Battery charging with solar panel

This might be a very basic question but: I have a battery holder holding 3 x 1.2 volt rechargeable batteries (so 3.6 volts total) and I want to be able to charge them with a small solar panel (some 100ma 0,5 volt panels, ill use 4 to get around 2 volt during full sun and use a converter? to get the power right.)while the panel also powers the Arduino, the battery will act as a buffer for the night or clouded periods. In my understanding I should power the batteries with a specific voltage but i don't know what that voltage would be. I am sure most of you understand this better then me at this point so I hope someone could let me in on the secret of battery charging.


You might find this link helpful: Many of the cheap solar lights etc. that people get (I was just looking at the cheap ones I got from Costco a while back) simply limit the current into the battery. NiMh and NiCd are funny because (see the link I gave) because, unlike lead acid batteries (car) where you can just float them at 14.3 volts or something like that, you have to handle them a bit different.


2v is not much. If you want to charge a 3.6v battery that sounds like three 1.2v cells in series. IF that is true, then you actually have to charge 1.5 or 1.6 times 3 which is at most 4.8v not 3.6v. That means you have to step up from 2v to 5v which is more than 100 percent, and even at 100 percent you'd have to supply 200ma of current min to charge at 100ma. With the actual setup you'd need probably 300ma from the solar cells.

It works so much better when you have enough cells to reach above the target voltage, and you can even calculate the approximate max power point and maybe size the array for that voltage. For example, since you need about 5v then a 6v panel may just make it, but an 8v panel would be better most likely. With a 12v panel (21v open circuit) you could easily charge even more cells in series, and those panels are more common for reasonable prices. That way you dont lose any charging current either, if the panel is 100ma then you get roughly 100ma, no down sizing required :-)

Oh yeah, another point is that you can not depend on the open circuit voltage to be the target charge voltage because that is the voltage of the panel when it does not put out ANY current. The max power point voltage is the voltage you want to go by, which can be obtained from the data sheet or experimentation measurements.

I could also use a 6v 740ma max panel. How would i wire that