It seems after a while playing with the forerunner 45 settings am able to increase - get really close to the advertised battery times (both gps as well as smartwatch).
So , if you have checked on how to improve the battery life on your watch and you found every time the same information provided ,am here to suggest you something that as i have seen no one yet has written.
The first thing which is mentioned everywhere is the GPS - use ''GPS only'' on your workouts.
if you need to use GPS + another satellite based on your location keep in mind that it will reduce your battery life significantly.
Other satellite's are used if there are a lot of clouds , humidity or bad weather in general for better GPS accuracy.
Secondly every widget that updates it self every few minutes - hours will also drain your battery.
Removing the weather widget for example will increase your battery life.
I currently have seen massive improvement on battery life using as widgets only the following :
Health stats , My day , Last Run , Garmin Coach.
Furthermore when it comes to Backlight its very important to disable it During the activity , Backlight is one of the reasons your battery is decreasing even when you don't workout.
By default backlight its on during and not during activities, this means every time you press a button the backlight is on but you don't see any difference except if its during night time.
So , having the backlight on during an activity is useless and you just reduce your battery. The only reason you need a backlight during an activity is only if you run AT NIGHT. Running in the morning - evening does not require backlight on since our watch display can be seen better if its in the sun.
Last but not least , auto laps, sounds , vibrate alerts , your watch Bluetooth , and how often you sync it with garmin connect affects your battery.
If you don't absolutely need anything from above just disable it.
Lastly , a most effective to sync your device is by using the garmin express app for computers by just plugin the device on the computer because if you sync with bluetooth to your phone as i said previously drains battery.
Sorry for the long paragraph , but i did not seen most of the above explained on why and when to turn them of. I added some of the things i found useful for the battery life which were not mentioned somewhere as far as i know.
I hope i helped some of you. If you need anything let me know.