solar battery charger project

Solar Battery Charger Circuit

This is the most simple and affordable solar battery charger that the hobbyist can make. It has a few drawbacks over other similar controls, but offers numerous advantages. It is intended for charging lead-acid batteries, but may also be used for charging any battery at a constant voltage. Voltage output is adjustable.

Advantages & Disadvantages of this solar charger

  • + Simple, small & inexpensive
  • + Uses commonly available components
  • + Adjustable voltage
  • + ZERO battery discharge when sun is not shining
  • — High drop-out voltage—may be marginal for 6V application
  • — Current limited to 1.5A
  • — No LED indicators—no bells or whistles

Solar battery charger specifications

  • Solar panel rating: 20W (12V) or 10W (6V)
  • Output voltage range: 5 to 14V (adjustable) (may be reduced further by shorting R2)
  • Max power dissipation: 10W (includes power dissipation of D1)
  • Typical dropout voltage: 2 to 2.75V (depending upon load current)
  • Maximum current: 1.5A (internally limits at about 2.2A)
  • Voltage regulation: ±100mV (due to regulation of series rectifier)
  • Battery discharge: 0mA (this control will not discharge the battery when the sun doesn’t shine)

Solar battery charger schematic

solar battery charger circuit schematic

6V Applicaton

  • Output Voltage: Set for 7V
  • Input voltage:
    • Battery discharged (6V): 8.75V Min @ 1.5A (this is a little high for panels that are characterized for 6V applications)
    • Battery charged (7V): 9V Min @ 10mA (e.g.)

12V Application

  • Output Voltage: Set for 14V
  • Input voltage:
    • Battery discharged (12V): 14.75V Min @ 1.5A (Available from solar panel characterized for 12V operation)
    • Battery charged: (14V): 16V Min


Related Products: Power Management | Battery Management | Special Purpose Voltage Regulators | Power Supply Controllers and Monitors | Specialized Power ICs and Modules


Minimum Head Voltage

This is also referred to “drop-out voltage.” The input voltage must exceed the output voltage by about 2.75V @ 1.5A. Fortunately, when the battery discharged, the output voltage is lower so the solar panel voltage will also be lower.

When fully charged, the battery voltage will be high, but the current is very low—at this point, the drop-out voltage reduces to about 2V and the open circuit solar panel voltage also comes into play. The schottky rectifier was selected to reduce this head voltage requirement—the voltage drop of the schottky is about 0.5V @ 1.5A or about half that of a typical silicon rectifier.

More advanced controls have a much lower head voltage requirement and will function better under marginal conditions.

Maximum Power Dissipation

In this solar battery charger project the power is limited by the thermal resistances of both the LM317T and the heat sink. To keep the junction temperature below the 125°C Max, the power must be limited to about 10W. If a smaller or less effective heat sink is used, the maximum power dissipation must be de-rated. Fortunately, the LM317 has internal temperature limiting so that if it gets too hot, it shuts down thus protecting itself from damage. Max power comes into effect when charging a 12V battery @ 1.5A: e.g. battery voltage = 12V, solar panel = 18V. P = (18V – 12V) * 1.5A = 9W. So thermally, it is carefully matched to the current rating.

If a solar panel that is characterized for 12V is applied with a 6V battery, the maximum current must be reduced to about 0.7A: e.g. battery voltage = 6V, solar panel voltage = 18V. P = (18V – 6V) * 0.7A = 9.6W. In this case, the solar panel power may not exceed 10W.

When charging, the heat sink normally runs warm. When beginning to “top off” or completing the charge at maximum voltage, the heat sink runs hot. When fully charged, the heat sink runs cool. This heat is not exactly wasted power—it is excess power that is unneeded in the process of charging a battery.

Current Limiting

Current limiting is provided by the solar panel—it is not a commonly understood fact that the solar panel tends to be a constant current device. For this reason, a solar panel can withstand a short circuit.

Therefore, the control does not need current limiting.

