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I've believed this since 1994 - that if you really want to turn off a cell phone, you need to take the battery out.

But why? What is a good technical reason to keep spending battery charge and occupying tower bandwidth despite the user having shut down?

Please, if you could refer to sources, alongside your provided reasons, I would love it very much!

Clarifications: I am aware of locking your screen, which (depending on your settings) can make the screen turn off. My question is clearly about shut down. Not locking screen, not screen locking itself after a minute due to inactivity... I am talking about pressing the power button and choosing "shut down" (Nokia) or "Power Off" (Android)

It's very hard to find discussion about this topic because of millions of people raising all sorts of power issues on their phones but I really don't know of anyone who doesn't believe the baseband processor remains on even when you shutdown the device, including experts in the field.

For example, here's a guy who appears to know a thing or two about Android phones (look at the most popular answer to the question): https://android.stackexchange.com/questions/215736/what-is-the-fastest-way-to-shutdown-unrooted-android-phone

Another example: https://www.zdnet.com/article/fbi-taps-cell-phone-mic-as-eavesdropping-tool/ "Some handsets can't be fully powered down without removing the battery" ...Again, I know these aren't design documents by Qualcomm but people seem to be adamant that it's a thing! Would be lovely if they cited their sources :D

Todd Messenger
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  • If somebody is trying to call you, how does the network know which base station is within range to tell your phone to ring? .... Sure if you don't want to be found, switch the phone off as Bimpelrekkie suggests –  Aug 18 '20 at 12:01
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    obviously I am asking about the BB on when user had shut down... I wonder why am I getting so much flak for this? I've heard it from my neighbor who helped design the SMS system... I don't have access or expertise to look into qualcomm's design documents but am I really talking about something people never heard before? :D – Todd Messenger Aug 18 '20 at 12:06
  • Nobody's giving you flak. It may be that you haven't clarified what you want to know, or it may be that you aren't understanding something. Being able to receive calls is a **good technical reason** for burning BB and bursts of RF power. It applies when the phone appears to be off, but not when the phone is switched off (long press and select "Power Off"). What's unclear about those cases? –  Aug 18 '20 at 12:15
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    being able to receive calls when the user has indicated to shut down? I don't know why people are talking about the state when the phone has locked screen. I didn't mention that or mean that. I will state it clearly in the question – Todd Messenger Aug 18 '20 at 12:20
  • Folks are talking about locked because they know that many typical users don't know the difference between "locked" and "off." – JRE Aug 18 '20 at 12:29
  • As noted by others: Modes are On / Locked = Standby / Off. Communications only happens in the 1st two modes unless some special software has been loaded. HOWEVER - if the battery is inserted then SOME circuitry is powered up. The phone is more vulnerable to immersion damage. If a phone is ever dropped into liquid then removing the battery ASASAP should be the first step (where possible). || – Russell McMahon Aug 18 '20 at 12:35
  • I'm just going to come out and say it, if the only proof you have that phones continue to transmit and receive data after completing the power off process (equivalent to a computers shutdown and halt process) is "some guy" on some message board, or claims to have had a hand in the SMS network (not how that works, btw), I'm going to have to say you don't have much proof they do. Meters to test this are cheap and easy to use. Yes, malware that fakes off states is a thing. But if it is still doing a thing a simple amp meter in line with the battery would show. It. None of mine have...ever. So... – Tank R. Jun 10 '23 at 02:59

2 Answers2

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that if you really want to turn off a cell phone, you need to take the battery out

No, that's not true, all the phones I know can be fully switched off, usually by holding down the power button. You're confusing switching the phone off with the standby mode of the phone where it looks like it is off (display off, no lights flashing) but it can still receive a call. In this state the battery should not be drained that much so the phone can stay for months or more in this state.

For receiving a call the "cellular communication part" (that includes the baseband processor) of the phone needs to be activated on a regular basis to check if someone is trying to call you. In this state the battery will be drained such that the phone can stay in this state for a few days or more (simple "dumb phones"can last for weeks) in this state.

All phones are shipped in the "power off" state and that makes sense as otherwise the customer would receive a phone with a drained battery. That's bad for the battery! Also: a new phone does not contain a SIM card so it cannot even fully connect to a network so there's no point to have the baseband processor activated.

