Solar Charger

Solar Battery Charger Circuit

In a previous post, we delved into a circuit schematic illustrating a 9V battery charger circuit that employed LM311 and SCR components. In this post, we will explore a different circuit configuration specifically designed for the purpose of charging lead-acid batteries using solar panels.

The notion of harnessing solar power is not a novel concept. Solar energy is gaining ever-increasing popularity due to the depletion of non-renewable energy resources. Solar energy isn’t confined solely to our planet; it also finds practical utility in space stations where conventional electrical power sources are limited.

Presented below is the circuit diagram for charging a 12V, 1.3Ah rechargeable lead-acid battery with the assistance of a solar panel. This solar charger is equipped with mechanisms for managing both current and voltage, incorporating an overvoltage cutoff as a safeguard. Furthermore, its adaptable output voltage allows for the efficient charging of various battery types while maintaining a consistent voltage level.


  • Specifications of the Charging Circuit
  • Solar Battery Charger Circuit Diagram:
    • Solar Battery Charger Circuit Design
  • For Charging 12V Battery
      • Output voltage
      • Charging current
      • Time taken for charging
      • Power dissipation
    • How to Operate this Solar Battery Charger Circuit?
    • Solar Battery Charger Circuit Advantages:
    • Solar Battery Charger Circuit Applications:
    • Limitations of this Circuit:

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:

The charge control circuit within the solar battery charger plays a crucial role in stabilizing the output voltage. It manages the flow of the charging current, channeling it through diode D1 towards the LM317 voltage regulator. The LM317 voltage regulator, in turn, employs its adjust pin to oversee and maintain both the output voltage and current at a steady level, ensuring that the battery receives a consistent and controlled charging current.

Solar Battery Charger Circuit Diagram:

Solar Battery Charger

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

To meet the requirement for an adjustable voltage regulator in the circuit, the LM317 variable voltage regulator has been incorporated. The LM317 is capable of delivering an adjustable output voltage ranging from 1.25 to 37 volts and can handle a maximum current of 1.5 amps.

Typically, an adjustable voltage regulator exhibits a voltage drop of approximately 2 to 2.5 volts. Consequently, the solar panel chosen for this setup has a higher voltage rating than the load. In this case, a solar panel with a voltage output of 17 volts and a power capacity of 5 watts has been selected.

The lead-acid battery employed here adheres to a standard rating of 12 volts and 1.3 ampere-hours (Ah). To effectively charge this battery, you will require the following components.

Incorporating a Schottky diode is essential to safeguard both the LM317 and the solar panel against potential reverse voltage generated by the battery during periods when it is not charging. Any 3-ampere diode can be utilized for this purpose.

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

  • Solar panel wattage/Solar Panel Voltage = 5 / 17 = 0.29A charging current
  • The LM317 can deliver up to 1.5A of electricity.
  • If your application requires a lot of current, it’s best to use high-wattage panels.
  • (However, in this case, my battery requires an initial current of less than 0.39Amps.) The battery also mentions this initial current).
  • It is not suggested to utilise LM317 if the battery requires an initial current greater than 1.5A.

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.

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

The thermal resistance of both the LM317 voltage regulator and the heat sink imposes a power limitation on this project. To ensure that the temperature remains below 125 degrees Celsius, the power must be restricted to 10 watts. Internally, the LM317 voltage regulator is equipped with a temperature-limiting mechanism that will automatically shut it down in the event of overheating.

As the battery undergoes charging, the heat sink gradually warms up. During charging at maximum voltage, the heat sink becomes notably hot. This increase in temperature is primarily attributed to the surplus electrical energy that is not essential during the battery charging process.

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:

This circuit uses solar energy to charge Lead-Acid or Ni-Cd batteries. (If you read the previous posts, you’ll get a concept of how a lead acid battery charger circuit works.)

Limitations of this Circuit:

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

Solar batteries serve as a vital power source, ensuring the smooth operation of devices. As non-renewable energy resources dwindle, there is an urgent imperative to amplify the utilization of solar power. Solar batteries play a pivotal role in accelerating this transition.

Nevertheless, when acquiring solar batteries, it is essential to possess electronic devices that are compatible with them. My foremost recommendation is to consider investing in Solar Lights Kits, which can be conveniently installed in home gardens, walkways, and on walls.

These kits are affordably priced and contribute to enhancing the aesthetic appeal and ambiance of outdoor spaces, particularly during the nighttime. With the presence of bright white light, you can enjoy quality time with your loved ones while basking in the captivating atmosphere it creates.


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