Solar Battery Charger Circuit

Solar concept is not new for us. As non-renewable energy sources are decreasing, usage of solar energy is increased. This solar energy is not only used on the Earth but also used in space stations where no electrical power is available. Here is the simple circuit to charge 12V, 1.3Ah rechargeable Lead-acid battery from the solar panel. This solar charger has current and voltage regulation and also has over voltage cut off facilities. This circuit may also be used to charge any battery at constant voltage because output voltage is adjustable.

Specifications of the Charging Circuit
Solar panel rating – 5W /17V
Output Voltage –Variable (5V – 14V).
Maximum output current – 0.29 Amps.
Drop out voltage- 2- 2.75V.
Voltage regulation: +/- 100mV

Solar Battery Charger Circuit Principle:

Solar battery charger (for example: SUNJACK USB BATTERY CHARGER ) operated on the principle that the charge control circuit will produce the constant voltage. The charging current passes to LM317 voltage regulator through the diode D1. The output voltage and current are regulated by adjusting the adjust pin of LM317 voltage regulator. Battery is charged using the same current.


Circuit Components
Solar panel – 17V
LM317 voltage regulator
DC battery
Diode – 1n4007
Capacitor – 0.1uF
Schottky diode – 3A, 50V
Resistors – 220, 680 ohms
Pot – 2K
Connecting wires

Solar Battery Charger Circuit Design

Circuit must have adjustable voltage regulator , so Variable voltage regulator LM317 is selected. Here LM317 can produce a voltage from 1.25 to 37 volts maximum and maximum current of 1.5 Amps.


Adjustable Voltage regulator has typical voltage drop of 2 V-2.5V .So Solar panel is selected such that it has more voltage than the load. Here I am selecting 17v/5w solar panel.


Lead acid battery which is used here has specification of 12v/1.3Ah. In order to charge this battery following are required.


Schottky diode is used to protect the LM317 and panel from reverse voltage generated by the battery when it is not charging. Any 3 A diode can be used here.

For Charging 12V Battery

Output voltage
Set the output voltage to 14.5 volts(This voltage is specified on the battery as cycle use.)
Charging current
Charging current = Solar panel wattage/Solar Panel Voltage = 5 / 17 = 0.29A.
Here LM317 can provide current upto 1.5A .So it is recommended to use high wattage panels if more current is required for your application.(But here my battery requires initial current less than 0.39Amps. This initial current is also mentioned on the battery).
If the battery requires initial current more than 1.5A,it is not recommended to use LM317.
Time taken for charging
Time taken for charging = 1.3Ah/0.29A = 4.44hours.
Power dissipation
Here solar panel has 5Watts
Power going into battery = 14.5*0.29 =4 watts
Thus 1 watt of power going into regulator.

Power Dissipation

In this project, power is limited because of the thermal resistance of LM317 voltage regulator and the heat sink. To keep the temperature below 125 degree Celsius, the power must be limited to 10W. LM317 voltage regulator internally has temperature limiting circuit so that if it gets too hot, it shuts down automatically.

When battery is charging, heat sink becomes warm. When completing the charging at maximum voltage, heat sink runs hot. This heat is because of excess power that not needed in the process of charging a battery.

Current Limiting:

As the solar panel provides constant current, it acts as a current limiter. Therefore the circuit does not need any current limiting.

Solar Charger Protection:

In this circuit, capacitor C1 protects from the static discharge. Diode D1 protects from the reverse polarity. And voltage regulator IC provides voltage and current regulation.