battery life

Hi, also i have noticed an increased battery drain after update, before was around 2%hour, now like 10%, as your

Your "backlight timeout" setting is "stay on." That means if it comes on it never goes off. Maybe that uses up the battery faster?

The "backlight timeout" setting is "stay on". yes but garmin says that with this setting the battery lasts 24 to 26 hours. you understand that between 24 hours and 10 hours there is a big difference...

Where does Garmin say that? In the 840 manual it indicates "up to 26 hours" which means 26 hours is the maximum, but depending on usage (backlight, backlight brightness, temperature, etc.) it could be less. Up to 26 hours is not a guarantee of 26 hours. Also, 7 degrees colder will shorten battery life noticeably compared to warmer environments. There is a battery saver mode also. 

Garmin instructions are available at the following link.

https://support.garmin.com/en-GB/?faq=in6koTl7dd0dPELNx7QKi9&productID=798777&tab=topics&topicTag=region_batterytips

My configuration is like "MID" but with 3 sensors (so lower than "DEMANDING"). In this period the only difference is the temperature which is around 8/10 degrees.

at the following link:

https://www.garmin.com/it-IT/p/798777#overview

Garmin declares:

EXTENDED BATTERY LIFE
The Edge® 840 bike computer has a battery life of up to 26 hours in intense mode and up to 42 hours in power saving mode.

My experience with batteries for the lights I use when cycling and for my 520 Edge cycling computer (I haven't had the 840 long enough to experience large differences in temperature) is that the batteries last maybe half as long at 10 deg C versus 30 deg C. The temperatures you mention 8/10 degrees are on the cold side and could explain the lower battery life.

I also thought about the temperature but in the link I attached the temperature range is indicated from 10 to 40 degrees so 8 degrees are not much less than the 10 indicated.
Also my daughter (we usually go out together) has been using a Bryton 750se for a year and the current consumption is not different from the summer ones...

The comparison to Bryton 750se is interesting ... but different batteries may perform differently, so unclear how to interpret. All my devices' batteries last much less in 10 deg versus 30 deg (or even 35 deg which we get here in the summer). I think it can be different for different types of batteries. Garmin says "UP TO 26 hours" but clearly this can mean less than 26 hours depending on how it is used, temperature, etc. You can always elect to replace the battery (Garmin has instructions) if you believe it is faulty or worn out. Batteries are "consumables," in other words each time you use it and recharge it, the chemical changes reduce its life slightly. I have an EDGE 520 I have been using since 2015 and its battery lasts about half as long as it did when new; it has been on for about 3000 hours total, cumulatively, about 1500 rides of about 2 hours each. For that reason I replaced it just recently (with the 840).

true that the garmin and bryton batteries can be different. but the technology is definitely the same so they should react similarly to temperatures.

If you look at the garmin table between the MID configuration and the DEMANDIG configuration there is little difference; garmin declares a TEMPERATURE RANGE of 10/40 degrees, so the differences should not be more than 50% (next round I'll keep it in my pocket and we'll see).

My 840 is new and has less than 20 hours of use so I don't think it's a battery problem.

I thought it was a configuration problem.

However, I have a doubt that the consumption could also be due to the new Firmware versions (too heavy for the device / even after the first installation it continues to configure itself and/or download additional components).

Sometimes batteries have a "knee" in their performance curves where performance starts to degrade rapidly.  I realized after reviewing our "conversation" that the average temperature of my rides (I have only 5 rides, 2 hours each, for 10 hours total on my new 840) has been about 15 degrees C, which is somewhat higher than I realized and higher than yours. After those 5 rides (10 hours), my battery was at 80% and I recharged it. Maybe I should have let it continue uncharged to see how long it would last. But my use is different from yours, I think: I have no sensors attached (no power, no cadence etc.), I am using just the 840 by itself. Yours sounds worse, perhaps you should make a warranty claim while it is still new. Although I'm not sure what the warranty coverage is for the battery, sometimes batteries are excepted, one would have to read the small print regarding the battery. My expectation is that the words "up to 26 hours" were used carefully to cover a wide range of conditions. It would be easy to test: simply turn it on and configure it to never sleep and measure how long it lasts? I don't understand the comment about Firmware: mine downloaded updates to the software and to the maps right away when I connected it to the computer and I haven't noticed any more of that activity since then. You should not have to worry much about "configuration," you should be able to just turn it on and use it with most of the default settings and get the advertised battery life. I think, however, use of the backlight can dramatically shorten how long these batteries last, based on some experiments I did with my old Edge 520. If you are riding during the day, that should not be an issue.