63johnw may well be right. You could also have a look at what's using your battery, although the built-in monitor (in Settings|About phone) only shows percentages. If OS services or screen show around 50%, that's normal. If something's higher, that indicates a problem. If it's part of the system doing it, a reset may help (put the phone on charge when you do this to avoid a power failure half way through the process). If it's something else, it may be a rogue app.
