A phone not in airplane mode and a high capacity lithium battery are not comparable at all.

Airplane mode is largely pseudoscience/an abundance of caution/solving a different problem than a safety one. There's approximately zero chance of a phone interfering with avionics, especially modern ones, with their very low transmission power.

Supposedly the real reason has always been that mobile network operators don't like the interference high-altitude phones can cause: They're in view of potentially many base stations, some of which might be using the same frequency (which is possible since far-away regular-altitude phones are below the radio horizon and therefore not an issue).

Some evidence for this theory: The "mobile phone ban" is an FCC regulation, not an FAA one, and many (non-US) airlines have been offering on-board microcells for decades without any issues.

Is a lithium battery installed in a device (allowed) comparable with a lithium battery not installed in a device (not allowed)?

That's the steelman version of the parent poster's question.

Danger probably primarily varies with capacity, production quality etc., not form factor (disassembled or assembled into a device), sure.

I'm not saying that current regulations of lithium batteries make sense; my argument is that the actual threat from lithium batteries seems larger than that of devices not in airplane mode (i.e. somewhere around zero).

An installed battery will have its terminals protected and connected to the device.

An uninstalled battery will have its terminals exposed, where they could be accidentally shorted by a stray paperclip, foil wrapper, or zip.

"the real reason has always been that mobile network operators don't like the interference high-altitude phones can cause: They're in view of potentially many base stations". This makes zero sense: the aircraft is on the ground and not moving when the ban is put in place. The ban is removed at altitude, when, you say, that the phones are potentially 'in view' of many 'cell towers' (not base stations). In fact, the plane is essentially a Faraday cage at altitude, and a phone has almost zero chance of connecting to a tower, even shoved into a window pocket.

> The ban is removed at altitude

Not in the US, where it applies throughout the flight.

And at least in Europe, the ban is due to the risk of distraction/disorientation in case of an emergency, in my experience.

> the plane is essentially a Faraday cage at altitude, and a phone has almost zero chance of connecting to a tower

Counterpoint: I have a whole collection of “welcome to <place>, your roaming charges will be <exorbitant>” text messages on my phone from countries I’ve only ever overflown at 30k feet.

This is from flights that do permit in-flight phone usage, but I believe my network has no roaming agreement with the microcell operator, so it keeps scanning and sometimes catches a bidirectional link to some long-range tower. (They’re specifically optimized for that in the North Sea and Atlantic for fishing boats, as far as I know, so for regular modern towers it’s probably less likely, but that separation hasn’t always existed.)

Supporting your counterpoint: I am a cell tower geek and I have an app on my phone that records the cell ID of every tower my phone attaches to. I once flew from Wellington to Auckland on an Airbus 320 and forgot to turn off Airplane mode. Arrived into Auckland and my app had logs around a hundred or so cells my phone managed to attach to. So it can happen. I've also had successful two-way text conversations while still in the air but low enough (e.g. when descending on approach to an airport). Saying a plane is a Faraday cage is a bit extreme but I do acknowledge the steel tubing around you will reduce the signal strength by quite a bit -- but not quite enough to 100% block out the signal it seems.

Plane windows are large compared to radio waves. They'd need to be no more than about 5 cm wide to create a full Faraday cage.

Supporting your counterpoint too: MH370 co-pilot's mobile phone did connect to a cell tower mid-flight after the hijacking. [0]

[0] https://www.dailymail.co.uk/news/article-6839167/MH370-pilot...

There's also the issue that the burden is on proving they're safe--and nobody has a reason to shell out the bucks to do so.

I do agree that interference is quite relevant. The general rule of radio is that you play nice, especially when on a licensed frequency (the cell companies have the licenses for those bands, the users do not), and a phone up high over multiple cell towers is most certainly not playing nice.

Small Li-ion batteries installed in a device are allowed in check luggage.

Loose/spare Li-ion batteries that are not installed in a device, and large batteries over 100-160 Wh are banned in checked luggage.

It's unclear to me what the official requirements are. For example this is from the TSA's site:

"Devices containing lithium metal or lithium ion batteries should be carried in carry-on baggage. Most other consumer electronic devices containing batteries are allowed in carry-on and checked baggage."

Taken literally, this is of course widely ignored. There are also various requirements around spare batteries that do include capacity limits.