I am interested in designing my own portable phone charger or power bank. I would ideally like to use a Li-Ion battery and be able to have interchangeable male connectors depending on what phone I have at the time (ie iPhone 4s, android, galaxy, etc). Can someone please provide a basic circuit design that would allow me to do this along with an explanation of what is going on. Is there common specs or regulations that most smart phones have when it comes to charging (ie common voltage, current, etc to the phone battery). Please HELP!!