The battery used in a solar power system is called a deep cycle or leisure battery. It can cope with being discharged to a much greater extent than a normal car battery, which needs to be constantly topped up by the alternator much like a reservoir capacitor in a power supply. The theory behind lead acid battery charging is very complex, but a simple rule of thumb to prevent cell damage is to not allow the voltage to go above 14.4V when charging or below 11V when discharging, the ideal charging voltage being around 13.5V. Systems can be employed that disconnect the battery if the voltage falls too low, but if you know the daily demand and stay within it there shouldn't be a problem. A fully charged battery should read more than 12.7V


An important part of any renewable energy system is the charge controller, which maintains the battery voltage at its optimum level. Commercially made units are available with varying prices, features and current handling, but if you want to build your own, the simplest method is to use a shunt regulator. Shunt regulators may not be as efficient as processor controlled designs, but they do work very well. The amazing fact that a solar panel can be short circuited without being damaged is put to good use here to create a simple controller that can charge a battery to an exact preset voltage level. The schematic diagram is shown below

The circuit uses an LM393N voltage comparator, which although not an opamp, can be thought of like an opamp as far as the inputs are concerned, but the output is very different. When the + input is lower than the - input, the output (pin 7) will be internally grounded, effectively connecting the gate of the IRF540A MOSFET and the lower end of the 10K resistor to earth. But when the + input goes higher than the - input, the output will 'float' open circuit, letting the 10K resistor feed the gate, turning it on. An important feature of the LM393N is that it has a low current consumption. The whole charger circuit only consumes about 3.5mA and so has a negligible drain on the battery. The low current part of the circuit is constructed on veroboard as shown below (OUTPUT goes to the gate of the MOSFET)

           

The IC is soldered directly to the board, a socket not being recommended in this application. As it is a dual device, and only one comparator is used here, the other is shown greyed out. On the actual veroboard, the unused pins were folded under the body of the IC out of the way, with the inputs being soldered together to make them happy! The reason for this is so the copper tracks can pass from one side of the board to the other unhindered. It just makes designing easier. The veroboard is fed via a small fuse in case of component failure, though I suspect veroboard tracks make good fuses anyway!

The inverting input (pin 6) is fed from an LM317L programmable regulator, its output voltage being set by a multiturn preset VR1. The non inverting input (pin 5) samples the voltage across the battery via a potential divider formed by two 47K resistors. As these resistors are equal in value, the voltage at pin 5 will always be half that of the battery, whatever it happens to be. So setup is achieved by adjusting VR1 to give half the required charge voltage on pin 6 (eg. 6.7V for a wanted charge of 13.4V). The formula for calculating the LM317L output voltage is: Vout = VR1 value 1000 + 1 x 1.25. With the values used this equates to 1.25 to 7.5 volts available on pin 6 of the opamp

Now if the battery voltage is below 13.4 volts, pin 5 will be less than 6.7 volts and so the output from the comparator (pin 7) will be grounded and the MOSFET will be off. The solar panel will be connected across the battery and it will charge. As the battery charges, its voltage will rise towards 13.4 volts at which point pin 5 will also rise and eventually reach 6.7 volts (the inputs of a comparator always want to balance). Because of the extremely high gain of the IC, pin 5 only has to go an immeasurably small amount higher than pin 6 to make the output (pin 7) go open circuit, which will allow the 10K resistor to turn on the MOSFET and connect the 2R2 'shunt' resistor across the solar panel, effectively dumping its energy into a resistive load instead of the battery. This continuous action maintains the voltage across the battery at the preset level

Note: In practice, and with sun permitting, setup can be achieved more easily and accurately by connecting just the solar panel without the battery or load and then adjusting VR1 for 13.4V across the battery supply rail (BATTERY+/- on the veroboard)

The MBRF1645 is a 16 amp schottky diode that prevents the battery from being 'shunted' by the 2R2 resistor. It also stops voltage from the battery being fed back into the panel, though most panels have a protection diode built in. This component is not critical as long as it can cope with the current. Normal silicon diodes can be used, but schottky types are better because they have a lower voltage drop across them, so less energy is lost. The IRF540A MOSFET is rated at more than 20 amps which means the circuit should quite easily handle the relatively low amount of power demanded of it, but even so, the high current components should still be mounted on a heat sink. With the 40 watt panel I am using, the 2R2 resistor gets extremely hot so make sure it's at least a 100 watt wirewound. The parts used here should allow another 40 watt panel (with an additional 2R2 resistor) to be added later if required

The circuit board is mounted in a junction box to protect against dust