946. How would you construct and wire a battery recharger using solar panels as a voltage source? — JW, Kingston, Ontario
First, you would need to put enough solar panels in series to develop a voltage greater than that of your battery. For example, to recharge a 1.5 volt battery, you would probably have to attach three or four simple solar cells in series because each one only provides a current passing through it with about 0.5 volts of voltage rise. Having assembled enough solar cells, you should then attach the positive output terminal of the solar cell chain to the positive terminal of your battery and attach the negative output terminal of the solar cell chain to the negative terminal of your battery. When you put the solar cells in the light, they will begin to push electric current backward through the battery and the battery will recharge. Whenever you send current backward through a battery, its electrochemical reactions can run backward and it can recharge to some extent. Unfortunately, some batteries recharge more effectively than others—the bad ones just turn the recharging energy into thermal energy. The only real subtlety in this business is in stopping the charging when the battery is fully recharged. You should check the battery voltage periodically and when it's close to the voltage of a new battery, it probably can't take any more charging.