I cannot be entire sure about this but it seemed to happen this way:

When the iPhone 5, 6, or 7 was out of power, I could plug it into my car's USB charger or a portable USB battery charger for 10, even 15 minutes, before the phone is willing to start up. A 15 minute wait, can be a bit too much -- if it were 2, 3 minutes, it would be more tolerable, but a good 15 minute, is too much.

But I noticed sometimes when I plug in the lightning jack for a charge, the iPhone will start immediately.

Is it due to some charger are supplying 5W, while some are supplying 10 or 12W?

I suspect iPhone might be doing some calculation: if possible power consumption is less than power input (10W), then be worry free and power up immediately. However, if possible power consumption is more than the input (5W), then don't power up first, accumulate first before powering up (and it turned out to be 15 minutes usually).

Does someone know for sure how this works?

P.S. Decemeber 2016: So today, I tried out an iPhone 5S, and saw that it was 1%, but too late, about 1-2 seconds after I plugged it in, it turned itself off. And thinking that it was a 39W 2 port charger with QC 3.0 (an Anker charger used in the car), I thought it should start up almost immediately. Not so, it took close to 15 minutes before it turned itself on, and I didn't look what battery level it was when it started up, but 10 minutes or so after the start up, I looked, and it was at 21%. So it looked like it could have turned itself on sooner.

  • can the down vote righteous police post a comment for the reason before they down vote, or else it seems like they just come here having fun – nopole Nov 23 '16 at 20:37

Many factors - dead phone/0% battery isn't an entirely fixed point, depending on battery age/health, power requirements, temperature (winter cold...), calibration state etc it can mean anything from a calculated shutdown by the device at a level before excessive wear starts to occur, to the phone just suddenly dying at a point where there's still supposed to be juice left and it's not so much protecting itself, more like a blackout. During discharge the voltage continuously decreases, which is what the phone keeps tabs on and cuts off at a safe level, but as it loses charge the max amperage/wattage also gives way, leading to further (short) voltage drops when it overextends itself.

As an example, if you're running low on battery and decide to take a bunch of photos before your phone dies, the high power draw will likely mean the phone shuts off a lot sooner than if you kept it in your pocket on standby. But that means that in the latter scenario... once it's dead, it'll be deader, and it will take longer to bring up to a healthy enough voltage to boot - turning on, mind you, is a lot more power intensive than just staying alive once on. If the phone tries to boot before enough power is available it will black out again.

Input power obviously plays a large role here, just as you say, but it's not so much the phone calculating anything fancy or being tactical. Plug an iPhone with a worn-out battery into a semi-glitchy cable and it will happily bootloop for weeks on end instead of waiting and building up enough charge to actually manage to power on and chill out.

You're on the right track, key thing being that the brick can't power the phone directly, it's feeding the battery which feeds the phone, again a battery at low charge is flaky so even plugged in there needs to be a buffer, and once you get going that where you get the non-linear response. Say the phone needs 2W and it's getting 4W effective from a 5W brick, so 2W left to build charge. Double it to 8W and the surplus triples.

  • do you mean then my best bet will be always try to use a 12W charger? I was looking into whether a 39W QC 3.0 2-port charger could make the iPhone start up within a few seconds every time... but also worried about whether QC 3.0 might be too much for the iPhone (damage or overheat the battery) – nopole Nov 24 '16 at 3:15
  • @太極者無極而生 as long as the charger can output 5V the max amperage is irrelevant, the phone handles the power draw and will only draw as much as it can use, which is less than 12W. So it wouldn't hurt, but it wouldn't help. – tolgraven Nov 24 '16 at 3:25
  • @太極者無極而生 but yeah, 10/12W is faster and perfectly safe, if you have that available there's no point using the 5W ones. But in general the best way to take care of li-ion batteries is to not let them drain if you can help it. Like I said they turn off before proper damage occurs obviously but still going 100-0-100 will cause a_lot_ more wear than 100-50-100-50-100. – tolgraven Nov 24 '16 at 3:39
  • So it is the exact opposite of NiCad batteries... and similar to a car -- never let the fuel tank drain to 0 – nopole Nov 24 '16 at 11:19

In fact the iPhone 6, 6s and 7 support being charged faster, as they are not limited to 1000 mA as smaller iPhones (iPhone 5s and earlier, iPhone SE).

So yes, you should see a faster boot with the newer models.

  • 1
    I think I experience this even with the iPhone 5. Also... 5V and 5W, so isn't that 1A? ( by the formula P = VI ) – nopole Nov 23 '16 at 20:50
  • Have you measured your car's USB port output? If you use a quality usb port you can plug into your car, then you should see the same results when its output is labeled as 2100 mAh. – oa- Nov 24 '16 at 15:18
  • not sure how to measure my car's USB... but when I charge using my car's built-in USB socket, then it feels really slow... not like a 10W or 12W charger or the Anker 4 port charger, which is quite fast – nopole Nov 24 '16 at 17:28
  • by the way my question is not about older model vs newer model (of iPhone), it was about how really do iPhone decide how soon it will decide it is confident enough to start booting up – nopole Nov 24 '16 at 17:29

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