tydude

Okay, I am making a solar powered battery charger and I need some help. I have the solar cell which outputs about three volts in direct sunlight. I am planning to hook it up to a diode and a resistor to the battery. My question is will my battery overcharge and get destroyed? What should my resistor reduce the output of the solar cell to? Any help would be appreciated.

Leftyretro

I think more information is need to really be a help to you. Size (capacity in amp hours) and voltage of battery is important. Amperage of the solar cell at full output is also needed. Also what load you are running and it's duty cycle will tell you if it's even a realistic proposition.

Lefty

audioguru

What type of battery? Lead-acid, Ni-Cad, Ni-MH or Li-Po.
I hope you are not trying to charge disposable alkaline battery cells.
You are not going to charge a 12V car battery.
See, you forgot to say what kind of battery and its spec's.

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Uncle $crooge

tydude

It is a 1.2v 900mAh 90mA x 14hr Ni-Cd battery.

Nigel Goodwin

You haven't mentioned how big your solar panel is?.

You also don't have your location filled in! - an important thing to know if you're asking about solar panels.

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audioguru

Ni-Cad battery cells are old. New ones are Ni-MH with a much higher capacity of 2500mAh for an AA size cell. Does your old Ni-Cad cell still work?

It needs a charging current of 90mA for 14 hours. The sun doesn't shine for 14 hours so a full charge will take longer than 1 day.

How much current can your solar cell output at noon in summer if it is pointing directly at the sun?
A window will reduce the current to about half and so will a time before or after noon, and if the solar cell is not pointing directly at the sun, and if it is not summer or if it is cloudy.

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Uncle $crooge

RadioRon

Here's a good link with some useful theory:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/484

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RadioRon

Thunderchild

I don't think you need the resistor, solar panels have high internal resistance and so you can even short circuit it with an amp meter and it won't damage the panel, if you use a resistor you might loose valuable power also consider charging 2 NiCD or NiMH batteries and use a skotchky diode as it have a voltage drop of 0.2 volts rather than 0.6-0.7 volts.

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