The battery is a clear image of stored energy, so it is read as how charged or drained you feel. A dead one is traditionally taken as a call for rest and renewal.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.