Long before the current fashion for sustainable development caused solar panels to blossom on roofs and terraces in non sun drenched areas of the world, numerous nomadic and Route 66 users were already using them on motor-homes or pleasure craft. In these situations, the primary rôle of a solar panel is not to sell power back to the local electricity board or utility, but to charge an array of batteries in our on the vehicle or craft to provide a source of electricity after dark. Even though such an operation might appear trivial, all the more so if you look at certain ‘charger’ circuits, it really is nothing of the sort, if you’re keen to look after your batteries.
Even though it does work, the solution of wiring batteries, supplied load, and solar panels in parallel is far from being satisfactory in at least two situations, which we’ll discuss below. When the load powered by the batteries consumes little or nothing at all, and the batteries are already well charged, and it’s also a sunny day, the batteries are in serious danger of being over-charged, which as everyone knows will severely shorten their life — and possibly your travel. ling distance. On the other hand, when the load powered by the batteries is drawing a lot of currentand there is little or no sun, the batteries can end up completely discharged, which as it may turn out is just as detrimental to their life as over-charging. Yet it takes only a handful of components to build our intelligent regulator, the circuit of which is given in Figure 1. It uses a PIC 12C671 microcontroller, which has the double advantage of being housed in an 8-pin DIL package and containing a multiinput analogue/digital converter (DAC). Potential divider R6-P2-R7 feeds a discrete voltage level to AN0 to set the battery voltage at which charging should be stopped, to prevent any risk of over-charging. Potential divider R8-P1 and R5, feeding PIC analogue input line AN1, this time defines the battery voltage below which the load should be disconnected to avoid excessive discharge. In this way, a voltage window is created for the PIC to maintain in the interest of the battery’s health and lifetime — and your peace of mind, of course. The voltage present at the battery terminals is measured via the potential divider – fixed this time – R1 and R2, feeding PIC port line AN2.
Zener diode D5 protects the microcontroller from any spurious external voltage that might appear on the terminals of the solar panels – during thunderstorms, for example. Depending on the above mentioned voltage thresholds, the circuit controls relays Re1 and Re2, via transistors T1 and T2. The first is used to connect the solar panels to the battery. Hence it is energized as long as the battery is not being over-charged, otherwise it is off. The second, T2, is used to connect the battery to the load being powered. So it is energized as long as the battery is not too deeply discharged, otherwise it is off. Diode D1, which must be a Schottky type to minimize the forward voltage drop, avoids the battery discharging through the solar panel in periods of weak sunshine. A normal silicon diode in this position will have a too high forward drop (about 0.6-0.7 V) to ensure optimum results hence is not recommended. Note the 4-pin connector at the bottom of the circuit diagram.
This allows the present charger to be connected to the Solar-powered Automatic Lighting Module described elsewhere in this issue. If this module is not being used, all you need do is connect a jumper across pins 1 and 2, as indicated in the diagram. To make this project easy to build, we’ve designed the PCB shown here. As usual, the copper track layout is contained in the free download available from the Elektor website. This PCB has been designed for 10 A Finder SPDT relays, which leaves plenty of freedom in terms of choice of panels and battery. When designing this charger, we planned for a maximum battery current of 2 A, as indicated by the fuse value given, but there’s nothing – apart, perhaps, from your wallet, for the cost of the battery and solar panels – to stop you from going higher.
The .hex file to be programmed into the PIC 12C671 is available free to download on the Elektor server, as well as from the author’s own website (see end of article). Once built, the project is elementary to adjust, and only requires a DC voltmeter and an adjustable PSU, even a very simple one. Do not connect any of the external elements to the charger, and replace the battery by your stabilized PSU set to 12 V, with a voltmeter across it. Then increase the voltage to 14.5 V and adjust P2 so that Rel1 just drops out. Then reduce this voltage to confirm that Rel1 is energized again at around 12.8–13 V (depending on component tolerances). Continue to reduce the voltage down to 10.5 V and then adjust P1 so that Rel2 drops out. Increase the voltage again to check that Rel2 is energized again around 12 V or just under. P1 and P2 do not interact, so it is easy to adjust them independently. Lock the wipers of P1 and P2 with a little sealant and fit your project into a case, taking care to protect it from damp if it is to be used outdoors. A sealed electrical junction box is ideal for this, at a ridiculously cheap price.