I have found with solar chargers that if the weather is bad, the voltage coming from the solar cells is half what it would be on a sunny day. So I want to use the cells that come with solar garden lamps in series to generate around 20v on a sunny day. I want to also be able to charge only one 1.2v battery if required. I assume the batteries will have to be lined up in parallel while the solar cells will be in series. In order to make sure the voltage does not exceed the upper charging limit, something will be needed to regulate this based on a setting. Any advice on what I will need in the circuit?