Newby here. I bought a pair of 100w panels over a year ago to play with. I want an emergency USB backup charging system for phones. I see USB ports on some of the solar charge controllers, but someone wrote they still need a battery to work. Is that true? Or can I use panel/controller to charge USB devices without a battery attached to the controller? I'm looking at the Epever Triron controllers with plans to expand to include batteries in the future. Thanks for your input.