Don't ever allow the battery to discharge completely. You are lucky you can restart phone after charge. I thought the phone will auto switch off after 4% as reported by another user. Inbuilt battery gadgets is best to resume charge after 5% left to avoid difficulty to switch it back on at one go. I have bad experience unable to restart my nexus n fonepad when battery drain to empty. I resort to open back casing n pull off battery connectors n reconnect it to restart it. Don't think you can do such for the XZU.