6V solar charge control

6V LDO Solar Charge Controller

This Low Dropout Voltage (LDO) solar charge controller is a variation of the previously posted 12V LDO controller. It is optimized for charging a 6V lead-acid battery with a 9V solar panel. Minimum voltage drop is less than 1V. It uses a simple differential amplifier and series P channel MOSFET linear regulator. Voltage output is adjustable. It may also be applied in two or four cell lead-acid applications (4V & 8V).
It is not recommended for 12V applications.

6 Volt solar charge controller schematic

6V solar charge control

6V Solar Charge Controller Specifications

  • Max solar panel rating: 50W (8A, 6V nominal) (open circuit voltage: 9 to 10V)
  • Output voltage range: 4.7 to 9.8V (adjustable) (not recommended for 12V applications)
  • Max power dissipation: 16W (includes power dissipation of D3)
  • Typical dropout voltage: 0.9V @ 8A (less @ lower currents)
  • Maximum current: 8A (current limiting provided by solar panel characteristics)
  • Voltage regulation: 80mV (no load to full load)
  • Battery discharge: 1mA (Chinese controls discharge at typically 5mA)
  • LED indicators:
    • RED: Solar panel active
    • GREEN: Series regulator limiting current (fully charged or topping off)
  • Reverse battery protection: Control shuts down if battery is inadvertently connected reverse

Operation at lower current/power

While designed for 8A, 50W, it will function just as well at much lower current /power.

LDO Solar Charge Control Photos

Perf board—sorry, no circuit board artwork at the time of publication.


LM317LZ—many readers may not know that this component exists in the TO-92 package.


Bill of Materials

Dropout Voltage

The input voltage exceeds the input voltage by 0.9V when charging at the maximum rate—the lower, the better. Low Dropout Voltage (LDO) is the catch phrase for anything under approximately 2V.

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 than the charge voltage, so to accommodate both voltages, a compromise is reached by simply reducing the voltage slightly—that is how ALL automotive systems operate. To obtain maximum charge in a 6V battery, set the control to 7 tp 7.4V.

Voltage Adjustment

To set the voltage, disconnect the battery and connect a 470Ω dummy load resistor across the output. The resistor is necessary to shunt potential MOSFET leakage current as well as the green LED current. The battery must be disconnected because the output voltage of the control cannot otherwise be set below actual battery voltage.

Circuit Operation

U1 is an LM317LZ TO-92 voltage regulator that is set to put out 3.1V. Low voltage zeners (below 6.2V) are too sloppy to use as voltage references, so the LM317 is used. Q1 & Q2 make up the classic differential amplifier that amplifies the difference between the reference voltage and the feedback voltage from the arm of potentiometer R6. The output is taken from the collector of Q2 and drives the gate of P Channel MOSFET Q3. Differential voltage gain is probably in the order of 100 to 200. For best performance, I selected Q1 & Q2 for matched hFE (approx 300). As the feedback voltage increases at the arm of R6, Q2 turns on harder and steals some of the emitter current away from Q1. The collector current of Q1 follows the emitter current and drops less voltage across R1 thus reducing Vgs of Q3 and turning it off. C2 provides frequency compensation to prevent the amplifier from oscillating.

Q3 (Fairchild NDP6020P) is a high current P-Channel MOSFET that has an Rds on of only 50mΩ. This is in the class of logic level controlled devices as it may be turned on fully with only a 4.5V gate to source voltage. To obtain these properties, a sacrifice is made in voltage rating. As a result, Q3 is rated for only 20V. Because Max Vgs is only 8V, a 6.2V zener (D1) protects the gate from potentially destructive voltage. Due to these voltage limitations, this control is not recommended for 12V applications.

When the battery reaches set voltage, Q3 starts to drop significant voltage and turns on Q5 which powers the Green LED.

Q4 is dormant unless the battery is connected reverse—should this happen, Q4 turns on and reduces the reference voltage input to zero thus turning Q1 & Q3 and preventing damaging battery current.

