Battery life during activity

During an activity backlight was set at 5% only

Was your backlight always on or did you specify a time-out after which the backlight was turned of?

Keeping your backlight always on, even at only 5%, will use quite a lot of battery.

No, display was on for 8 seconds only, when button has been pressed. I have meant that I reduced display brightness as well to minimum to save battery life. And I was not using it that much, and kept time only on display so it helps with battery life as too many numbers changing and display updating data all the time may reduced battery life. So if I wanted to have i.e. distance, time, HR rate on screen all the time, the battery life would be even worse...

Hmm.. perhaps any CIQ third party apps on your watch? Those are also notorious for battery consumption.

Otherwise I don't know. could be a bug in the firmware. Garmin software has loads of bugs.

No other apps or watch faces, all standard. Never installed 3rd party apps at all :/

Don't trust the numbers in Power Mode and always have a power bank with you on longer activities so you can charge the watch.

Before you charge the watch be sure to set USB Mode to Garmin otherwise the activity will stop...
System -> USB Mode -> Garmin

Here's one of my Ultra Running Settings (78 hours) that I used in a 100K Run. After 14 hours and 26 minutes I had 63% battery left.

Power Mode Setting:
- GPS = GPS + GLONASS
- Music = Disable
- Phone = Disconnect
- Wrist Heart Rate = Turn Off
- Pulse Oximeter = Turn Off
- Map = Disable
- Display = Always On
- Backlight = Turn Off (I used a headlight at night so I didn't need the backlight)
- Accessories = Disconnect

If I do a calculation with the above numbers I get this:

14 hours x 60 minutes + 26 minutes = 866 minutes

866 minutes / 37 (%) = 23,4 minutes per %

23,4 minutes x 100 (%) = 2340 minutes

2340 minutes / 60 minutes = 39 hours

According to the above calculation my 78 hour Power Mode was actually 39 hours... but I don't know for sure if it would've last for 39 hours...

Roland Meijer said:

Map = Disable

that seems to me to be an important point. Or that you don't stay on the map all the time. This consumes a lot of electricity because the screen is constantly updated. I wonder if the OP was constantly on the map screen.

I did not have map screen in this activity, I did not need it, and as I wrote earlier I have had time data field (lowest battery consumption as it updates only once a minute when time changes) on a screen for 99.5% of time. Please read my post first.

And most important issue is why watch is showing that battery is charged for 60 hours of battery life and it only lasts for 20 hours only?

I have Taxtic Delta too. I also have a battery problem.I think it's a firmware issue.

I had a similar situation Saturday.  Went on 19 mile bike ride. Started with 100% battery charge, and an hour and a half the battery level showed 50%. I am using a fenix 6 sapphire. Like you I did nothing special. No map, back light, and no changes except recently updating to latest FW. First time I experienced this.

Interesting, your bettery life seems to be even worse than mine. Surely it will not be precise but if it shows 60 hours of use, it should be close to this number and not three times shorter. Last night I have started an activity with GPS turned off and battery consumption was ok, 110 hours to go and in the morning after 12 hours of use it was showing 100 hours to go. But wrist HR was inactive as I have left it on my desk. Next I am going to do the same but with watch on my hand so wrist HR could stay active all the time. But it looks like it is GPS related thing. 

I am a little bit disappointed that my Delta only lasted 20 hours on full battery charge when it was showing 60 hours of use, while for 99.5% of time it was showing just time of day.