How do you charge a battery with a solar panel?

Question

For part of a school project how would I charge a car battery with a solar panel? The panel is 48V and 6A. This needs to be low budget so I can't buy an expensive controller, Is that the only way or are there other ways of doing it?

Answer 1

Solar panels behave as constant current sources when the load draws sufficient current. The current will proportional to the illumination. Therefore connecting it to the battery in the correct polarity will charge the battery. There are several issues.



1) The panel is capable of delivering the current at a higher voltage than the nominal 12V, so it can charge a higher voltage battery the same way. Much of the power that could be provided is wasted. A suitable charge controller could step the voltage down and increase the current, so the total power remains similar, and none is wasted. As you already have plenty of current, I guess this can be ignored. It is just inefficient. The power ends up heating the panel, and it may become too hot.Normal maximum operational temperature is 50 or 60°C. A suitable voltage panel will have 36 cells in series, and produces around 21V with nothing connected. Yours is probably about 3 times more voltage.



2) The second is that the current is probably more than a smaller car battery can deal with continuously. Normally batteries have a continuous current rating around 1/10 their ampere hour (Ah) rating, which is more like 4A for a 40Ah battery.. A larger battery would handle the 6A okay. The issue here is that the battery could overheat, depending on ambient temperature.



3) When the battery is fully charge it needs to be disconnected. A simple approach that works well is to disconnect the battery when its voltage is approximately 14V. THis voltage is actually around 13.85V to minimise overcharging and hydrogen gas generation. The charger is reconnected when it is approximately 13V. This does require a certain amount of electronics which you would have to make yourself. The connection and disconnection could be using a relay, but works better with an electronic switch.



4) Charging a battery has some hazards. In this case the battery will be charged at around 6A whenever there is sufficient sunlight.This battery should be well ventilated and located outdoors. I don't like the relay idea because there could be sparks as the contacts close - ooops bang. This is also possible if you disconnect the charging wires from the battery.

Answer 2

48 volts is rather uncomfortable. I would be inclined to modify the panel by cutting traces to give me a number of 15 volt sets.



Most PV cells output about 700 mV, but check to see what yours does. Measure one cell, or count the number of cells and divide into the panel voltage.



If you work with the 48 volt output, you are throwing 70% of the output away.



The classic method of charging a lead/acid battery is to use the 10 hour rate...a 200 AH battery is charged at 20 amps. However if the charge is to be near continuous during daylight, I think I would go a little lower...say 15 amps in order to be kind to the battery.



I note that your panel has a 6 amp output, so when you modify it, it should be able to deliver about 18 amps.



A useful trick is to use car headlamp bulbs in series. The filament has a high positive thermal coefficient of resistance, and tends to act as a current regulator. In this case, I would use two dual filament bulbs with their filaments in parallel, arranged in series.

Answer 3

Homemade Solar Power Videos - http://SolarPower.siopu.com/?rKz

Answer 4

Here's a simple DIY Universal Solar Powered charger that you can try: http://www.eeweb.com/blog/circuit_projects/building-a-universal-solar-powered-charger



Hope that helps