As I don't have the time or money (or knowledge) to design my own electronics I'm taking an existing product and modifying it :-) It runs off three 1.5V watch batteries in series. I'd like to replace this with a single4.5V rechargable battery and charge it from a USB port. Does a 4.5 volt battery exist that would take up a similar amount of space? What sort of cicuit would I need to charge the battery (if any).
Many thanks, Michael