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Comment by Scene_Cast2 | Hacker News Reader

Would anyone have details on how Pixels can be found even when out of battery?

"Out of battery" isn't really what we think it means. The phone shuts down (and refuses to start back up) when it still has quite a bit of battery left. If it truly allowed the battery to go flat, that would damage the battery, and you'd have a really hard time charging it again.

The amount of juice your entire phone needs to run for, say, a half hour can probably power just the bluetooth LE chipset for occasional wakeups for beacon-sending for... weeks? Months? (Completely made up time spans, but the BTLE chipset really does need a tiny amount of power for infrequent wakesups compared to the entire phone.) Certainly this can't last forever: if your phone is missing for long enough, the battery will eventually run down to a point where the battery controller won't let even the BTLE chipset draw any more power.

This definitely requires hardware, firmware, and software support in order to set up, which is why only Pixel 8 is supported so far. Clearly when they were designing the Pixel 8 hardware, they already knew this feature was in the pipeline, and wanted to be able to launch with a supported phone.

That's how the iPhone does it as well, btw!

As far as I remember there's a separate microcontroller that controls "Find my phone" beacons without having to boot up iOS on the main application processor.

> That's how the iPhone does it as well, btw!

That explaines why my work iPhone has an empty battery after 2 weeks of being turned off. Good job Apple. /s

The CR2032 that AirTags run on for over a year has about 200mAhr of capacity. That's down under 5% of a typical 5000mAhr phone battery. So 1% of the contingency low battery capacity could probably run the BLE stuff for a few months if needed.

But also keep in mind that lipo batteries in phones self-discharge tens of times faster than CR2032s.

I'm guessing this works exactly the same way as Apple's "Findable After Power Off" feature [1]. If the phone "dies" the battery still has some reserve to display the "charging needed" screen when the power button is pushed, along with sending location. Additionally, it can use the Find My network [2] where other devices that are powered on can relay the proximity device's location using their own location.

1. https://www.theverge.com/22697218/iphone-apple-ios-15-find-m... 2. https://developer.apple.com/find-my/

Same question here. I have to assume this would either work by the device sending out a last gasp "I'm about to die" GPS location call home (for finding it when it's far away), or else there's a mostly passive RFID-like thing in the phone that makes it findable by nearby devices (for finding it when it's somewhere in the house/car/office with you).

How much energy would be needed to power a minimal wireless network node, of some sufficient capability, for let's say 1 day? Any IoT devs out there?

If it is using a similar technology to AirTags, then it could broadcast signal for a long time on remaining battery. Even Bluetooth can do months on coin cell battery.

AirTags get about 12 months on a CR2032 - which is about 200mAhr

Moto g30 without cell modem but with enabled WiFi easily gets two weeks of the runtime without much of display time.

My RAZR MAXX could sit for months on the one charge (with already busted battery) without any cell activity.

So if you don't need the display and a hungry wireless tech to keep on constantly you can have enough juice for days, even when the phone is no longer in the power profile to have a full run.

I read that they use bluetooth beaconing for this. A simple bluetooth beacon like a tile can run for months with minimal power. The phone isn't actually 'dead dead' it's still powering the bluetooth beacon.

It says they have specialized hardware so I imagine they use some ultra-low power chips and perhaps even a small extra battery?

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