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.