Messenger AWFUL battery life

Not likely to be hardware. You CANNOT test an inReach device indoors. The device automatically does an active "message/mail check" once an hour. The message check will almost certainly fail indoors because the unit cannot connect to the Iridium satellite network. But once the unit starts a mail check, it (re)tries very, very hard to complete it. Forever or until the battery dies. The Iridium modem is a pretty big battery hog. These repeated retries run the battery down quickly.

The M2 has a status line indicator (up/down arrows) which tells you the iR modem is on. I don't think the M1 has this feature.

Very interesting! 

So in that case leaving the bluetooth on and the device connected to a phone with internet should reduce the battery consumption indoors? 

If I understand right, the mail checks will happen via 5g and not iridium and should not be as costly?

Possibly, but only if the MESSenger app is involved in the environment. Personally, I treat iR devices as emergency SATELLITE communication devices. I stay as far away from the MESSenger app (and all the other Garmin apps) as possible. So I know very little about what or might not work with any given device/phone/app combination. IMO, if I  have a data connection, why not just use my phone. Why drag the inReach device into it at all

I only use the inReach device for messaging when I am out of cellular range.

So maybe I'm doing something non-standard, but I'm going on a multi-week trip. It will include a lot of wilderness, but also staying in shelters/hotels. 

I really want to avoid a situation where I lose all of my battery between 10pm and 8am next morning, just because I slept under a roof one night. 

Should I switch the messenger off for such cases? What would your approach be?

If you don´t need the messenger during 10pm/8am, I would switch it off to save battery. From the manual: "The device also checks for new messages every hour." If you have bad or no satellite connection, it will try to get a good connection again and again and that will burn a lot of battery (if I am not wrong).

You should ALWAYS turn iR devices off when indoors. You should also be careful not to bury the device in your pack, or otherwise interfere (for long periods of time) with a decent sky view.

If you have cell data coverage (not just SMS or voice, but DATA), I believe that the MESSenger app will continue to work - even if the device is off. Again, I never use this, so YMMV.

Indeed, it does seem to a problem with the fact that indoors without bluetooth the Messenger does not have the ability to send either via IR or via the regular data connectivity. 

I've repeated the experiment indoors but *with* a phone connection and I lost 3% over 10h, which is close to the 7%/day, which I have been reading about.

inReach Messenger is just stuck in a deathloop of checks.

An idea for the software engineers at Garmin - could you please implement it better? If there is no connectivity the Messenger should *not* check in a loop, like a maniac. There should be some sort of event-driven structure, that re-enables the message checks once a form of connectivity is re-established (be it phone or IR). The checks should also implement exponential backoffs of sorts. As in, if I failed 250 times in the last 3 minutes, there really isn't a good motivation to try 250 times in the next 3 minutes. I could just try 5 times or just once. And maybe messaging along the lines "no connectivity, will try again in x minutes", or "no connectivity, will try again once connectivity is back", so that people are aware of the situation.

You can submit suggestions here: https://www.garmin.com/en-US/forms/ideas/

However, you can't have it both ways. The only way an iR device knows it HAS Iridium connectivity is to try to connect. The point of the aggressive retry is that this is an emergency satellite communication device. This retry applies to the mail checks. It also applies to things like sending a track point, or sending a message. It is not intended for use indoors, or buried in your pack, or under water, or whatever. This "problem" is well known. All iR devices have behaved this way since inception. The product managers at Garmin have apparently decided that this implementation is the least of the available evils.

IMO, if you insist on using the device in other than the intended environment, you get what you get.

Btw: depending on how much weight I want to carry on a hike trip, I normally have a power bank and/or a foldable solar panel (55x25cm, 300g) with me. In full sun I have gained 12%/hr during a break for my Messenger with the solar panel.

Btw: I've had very good experiences with the messenger's battery consumption so far (the battery lasts about 50% longer than that of a Mini 2) but for my needs, the Mini 2 is the better choice because it has significantly more functionality,  great mount solutions, is very handy, the display is more usable (or better to say really usable) as the messenger display. The messenger has a better antenna and the better battery life. But I hate to be dependent on the cell phone/messenger app in order to be able to use the full range of functions of the messenger (full ir weather view...).

It would be a great thing, if the ir widget on the Garmin wearables would get more funtionality (order/view ir weather etc.); this would really upgrade the messenger (for me). But messenger and mini2 are made for a different target audience.

If you lost the orientation and your cell phone is out of battery (and the messenger has not enough battery left to charge it a bit (or you forgot the cable), it is a pain to enter messages on the messenger and you have only tracback on the messenger, which isn´t very (let`s say) "comfortable". Thatfore I love the mini 2 because of its navigation functionality and bigger display! The Mini2 is definitely worth the extra price (for me).