Blocking diode, D3, prevents the battery voltage from appearing across an inactive solar panel.


If this circuit appears too complex, strip off the unnecessary components and things get much easier—drop (or add later) R8-11, D2,4,5, Q4,5 thus saving 9 components.

Thermal Management

This is a linear series regulator that dissipates significant power when the pass transistor is both conducting current and dropping voltage simultaneously—during maximum charge rate when the voltage drop is low, the heatsink runs warm—when the battery is fully charged and there is low charge current, the heatsink is cold—but when the battery starts to top off at maximum voltage, the heatsink runs very hot—such is the nature of a linear regulator. At 8A, Q3 drops 1.45V (assuming solar panel voltage is 9V). The remaining 0.55V is the D3 voltage drop. P = 8A * 1.45V = 11.6W. The heatsink is rated at 3.9°C/W, so heatsink temperature rise = 11.6W * 3.9°C/W = 45.2°C. Adding the 25°C ambient temperature results in a heatsink temperature of 70.2°C. While this may seem very HOT to the touch, it is still cool to the transistor that is rated for a junction temperature of 175°C.

Heatsinking D3

D3 dissipates 4.4W @ 8A. This requires a heatsink. In an etched circuit, a large foil area is required to dissipate the heat. In the perfboard version, I added copper heatsink bars fabricated out of AWG #14 solid copper leads that were hammered flat to increase the surface area.

Testing the 6V LDO Solar Charge Control

My apparatus cannot simulate solar panel current above 6.6A. While the control is designed for 8A, it has not been actually tested at that level. Actual measurements indicate a voltage drop of 0.51V @ 4A and 0.64V @ 6.6A. Voltage regulation measures 80mV (NL to 6.6A). I do not know why the 12V LDO control performed much better in this regard.

For the future

Properties of the differential amplifier


Join the conversation!

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  • Vijay Zagade

    Hi Jim,

    How are you?

    I am making this 6V LDO Charge Controller.
    I am from India and I am not able to buy the MOSFET NDP6020P near around me and also the Schottky Diode 80SQ045N.

    Could you please suggest me any alternatives for the two Components?

    Thank you


  • vitor

    What will hapens on the night!
    The battery will discharge across the resistor R6 e R7!

    • Jim Keith

      Good observation, but the discharge is far below 1mA. This still compares favorably to commercial units that typically self-discharge at about 5mA.

      If you wish the discharge to be lower, simply move the top of the voltage divider to the anode of D3. The drawback is softer voltage regulation.

  • darrencch

    Sir, I actually not that understand what’s the function of the green LED. Is the LED will light up when the input is connected? Besides that, I only have 5W 18V solar panel, don’t have 5W 9V solar panel. Can sir suggest solutions to solve this problem?
    Thank you Mr. Jim.

  • darrencch

    Hi, Mr.Jim
    Can I get your opinion for my problem here? There are 2 components that I can’t find around my area which are
    NDP6020P and 80SQ045. So, I get F9540N and IN5822 to substitute them. Then, I face a problem with the green light, it lights once I connect the input to the circuit and it doesn’t work as it suppose to be. Sir, can you give me some suggestion or solution to this? And is it the problem occurs because I use the different components as listed? Thank you sir.

  • mustapha_kazaure

    hi mr. jim
    pls if i want a charger controller that i can use a 4 80w solar panels and i want the charge controller to withstand up to 20A current pls how will i upgrade this ur schematic. thank u sir

    • Jim Keith

      The IRF9Z34N is a poor substitute for the specified NDP6020P for these reasons:
      1. Increased Rds ON: 0.1Ω vs 0.05Ω
      2. High Vgs threshold voltage: 4.5V vs 1V
      (standard vs logic level device)

      The FPQ27P06 is not much better than the IFR9Z34N in this 6V application.

