Solar Charger

Solar Battery Charger Circuit

We saw a circuit diagram of a 9v battery charger circuit using LM311 and SCR in the previous post. Let’s look at a circuit for charging a lead-acid battery with a solar panel in this post.

We are not strangers to the solar concept. Solar energy is becoming more popular as nonrenewable energy sources become scarce. This solar energy is employed not just on Earth, but also in space stations where there is no electrical power.

The circuit to charge a 12V, 1.3Ah rechargeable Lead-acid battery from a solar panel is shown below. This solar charger features current and voltage management, as well as an overvoltage cutoff. Because the output voltage is adjustable, this circuit can be used to charge any battery at a consistent voltage.


  • 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 of a solar battery charger ensures that a steady voltage is produced. Through the diode D1, the charging current is sent to the LM317 voltage regulator. The adjust pin of the LM317 voltage regulator is used to control the output voltage and current. The same current is used to charge the battery.

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

Because the circuit requires an adjustable voltage regulator, the LM317 variable voltage regulator was used. The LM317 can generate a maximum voltage of 1.25 to 37 volts and a maximum current of 1.5 amps.

The voltage drop of an adjustable voltage regulator is typically 2 V-2.5V.

As a result, the solar panel is chosen to have a higher voltage than the load. I’ve chosen a solar panel with a voltage of 17 volts and a power of 5 watts.

The lead acid battery utilised here has a 12v/1.3Ah standard. To charge this battery, you’ll need the following items.

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

  • 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

Because of the thermal resistance of the LM317 voltage regulator and the heat sink, power is limited in this project. The power must be limited to 10W to maintain the temperature below 125 degrees Celsius. Internally, the LM317 voltage regulator features a temperature limiting circuit that shuts it down automatically if it becomes too hot.

When the battery is charged, the heat sink warms up. When charging at maximum voltage, the heat sink becomes very hot. This heat is caused by extra electricity that is not required during the charging of 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:

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 are one of the power sources that allows the device to run smoothly. As non-renewable energy sources become scarcer, there is a pressing need to boost solar power utilisation. Solar batteries play a critical part in achieving this in a short amount of time.

However, you must have an electronic gadget that supports solar batteries when you purchase the solar batteries. My greatest recommendation is to purchase Solar Lights Kits, which may be attached to home gardens, sidewalks, and walls.

They are quite reasonably priced and enhance the beauty and romance of the outdoors, particularly at night. In the presence of bright white light, you can spend some time with your loved ones.


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