Libra H2O suddenly discharging rapidly

My Libra H2O has suddenly gone in the last few days from requiring a charge every ten days or so - I use it quite a lot - to needing a charge almost daily. It's been running the same firmware (4.35.20400) for months, and I haven't put any new books on it for several weeks. The sleep cover still works - I can see it come out of sleep when I open the cover - and I haven't altered the screen brightness, which I run at around 9-10%.

Any suggestions on where to look for a solution?