Car batteries rely on a running engine to charge them, and an engine requires a charged battery to start. Keeping a car battery charged during periods of inactivity can save you time, money and trouble when you are ready to start it again. A simple solar charger will help to keep your car battery strong enough to ensure you can get the engine running.
Acquire a 12-volt rated solar panel, or solar collector. This outputs slightly more than 12 volts when it's not connected to the battery. You can test its output voltage using a multimeter, which should read around 15 volts in direct sunlight.
Connect to the car battery in one of two ways: through the cigarette lighter socket using an adaptor, or by placing crocodile clips directly onto the battery terminals.
When you use the cigarette lighter socket, you can lock the solar panel and its connections inside the car for extra security. If you choose crocodile clips, you'll need to have an external solar panel or find a route for the wires from under the bonnet to inside the car.
Connect a red wire from the positive terminal of the solar panel to the anode--the leg attached to the large black part of the diode, the side farthest from the white band of your blocking diode. (The blocking diode prevents the battery from feeding power back into the solar panels when the light level drops. This prevents potential damage to the solar panel and drain on the battery.) Connect the diode's cathode (the leg next to the white stripe) to the central pin of the cigarette lighter adaptor or to the positive terminal crocodile clip.
Connect the negative terminal of the solar panel directly to the barrel of the cigarette lighter adaptor (the spring connections on the sides of the plug) using a black wire, or to the negative terminal crocodile clip. Using coloured wire can help you distinguish between them, but you can use any wire.
Put your solar panel in the car window where it will catch the most sunlight. Plug in the cigarette lighter adaptor or connect the crocodile clips, and the solar panel will trickle-charge the car battery whenever it receives sunlight.
House the diode inside the cigarette lighter adaptor or crocodile clip so you can connect the cathode leg directly to the terminal without using another wire. Make sure it's insulated so it doesn't touch the nearby negative connections. Constructing your own charger can be more expensive than buying a pre-built, car-specific solar charger. Investigate the ready-made kit option before buying all the parts to make your own.
Pay close attention when installing the diode. Make sure the output voltage of your solar panel isn't too high (above 17 or 18 volts) to avoid impairing your battery. Unless you're using a huge solar panel (half the size of the windshield or more), you don't really need to worry about overcharging. Smaller, domestic solar panels can't provide enough current to harm a car battery, but it's still worth running the engine every now and then to provide a proper charge for the battery. Unplug or disconnect the solar charger when running the engine. Leaving it connected won't harm anything, thanks to the blocking diode, but the solar panel will be ineffective when the engine's alternator is charging the battery.