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# Solar charge circuit question Answered

Hi, I have a 12 volt / 0.5 watt solar cell and I am hoping to charge a 9 volt / 200 mAh rechargeable battery with it. I think I have a suitable diagram for the circuit here...

http://www.flickr.com/photos/oskay/2963306721/

I was wondering if using a 12 volt cell was ok for charging the 9 volt battery? Would I need to add resistance to the solar cell to bring it down to 9 volts? If so where would I add the resister to that diagram? If it's relevant, the load will be a fan.

Thanks.

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## Discussions

Only just seen this . . .
No voltage dropping required - You want to get as much to your battery as possible and the battery will serve to regulate the voltage.  Use a schottky or germanium diode as these have a lower voltage drop than the more common silicon ones.
Unless you're in a very sunny part of the world, you will find that you get less power than you expect from the solar cell most of the time.  To maximise output, it needs to be orientated directly at the sun which is tricky as it doesn't stay still!

Thanks Andy. I'll wire this up then. I was wondering if there was some way of knowing the limit to what a rechargeable battery could take from a solar cell or array of cells?

The rule of thumb is that you can charge larger NiMH cells continuously at up to 0.1C, where C is the capacity of the battery, but smaller ones like yours I'd limit that to 0.05C.  So your 200mAH battery could be charged at up to 10mA continuously without damage.

Maximum charge rate you'd get from the battery data sheet and a typical charger for a PP3 size 9V battery charges in 4-5 hours, which is probably around C/4, allowing for the battery not being fully discharged.  Rule of thumb here is you need to put in 120% of what you get out due to losses.

Best thing to do is to put an ammeter in the circuit to get a feel for the sort of charge rate you're getting under different conditions. (By this method I found it's always less than you expect #;¬)

I think he should be concerned about what happens when the battery is fully charged....he could be putting 30-40 mA into the battery, which is OK for a charging current, but a bit high to keep pumping into a fully charged battery.

I'm sure there are standard solutions for this of varying complexity....not being familiar with them, I would suggest a simple arrangement might be some zener diodes with the appropriate power handling capbility. Thoughts?

I agree with AndyGadget it'll be ok.