Garmin forerunner 45 turning off during run

Try deleting unneeded files from the watch over the US cable - old activities, workouts, courses, language files, etc. If it does not help, I suggest performing the master reset to factory defaults. If still no go, contact the Product Support.

The same thing happens to me when it is at 60% my battery goes off and when I turn it on I see 40% and when I go to charge it I see 60% again

Like trux mentioned, please reach out to your local Garmin Product Support team since your watch is up to date:

Garmin Product Support

It is really annoying. The baterry pack is alway completely loaded before running. In the holidays in Costa Rei I alway run 10 -20 km and the watch doesn't work like I wish.

Good evening, the battery is to blame, I installed the previous software version and it turned off while the battery was at 60%

my mistake for the software version (it's 5.40) and it turned off while the battery was at 60%

I must say that when i turn in clock mode, then all seems to be ok. Battery at 60% and works for 3-4 days. I think that the problem appears when i am in workout mode (running).

I have the same problem, and if you look at this related entry on the forum (https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-45-series/310828/it-turned-off-for-me), others clearly do too - 7 pages of people with similar issues.  I would say it must be something to do with the latest software (or worse some built in obsolensence), but Garmin support don't seem interested - just suggested I return it and pay for a reconditioned one - but if it is software it will just happen again.  Tempted to raise it with a consumer program such as You and Yours on Radio 4.  

Have been having the same issue with my wife's Forerunner 45s (Emailed Garmin customers services but no reply so far) however, my wife noticed this appeared to be linked to the step counter function.

Even though she'd never set a daily step target there appears to be a default target and when reached the watch switched off - and then repeatedly switched off until the following day i.e. the step counter reset itself.

As with all of the previous reports - the battery level was full or good when switched back on.

She has now set the step target to a completely unreachable number and so far the watch has not switched itself off. This does appear to be something that happened after a software update and as such it should be something Garmin can fix with simple patch.