Float Charge of Lead-Acid Batteries

This control charges the battery at a constant voltage and also maintains a charged battery (float charge). The float charge voltage specification is a little lower, so to accommodate both charge and float charge voltage, a compromise is reached by simply reducing the voltage slightly—that is how ALL automotive systems operate. To obtain maximum charge in a 12V battery, set the control to 14.6V. Automotive systems further reduce voltage to 13 to 13.5V in order to accommodate high temperature operation as the battery is usually located in the hot engine compartment—battery has a negative thermal coefficient of voltage.

solar battery charger project

Application with other types of batteries

It is difficult to determine how to set the voltage in this case. The easiest way to do this is to charge the battery fully using other means and then transfer the charged battery to the control and connect an ammeter in series. Increase the voltage setting until there is significant current and then back off the potentiometer until the current drops to perhaps 10mA or so. Some types of batteries such as lithium ion types must be disconnected after charging to prevent degradation.

Solar Battery Charger Protection

C1 provides substantial protection against static discharge.
There is no protection against reverse polarity or mis-wiring. It is protected if solar panel is connected reverse without battery connected, or if battery is connected reverse without solar panel connected. However, if the battery or solar panel is misconnected simultaneously, who knows? anyone care to experiment?


Join the conversation!

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  • Jayakumar

    I have a 12V 50W solar panel.
    I want to charge a 12v battary.
    How much ‘Ah’ Battary can I charge using that solar power.
    I want to charge it upto 6 hours a day.

    Thanking you.

  • Craig

    Can I add something like an arduino microcontroller as a second output in parallel with the battery output so its powered by either the solar pannel or the battery?

  • Jim Keith

    The output voltage is on the high side for the typical 12V battery system –what this does is to generate more heat in the series regulator device. This is no problem if the heat sink is large enough. As a result, I recommend that you use a single solar panel. Peak power = 1.25A * (23V – 14V) = 11.25W. For a 4°C/W heatsink, Trise = 11.25W * 4°C/W = 45°C. Add this to your max ambient temp and also factor in the thermal resistance of the semiconductor junction to case thermal resistance and you will get a maximum Tj of about 90°C. It is rated for a max Tj of 125°C.

  • bill

    Hi Jim now i was wondering and that takes a lot for me to do….i have 2 Sun panels with a rating of VMP 23v..VOC 30.5..ISC 1250mA can i useone or both panels on my 5x 7.2AH system or would it be better to buy new panel as its very difficult to find solar panels in our area thanking you again for you trouble

  • bill

    Jim thank you so much for your help i really appreciate it…in time i will let you know what happened

  • bill

    Hi Jim im 74 years young just started electronics and solar so the question is…im trying to charge 5×7.2AH batteries my regulator is a 12v 10amp …..what size solar panel or panels do i need….please help me thank you

    • Jim Keith

      I would go with a 40, 50 or 60W solar panel. Search ebay for “solar panel 50W” etc. Some of the best are made in China and ship free to anywhere. Connect your batteries in parallel. Good luck.

  • Syed


    I am new to circuit design, need help to build a small Solar powered demo car for science project.

    My plan is to build a circuit where I will charge a battery from Solar panel and will be connected to two DC small motors.

    Would like to make use of the components which I have already bought

    2 small DC Motors with specification as
    1.5 – 9 V
    0.01A -0.3A
    Torque 36g cm
    speed 300rpm

    1 – 6V 100MA solar panel.

    Please suggest a circuit and components requirements

    Thanks in advance.

  • camilus. fernando

    Hello dear, I am doing experiments to run DC BLDC motors(nidec 50M1862010 dc 24v 2.7 amps photo copy machine)with MOSFET SLA 5060 circuit, some nidec motors are run 24v dc &5v dc supply volt,above motor need dc pulse volt supply. did you try to run this type of motors. (try to send photo) Thanks! Camilus.

  • Brian

    I am VERY new to circuit building. I like this circuit, very simple. What I’m needing to do is to charge a 9v 170mAh NiMH battery for a usb charger I have built. What do I need to change on this schematics to achieve this? Thanks!

  • William

    Wow, a reply already this is amazing!

    Thank you James

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