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Comment by nfjstjstns | Hacker News Reader

Actually, regardless of whether or not smartphones have a removable battery, there's always a small second battery connected to the baseband for emergency purposes. I've confirmed this firsthand by taking apart several models, and you make out small coin cells on most online disassembly documentation as well.

The one in my old Atrix was a good 250mAh.

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I don't mean to call you out, but I'm fairly positive this is wrong. I ran the baseband software team at Apple and worked on the first two generations of iPhones. I'm intimately familiar with that hardware having done reworks by hand myself on the first prototypes and being in all schematic reviews. They definitely didn't have secondary batteries connected to them. While this may exist on some new models of phone, I've never heard of it.

This doesn't even make much sense to me. Assuming you're talking about emergency calls, the application processor is necessary for dialing. You'd need to power the entire phone including the screen to make an emergency call.

Always ? from the few (~10) cellphones I disassembled I remember nothing that looked like a cell.

Also how do you trigger an emergency call ? manufacturer embeds a special firmware so you don't have to boot the main OS and only use energy for the call ?

It could have been located away from view on the other side of the motherboard, as is sometimes the case.

As for what the full intended purpose and potential capabilities of these secondary batteries are, I have no idea since that's not documented. I do know that it's for a whole lot more than an RTC, however.

I stripped them completely. Unless it was a very slim cell that could be confused with an inductor, or hidden into some module (say camera block) ...

I'd love for it to be true though.

What would the baseband transmit in an emergency and how would it know that there was an emergency, if the phone itself had lost power?

When a phone is off or out of power, is the baseband periodically pinging cell towers?

I could see coordinates, e.g. of a lost party, or someone who called 911 but got disconnected by a low battery, that kind of thing. Cell phone pings can be extremely handy for SAR.

If it has a 250mAh battery then that is good for a continuous 1/4W transmission for 1 hour. But I imagine the batter is mainly there for the watchdog and RTC. Which is why you never have to set the clock every time you turn the phone on.

You don't need to set the clock when you turn on the phone because it gets the time from the network when it registers.

Pull your sim and battery. Reinsert the battery and turn the phone on. The time is still there, network or not.

Sure, I said as much in the comment above, and I also know and have tested that at least one common smartphone works just fine with this battery physically removed.

then store your phone in a Faraday cage like you have in your kitchen (probably)

Took me a second to get what you meant. But yes, I do in fact have one. The seals around the door are meant to be airtight rather than RF-tight, though, so it may or may not be good enough, depending on your purposes. If you want your phone to not transmit its position, it may not be good enough, but if you want it to not eavesdrop on you, it probably works.

> meant to be airtight rather than RF-tight

You are thinking about the wrong one.

They mean the microwave, not the fridge/freezer.

I thought he meant aluminum foil.

I've heard about this emergency capability for years, e.g earthquake, tsunami etc...But has anyone ever heard of it actually being activated for real?

Yes. Japan for earthquakes. But no evidence battery is not required.

citation please

(good luck, as this is false)

https://security.stackexchange.com/questions/65382/is-it-pos...

I'm not sure I like your attitude. This is readily available information

That's talking about a backup battery for the real-time clock on the phone. That type of battery is usually not rechargeable and doesn't support enough power to make any sort of call. I've never heard of an emergency backup battery for just the 'baseband' in any phone, but I am open to being proven wrong.

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