I have some 600mAh NiCd batteries from an old RC battery pack. I'd like to make a solar garden light with one, and I have a 2V 200mA solar panel for the project. Its open-circuit voltage is 2.4 volts.

I'm not sure how to safely charge the battery from the solar panel. My first idea was to use a 1N4005, but its forward voltage is just under 0.8 volts, which would charge the battery to a maximum of 1.6 volts (if I understand correctly).

What else could I do to charge the battery? I'm not sure how much capacity would be lost by charging it up to a lower voltage, but that probably won't be an issue.

 

Is the battery voltage 1.2V nominal?

 

Is the battery voltage 1.2V nominal?

Yes

 

The simplest way to charge the battery is to add a resistor and diode in series to limit the maximum charge current (full sunlight) to about 1/10th the battery mAh or 60mA.
To minimize the voltage drop from the diode, you could use a Schottky diode, such as a 1N5817, which will only drop about 0.2-0.3V at the charging current.

 

The simplest way to charge the battery is to add a resistor and diode in series to limit the maximum charge current (full sunlight) to about 1/10th the battery mAh or 60mA.
To minimize the voltage drop from the diode, you could use a Schottky diode, such as a 1N5817, which will only drop about 0.2-0.3V at the charging current.

That was the general idea initially, but wouldn't that potentially allow the battery voltage to go way too high? Or would that not be an issue, because it can't charge fast enough to get that high in the first place before it's discharged?

 

That was the general idea initially, but wouldn't that potentially allow the battery voltage to go way too high? Or would that not be an issue, because it can't charge fast enough to get that high in the first place before it's discharged?

They use nicads in these applications because they can tolerate a constant trickle current without damage. So as long as the solar panel is “small” compared to the battery there’s nothing to worry about. The panel cannot force enough current through the battery, which is resisting more charge, to damage the nicad. It’s typical for the panel open circuit voltage to be about double the battery nominal voltage.

Solar light batteries are usually only 300-600mAh capacity. If your battery has higher capacity than the one originally in the solar light you should be fine.

 

They use nicads in these applications because they can tolerate a constant trickle current without damage. So as long as the solar panel is “small” compared to the battery there’s nothing to worry about. The panel cannot force enough current through the battery, which is resisting more charge, to damage the nicad. It’s typical for the panel open circuit voltage to be about double the battery nominal voltage.

Solar light batteries are usually only 300-600mAh capacity. If your battery has higher capacity than the one originally in the solar light you should be fine.

I'm looking to build a light from scratch. I might be able to dig up a light with a similar battery and solar panel and see how it works.

 

I'm looking to build a light from scratch. I might be able to dig up a light with a similar battery and solar panel and see how it works.

Ah, I missed some of your details. Just use the Schottky blocking diode as already recommended and you’ll be fine. The current of the panel against a charged battery will be just a fraction of that short-circuit current.

 

Or would that not be an issue, because it can't charge fast enough to get that high in the first place before it's discharged?

Yes.
At that low current, the battery charging voltage will never get too high.