The battery in my Garmin Forerunner 305 got to the point where it wouldn't hold a charge long enough for a 6 mile run. A Google search at first didn't result in much, but finally I came across a couple pages that looked promising. One guy used a phone battery with success, but I don't have any phone batteries lying around.
Garmin Battery Life Expectancy. If maintained correctly, your Garmin device can last several years. However, the battery life of any device reduces over time, and Garmin devices are not exempt. For example, a Garmin Forerunner 235 battery lasts up to 11 hours with GPS tracking and 9 days in SmartWatch mode.
Garmin's new Forerunner 45S debuts with a bevy of new metrics and a fresh new look. Battery life on the new Forerunner 45S in smartwatch mode tops out at 7 days instead of the 9 days promised 5LyqbRa.