Occasionally someone thinks they got a bad battery but so far out of 300+ I haven't had one. Please try this procedure from Garmin.
To recalibrate the Forerunner 405 software:
1) Charge your Forerunner 405 to 100%
2) Remove the Forerunner 405 from the charger
3) Start the timer on your Forerunner 405
4) Allow the Forerunner 405 to completely drain (until it shuts OFF)
5) Charge the Forerunner 405 back to 100%
6) Wait one additional hour (on the charger)
7) Remove Forerunner 405 from the charger
8) Run!