Forerunner battery dead?

Before yesterday's run I'd changed my Forerunner 100 overnight but half way through it showed 'Low Battery'. I put it down to the cold, and perhaps poor connection with the charging clip. Last night I made sure the clip was well positioned and by this morning charging was complete. I disconnected the charger and within an hour I heard the beep of 'Low Battery' again even though the watch was on standby. The battery indicator is low, of course. I've tried rebooting the watch and it still shows an empty batter but no inclination to charge. Maybe it's the end of the line for it? Although batteries are available online I wonder how easy it will be to get a good outcome. These things aren't built to be repaired.