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Solar Battery Charger Circuit

In the previous post we have  seen the circuit diagram of 9v battery charger circuit using LM311 and SCR .In this post let us see the circuit for recharging Lead-Acid battery using Solar panel.

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 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.


Solar Battery Charger Circuit Diagram:

Solar Battery Charger Circuit Diagram
Solar Battery Charger Circuit Diagram

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

LM317 Datasheet

Do you know about the concept – How an Automatic Battery Charger Circuit Works?

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.

All the above mentioned parameters have to be taken into account before charging a battery.

For 6V Application

Set the output voltage to 7.5-8 volts as specified on the battery.

calculate the charging current ,power dissipation as shown above.

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.

Solar Charger Specifications:

  • Solar panel rating: 20W (12V) or 10W (6V)
  • Vout range: 5 to 14V
  • Maximum power dissipation: 10W (includes power dissipation of schottky diode)
  • Typical drop out value: 2 to 2.75V (depends on load current)
  • Max current: 1.5A (internally it limited to 2.2A)
  • Voltage regulation: +/- 100mV

How to Operate this Solar Battery Charger Circuit?

  1. Give the connections according to the circuit diagram.
  2. Place the solar panel in sunlight.
  3. Now set the output voltage by adjusting pot RV1
  4. Check the battery voltage using digital multi meter.

Solar Battery Charger Circuit Advantages:

  • Adjustable output voltage
  • Circuit is simple and inexpensive.
  • Circuit uses commonly available components.
  • Zero battery discharge when no sunlight on the solar panel.

Solar Battery Charger Circuit Applications:

Limitations of this Circuit:

  1. In this project current is limited to 1.5A.
  2. The circuit requires high drop-out voltage.

Solar batteries are one of the power tools to make the device function efficiently. As the non-renewable energy sources are decreasing there is a need to increase the usage of solar power. Solar batteries play crucial role to make it happen within no time.

But the thing is when you get the solar batteries you need to have the electronic device that supports the solar batteries. My best suggestion is to purchase the Solar Lights Kits that can affix to home gardens, walkways and on the walls.

They come at very affordable prices and make the outdoor look more beautiful and romantic especially at night times. You can spend some time with your beloved ones in the presence of bright white light.

43 Responses


  2. I require circuit diagram of solar current controller of 48 volt 30 amp solar current controller.will you please provide me circuit diagram of above. Thanx in advance

    1. Here Schottky diode is used for protection.When the panel is in dark battery may discharge back.So to protect solar panel diode is used.

  3. Can i alter the circuit? If i can, what can i change. Im doing a project for my HNC level 4 Electrical and Electronics engineering so i will need to alter some parts.

    I hope someone could reply really quickly.


  4. how come ??? the input voltage are 12v and the out but are 15v ???
    the lm317 work as regulator not as generator !!!

  5. can i use 12v /5w solar panel for charging 6 v /1.3 A battery? What will be values of capacitor & resisters for that.please help it is needed for my project .help me as early as you can .thank you

  6. A current limiting resistor of 100 E 2 watt should be added in series to the positive supply after the shottky diode for the safety of the battery.

  7. “If the battery requires initial current more than 1.5A,it is not recommended to use LM317”

    Then what should i use? I have a 1.4A (initial current) battery

  8. Hi,
    Thank you very much for this schematic and those explanations. Is this circuit can be used in UPS source (Uninterruptible power supply), that is to say : can i add a burst regulator and my MCU just after the battery with the solar charging circuit ?
    Best regards,

  9. hi sir i want charge 3S 3.7v 2200mah lithium batteries with solar panel ,i need controller or protector circuit in between batteries and solar panel at the same time i am connecting load from batteries ,what can i do sir please guide me to design circuit ,
    Thank you in advance,

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