TLDR: you're confusing "power off" with "standby" which are different things. So the batteries do not need to be removed. Switching the power off (you might need to hold a button or use a menu) will stop all communication.

Bimpelrekkie
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    It depends whether you have an ordinary commercial phone, or whether some government or military body has been at your phone and installed spyware. Then, then phone can look like it's been turned off fully, but still be sending audio or GPS logs back to big brother. Taking the battery out or putting in a metal box are then the only ways to stop it communicating. – Neil_UK Aug 18 '20 at 11:54
  • @Neil_UK - tinfoil headgear helps in those situations too ... – brhans Aug 18 '20 at 11:58
  • @Neil_UK Of course my answer only relates to non-modified phones such as the average punter would buy/own. But I agree with your point. – Bimpelrekkie Aug 18 '20 at 11:58
  • I wasn't able to understand your examples, you say that in standby, one can receive calls and stay in standby for months. Then, you say that "for receiving a call.. the BB needs to be activated... the phone can stay in this state for a few days or more". So far, I think about 3 states: Battery out, shut down and locked. Battey out is obvious, shut down is when I choose to shut down from the power menu and locked is when I quickly press the power button to turn off the screen but keep everything else running... Is standby one of those or a 4'th state? – Todd Messenger Aug 18 '20 at 11:59
  • *in standby, one can receive calls and stay in standby for months* Only on a **very simple** phone, not a smartphone! From the baseband's point of view "locked" and "standby" **are the same thing!!! The "locked" function only switches off the display and demands a pin or whatever to get the phone out of standby. So no 4th state exists, only 3: 1) off 2) standby 3) active (making a call). Locked is the same as 2) – Bimpelrekkie Aug 18 '20 at 12:02
  • dude I appreciate you spending time trying to educate me! I am humbled and grateful! Are you saying that "standby" is a mode that smartphones don't usually have, it's more of an old-phone thing? And that mode is somewhere between "locked" (can receive calls, can last for a week) and "power off"? If so, I wish I knew that back in my Nokia days, I only ever needed to receive calls! – Todd Messenger Aug 18 '20 at 12:11
  • *Are you saying that "standby" is a mode that smartphones don't usually have,* Think about that, in what mode will your phone be when it is sitting there in your pocket, ready to receive a call? If there was no "standby" then the phone would either need to be off (and cannot receive any calls) or it would be busy in a call. That would only work if you power up the phone seconds before someone calls you. Do you always know when you get a call? I don't. I also think you're **overthinking** this whole thing. There are only 3 states as I mentioned. – Bimpelrekkie Aug 18 '20 at 13:49
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Regarding a phone which is off - the state it ends up with after you press down the power button a couple of seconds:

Most kind of electronics are powered by voltage regulators. The whole concept of digital systems assume a fixed voltage level, so you can't power digital electronics directly from an analog battery with its ever-changing voltage. You need a regulator in between and it needs to be powered.

This means that the voltage regulator electronics are always on, or at least sitting in some effective sleep mode, until you press the power button. Same thing with battery charger electronics and Li/Ion supervisory circuits.

I doubt that anyone would design the system so that a SoC or radio chip is powered when the phone is supposed to be off, because that would drain the battery relatively quick.

More importantly, the phone isn't allowed to send radio signals when it is off - I'm pretty sure that would violate radio regulations regarding spurious emissions and it would be easy to detect with a spectrum analyser.

When the phone is in standby, it does of course send data to the nearest base station on regular intervals, or otherwise it wouldn't be possible to know where in the world the phone is located. Which in turn means we wouldn't be able to receive calls.

If more than one base station picks up the signal, it is possible to "triangulate" the phone - determining its rough location based on receiver signal strength from each base station that picked up the signal.

Lundin
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  • *This means that the voltage regulator electronics are always on* It depends, some voltage regulators have multiple modes: 1) off/disable (**zero** power usage) 2) low current mode (low power usage) 3) normal mode (when the circuits it feeds are highly active). There are voltage regulators which have such a low "standby current" that they can be left on standby all the time. A **disable** function on a voltage regulator is very useful, that way there is no need to add a (MOSFET) switch in series with the battery, a volt. reg. will already have a MOSFET so then it can do two functions. – Bimpelrekkie Aug 18 '20 at 13:55