# Turn Off Battery Charging from Solar Panel at Nightfall

When loading a battery during the day from a solar panel it can be partially discharging through the panel after nightfall. This solar panel power switch circuit replaces the diode and connects the panel to battery through a relay contact.
When the power supply voltage is too low the relay is not ON, so the battery is not connected to the solar panel.

When the voltage is high enough to engage the relay and the LDR receives enough light in order to open T1, the relay will switch and the battery will charge.

The relay remains ON even when the solar panel voltage starts to decrease. A connected and charged battery can not action the relay when the light intensity decreases because R2 will block T1. The brightness at which this occurs is set by P1.

Because the power consumption is determined primarily by the relay, it is important that the relay should be a miniature one, with high coil resistance but also be capable to switch up to 10 Amps.

## Solar panel power switch schematic

Components list for solar panel to battery switch

• R1 = 100Ω
• R2 = LDR05
• P1 = 25K
• D1 = zener diode 9.1V / 1W
• D2 = 1N4148
• T1 = BC557

## Related Tutorials

• raju

I want to charge from solar pane 12v 15 w of 2v 7ah battery

• Colin Mitchell

How can a 12v relay work though a 25k resistor ?????

• Ted W

The solar panel turns itself off when it gets dark. A diode from it to the battery will make sure that the solar cell does not drain the battery. Throw the rest of that circuit back into the junk box!

• burger2227

Bad idea as the relay will drain current too

• Pentaconto

Looking at the circuit diagram, I think this circuit will not work. The transistor T1 will only open when the base-emitter voltage reaches 0.6 volts. In the diagram, base and emitter of T1 will always be at the same voltage. It would take a bias resistor between the base of T1 and P1-D2. Also, not is informed of the characteristics of the LDR R2. I think this circuit is a theoretical design published without checking its operation.

• Ted W

PS: The diode cathode goes toward the car.

• Ted W

1) Once the solar panel is isolated, the relay drains the battery.

2) Why not just use a diode? Then current only goes one way. You can fit the diode inside of the cigarette lighter fuse plug too.

• solar panel reviews

can you tell me what the best solar panel?

• HENRY BASTIAN.C

SIR..I WANT TO CONSTRUCT A TRAVEL TIME MOBILE CHARGER USING SOLAR CELLS..MOBILE:5V,350MA

• Wisconsin Dave

a little more info on what ldr05 could be used. what is the resistance on this photocell?
Thanks,
Dave

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