battery – Ve- Collector of T1- Emitter of T1- R2 – R3 – Negative of Solar panel.
How to connect?
Assemble the circuit on a Common PCB and enclose in a small plastic case with a hole for LED. Connect the circuit with the Battery using Crocodile clips. You can Tap the battery voltage from the battery terminals to power inverter or charging devices such as Mobile phone.
Use 1 mm Copper wire to connect the Solar panel with the Battery.