12V SSS Solar Charge Control Circuit

6A, 12V SSS Based Solar Charge Control

This is a Solid State Switch upgrade to the 6A, 12V solar charge regulator. Solid state switching provides longer life, smaller size and higher efficiency than a relay. In addition, the clock frequency may be doubled in order to better track battery load /charging requirements. The parts cost is only slightly higher. One drawback is the loss of the secondary output feature. For more information go the above link.

The application for this type of charge control is one in which the battery capacity is large in respect to the charging current; e.g. 6A solar panel charging a 60AH battery in which it takes perhaps a full day’s sunshine to fully charge the battery. For much smaller batteries, the high charging rate and relatively high battery internal resistance results in excessive terminal voltage so that the control immediately interrupts charging –the end result is that the battery cannot charge fully –a linear charge regulator is more appropriate in such cases.

Schematic of the Solid State Switch Solar Charge Controller

12V SSS Solar Charge Control Circuit

Bill of materials


Eliminate the relay life issue

While relay life is probably OK, it is definitely limited. Check out the following Omron relay endurance specification. Mechanically, it is good for 100million operations. With a 10A, 250VAC load, it is rated for only 36,000 operations. Note that a form C contact is used, so the form A (normally open contact) specification does not apply. I believe (guess) that it is good for at least 400,000 operations in the 6A solar charge regulator application –this equates to approximately 1year’s operation. So you can see the advantage of going solid state.

Omron G5LE specification

Omron G5LE Endurance Spec

Driving the MOSFET gate

Obtaining high speed MOSFET operation can be a challenge, but at such a low frequency (0.067HZ), driving the gate is a piece of cake. MOSFET turn-off is generally the issue, so I obtained an oscillograph. Turn-off occurs within about 80uS, and the active portion of the gate voltage is indicated by the step on the transition. R14 is what discharges the MOSFET gate capacitance, including the Miller capacitance in which the change in drain voltage is coupled to the gate thus slowing the transition. The gate turns on via R9 and the gate voltage is limited via zener D3 to 10V –otherwise the full solar panel voltage could appear between the gate and source and potentially damage the MOSFET.




To set the Max Voltage Adjust potentiometer (R12), start with it turned CW, monitor the voltage and wait for the battery terminal voltage to reach the desired voltage (e.g. 14.5V). At this point, turn R12 CCW until the LED extinguishes. When the LED comes on again at the next clock cycle, recheck the voltage at which it extinguishes.


For the future

  • 6A, 6V relay based solar charge control
  • 6A, 6V SSS based solar charge control
  • MPPT buck converter solar charge control
  • Selection guide for the various solar charge controls

Undocumented words and idioms (for our ESL friends)

piece of cake –idiom –easy, or easily accomplished


Join the conversation!

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  • Jim Keith

    This is a good application for this solar charge control–should work well. Good luck with your project.

    • irfanisarwadgmail-com

      I need a circuit to charge my 24 volt 14 Ah battery system.
      There is no issue with the input side i can make use any panels from 50 Watt to 250 Watts. I would be very great full to you if find the desired circuit.

      My email-Id-
      Mobile Contact – +91-9880452887

    • thaiorlandogmail-com

      Thank you sir,

      I write you this message to ask for your permission to use your circuit as the reference design for our group’s project.
      I will ensure to include all the credits to you as the author of this work. Our project showcase will be in the end of July, I can send you a copy of our project document as a proof of our work sir.
      My school is: University of Central Florida
      My Email is:
      I really hope to receive an email from you soon,
      Thank You again

  • thaiorlandogmail-com

    hello Jim,
    I am working on a senior design project, and it involves solar power system. I am wondering if I can use this circuit for my project,
    My project is about Intelligent Bike Rack system with Offgrid Solar Power Supply. I have been struggling with designing a charge controller for my power system.
    I have two 50 Watt solar panels (each panel has 12V, 2.5A).
    I also have two 6V 215Ah batteries (connected them in series produced 12V 215Ah).
    Is it okay for this charge controller to work for my current system?
    I hope to receive a response from you soon
    I greatly appreciate for all your efforts showing us the charge controller that you designed

  • Newttt

    Thanks sir for this circuit,what wattage do i need to to charge 12v,50AH (lead acid )?

  • labh singh

    I have installed solar system at my home. Three kW invertor fourty eight volt , Thirty amps.suggest me solar controller circuit diagram. Thanks

  • shepherd

    charging a 6Oah what solar wattage do i need?

    • Jim Keith

      I would use a 100W (8A) panel. 8A will fully charge your battery in about one day.

  • Hennie

    Jim, this circuit works very well. I only had a problem to set it up (to many variables while on a life system), but I solved it by disconnecting D5 and connect a bench power supply set at +- 18v on the solar input and another bench power supply connected to the battery output side set at 14.4v. I then adjusted R12 to cutoff at 14.4v, I then lowered the voltage on the battery side and checked D4 status. I also adjusted the power supply on the solar side between 16v and 20v while measuring the cutoff voltage at the battery side by increasing and decreasing the voltage at the battery side while observing the status of D4. The circuit was very stable. I then connected D5 again and connected the controller to my solar panel and battery and took some readings on the battery. Great results.

  • Tokkie

    Jim, tanks for your articles.
    1. Do you have 24v solar battery charger ccts or how can I modify this cct? I want to charge 2 105ah batteries in series.

    2. Can I use this cct to charge 2 or more 105ah 12v batterys in parralel, is there any drawback?

  • Akihanga Manyego

    Well documented article. well done Keith

  • Asif



    • Jim Keith

      Yes, this circuit prevents battery over-charging. Almost any high current P Channel MOSFET will do the job. Another possibility is the IRF9540N.

  • facu

    some protection against shorted or improperly connected battery? that you recommend