Use the power it produces. Basically, design the entire system so that you break even or a little less. If you design a system that overcharges the battery, you're wasting energy.How do I keep it from overcharging?
It would if you only wanted 3.6 or 5.2 volts and could keep the Zener diode from burning up from the current. When you pack it together with voltage divider resistors to get the right voltage and a current carrying transistor and a heat sink and all that, it is called a "voltage regulator" and it shunts excess voltage off as current to ground. Costs as little a $5 bucks brand new for a cheap Ebay sample or $50 for a KTM product that does the same thing.Wouldn't a Zener keep it from overcharging?