      If you must use this device, parallel two devices and use this circuit that delivers greater gate drive voltage:

    • mustapha_kazaure

      ok, thank u sir,
      but sir in my area (country)i found it difficult to get dis IC fqp27p06 but the IC IRF9Z34N is available in the market, as u describe in the other 12v/6v charge controller schematic u describe it as one of replacement of fqp27p06, so in dis case of my design of 80w panel will it work also perfect does fact the fact that i use the heat sink u specify for me?
      thank u sir

    • Jim Keith

      The best way to do this would be to make (4) charge regulators–one for each panel, and tie the outputs together. Perhaps go with the 2.5″ high heatsink to handle the increased power (80W) panel.

  • blastboot

    Hello.First of all thanks for showed and explained your circuit.It suits to charge my battery so i build it.But i have some questions:1- How do you calculate de 75% of battery level which green led lights and how it works? 2- I couldn’t understand very well how to regulate the right voltage to charge my 6V battery. I know it must be charged at +/- 7,45V (max) but what conditions do i have to make to regulate this voltage?Thanks in advance!!

  • blastboot

    If i connect a circuit to the battery (to be supplied by it) can have any problems even if minor problems (like green led maybe could loose accurancy indicating the 75% charged battery, maybe because the circuit connected to the battery will always have the same current consumption) and others?

  • Jim Keith

    This will work and is the most simple.

  • pmadithya


    I have a Solar Panel having rating
    5 Watt,
    V(open circuit) 21.6 Volt
    I(Short circuit) 0.32 Amps
    V(mp) 17.3 Volt
    I(mp) 0.29 Amps
    Please suggest a suitable Charge controller to charge my 6 Volts 4.5 Ah battery.

  • Jim Keith

    Your 6V solar panel may be specified or “characterized” for charging a 6V battery. 6V may be the “nominal” rating, but actual voltage output may be up to 9V in order to easily charge a 6V battery to full charge (7V). The additional 2V is extra voltage and is called “head room” or “head voltage.” To prevent this additional voltage from eventually overcharging or damaging your battery, a charge regulator like this circuit is required.

    Place your solar panel in full sunlight and measure the open circuit terminal voltage. This measurement will determine if there is indeed sufficient head voltage to charge to 7V or higher.

    • Antony John

      Dear Sir,

      Than You For your valuable instant reply. In full sunlight, from my 6V solar solar panel I am getting 8 to 10V output (open circuit-no load). In the evening or cloudy time, I am getting only 4V to 6V only. Whether this is sufficient for this circuit. Also How to connect a 1W (I think 3V) High power LED to this circuit (resistor is requiremet ).
      Thank you
      Antony John

    • Antony John

      Dear Sir,

      Than You For your valuable instant reply. In full sunlight, from my 6V solar solar panel I am getting 8 to 10V output (open circuit-no load). In the evening or cloudy time, I am getting only 4V to 6V only. Whether this is sufficient for this circuit. Also How to connect a 1W (I think 3V) High power LED to this circuit (resistor is requiremet ).
      Thank you
      Antony John

    • Jim Keith

      If the sun does not cast a shadow, the charging current out of the solar panel will be insignificant.

      On your 3W LED:

      I = P /E = 3W /3V = 1A

      R = E /I = (12V – 3V) /1A = 9Ω
      (use 10Ω because it is a standard value)

      P = I²R = 1² * 10Ω = 10W
      (use 20W so it will run cooler)

      Now, you could string (3) 3W LEDs in series and get 3X as much light and drop the voltage and power of the series resistor. In this case:

      R = (12V – 3 * 3V) /1A = 3Ω

      P = 1² * 3Ω = 3W
      (use 5W)

      A way of conserving power would be to power a single LED via a current mode buck switching regulator. This is a more complex circuit that requires a pass transistor, inductor and free-wheeling diode. However, the efficiency would be high and the average battery current would be approx 300mA. This would be a nice project for the future.