What is one problem William encounters, after inventing the windmill, that he uses curiosity and ingenuity to solve?

William's initial windmill was successful, but it was not perfect. When there was no wind, for example, the electricity stopped, and so William began to look for a battery. To earn money to purchase one, William began to use his windmill for other purposes, like charging the neighborhood's phones for a price. Phones, however, needed more voltage than a lightbulb, so he had to build a transformer to increase the voltage. This worked well, and soon the people of the neighborhood came to his windmill to charge their phones.