Solar Panel Charge Regulator

This project is under development so it cannot be guaranteed to work perfectly without modification. In theory it should work fine, but until it has had a thorough testing it is presented at the moment as an idea...

How it works

The circuitry around the op-amp IC1 is used to detect the incoming voltage from the solar panel/s and adjusts the threshold point depending on the setting of VR1, the normal/equalize switch and the thermister TH1.

The thermister TH1 is fixed on top of the battery and adjusts the cut-off voltage depending on the battery temperature. This along with C1 C2 and R3 are optional.

The voltage from the solar panel is fed via transistor Q1, through the 78L05 voltage regulator to give a fixed 5V reference and power to IC1, which is only powered when there is sufficient voltage from the panel. This voltage is also sent via blocking diode D1 and resistor R2 to variable resistor VR1 for detection of the voltage and triggering the cut-off point.

IC1 is configured as a comparalor which switches its output depending on which input is higher so once the panel starts producing electricity IC1 powers up and switches on the mosfet power transistor Q2, thereby connecting the negative rail or the panel to battery negative. The capacitor C5 prevents multiple triggering or oscillalion when IC1 is near the cut-off point. This can happen if passing clouds or aircraft or even birds temporarily reduce the panel output.

Mosfet Q2 can be any suitable mosfet transistor that can handle the current from the panel, the one specified is cheap, very robust and easily available in the UK. Suitable mosfets should be found in practically all countries where electronic parts are on sale.

The green LED1 switches on to show that the panel is charging and the red LED2 comes on when the circuit cuts off the panel when the battery is fully charged.

D2 is used to connect the positive output of the panel to the battery and prevents the battery from leaking charge back into the panel, this may be already incorporated into the panel and if so is not needed.

C8 prevents switching spikes and the C6 - C7 combinalion are good practice and help prevent any interference.

Transorb diode TZ1 is also an option but it is well worth considering. These devices are designed to absorb large amounts of energy for a very short time and are a safety precaution against lightening and EMP.

The fuse is an essential safety requirement ahd the value should be chosen to suit the panel output.

Switching SW1 to equalize gives a higher voltage for when the battery has become over discharged or has had a lot of light cycling which causes the cell voltages to become unequal. Charging the battery for a few hour al this higher voltage helps to bring them back in line again and can help to prolong the battery life.

Semiconductor Pin-Outs:

Setting up

This is best done with some sort of variable power supply. An easy way to do this on the test bench is to use a pair of 9V batteries and a simple variable regulator built with the ubiquitous LM317 variable voltage regulator.

Connect the supply between PV+ and PV- and adjust the voltage so that the circuit turns on and LED1 lights up, a small 12V light bulb could be used to check that the mosfet is turning on. The thermister, if used should be at room temperature. Make sure SW1 is set to the normal position.

With a volt meter connected to the input turn up the voltage to 13.8V and slowly adjust VR1 until the circuit triggers and LED2 comes on instead of LED1. Back off the voltage to 13.7V and ensure that the circuit triggers on again. This may take a moment to happen due to C5. if it doesn't then tweak VR1 again so that it does and then slowly adjust the input voltage up to check whether it switches off again. If the voltage goes up to 13.9V to switch off it is really no problem, but is a little above spec.

LM317 Test Circuit

Above is the basic LM317 circuit.

Components:

R1 = 180R

R2 = 1K5

VR1 = 470R

C1-C3 = 100n

C2-C4 = 10uF

D1-D2 = LM4001 or similar

Keeping to easily available standard values as shown, R1, R2 and VR1 will give an adjustment between 11.67V and 14.93V. (Replacing R2 and VR1 with a 2k2 resistor and making R1 220R will give 13.75V but not adjustable of course, but this could be used to calibrate if need be).

This circuit can be fed via a pair of 9V batteries wired to give 18V or any suitable dc power supply under 40V. For this application a heatsink will not be needed. (as long as any load on the regulator chip is kept low!).

Layout details will follow in due course, but those with electronics experience should have no problem building either of these circuits on some stripboard.