Solar NiCad charging

[Resilient]

I am wanting to set up a circuit to charge two batteries via a solar cell and I can't find a whole lot of good information on it.

I found this:

http://www.solorb.com/elect/solarcirc/aacharge/index.html

but I don't quite understand how that doesn't overcharge the batteries. A fully charged NiCad is around 1.5 volts (so two would be 3v), but 7-solar cells will give 3.5 volts. So left alone wouldent that over charge it?

I also assume the resistance of the battery while charging increases, but I cant find anything that talks about the charging characteristics, only discharging.

I have done some poking around but all I can find is information on how to charge the batteries assuming you have a charger. I want to know the theory behind the charging/charger to make my own solar charger.

Thanks!

[lnxfergy]

The current is so low, its effectively a trickle-charger. I think that most NiCD/NiMH cells can pretty much trickle-charge for a long time without adverse effects.

-Fergs

[bodhibuilder]

I read so too; when you trickle charge a NiCd battery, you could keep it plugged/charging for a long time without harming the cell.
The trickle charge delivery recommended is at the highest a 10th ampere an hour of the battery's ampere.

P.S - I think a NiCd cell's voltage is 1.2 V.

[Resilient]

I guess the current is what I am a little confused about.

I assume the battery acts as a resistor as it is being charged (?) but I cant find anything that talks about the behavior of a battery as it is being charged.

So I know to trickle charge, the current should be less than C/10. What I would like is some way to actually estimate what the current is, short of hooking up a amp meter after the circuit is built.

I also don't see anything that says what voltage my source should be to charge a nicad cell. I know it has got to be more than 1.2v, but how much more?

[lnxfergy]

With a solar charger, the current is a function of the panels themselves -- and furthermore, a function of the amount of sunlight falling on them.

-Fergs