I am currently working on a project in which I am using piezoelectric material to charge a battery. The voltage output of the piezoelectric material is variable depending on how much mechanical energy is applied to it. I don't want there to be too much voltage or it will fry the battery, and if there is too little, the battery won't charge well. I am using a 9V battery. How can I limit the voltage input to not be too high or too low?