I cannot be entirely 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 off 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 Nov 23, 2016 at 20:37

3 Answers 3


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) Nov 24, 2016 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, 2016 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, 2016 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 Nov 24, 2016 at 11:19

The reason the more powerful QC 3.0 charger wasn't charging your iPhone any faster is because iPhones are not compatible with QC. iPhones use USB-BC, USB-PD, and Apple's own BrickID for power.

USB-BC and QC both use the USB D+/D- pins for the power brick to advertise how much power the brick can supply. But they do so in ways that are not compatible with each other.

Apple devices will check a USB-A port for a given set of DC voltages on the data pins, the power brick has no 2-way communications with devices. Based on the voltages detected it will know how much current is safe to draw. The voltages on the data pins may differ but power is always delivered at 5 volts. USB-BC and BrickID both use this method of advertising available current with voltages on the data pins, BrickID extends this set of voltage combinations to allow advertising available power above what USB-BC specifies.

QC uses some 2-way communication to tell a compatible device how much current it can supply, and at what voltage. A QC device can then request a voltage between 3.6 and 22 volts, but always starts out with a USB compatible 5 volts. If there's no communication from the device then the QC power brick stays at 5 volts.

Apple devices will assume that the power brick it is plugged into can only provide 1 amp if it can't communicate with the power brick. That's how USB-BC was designed and QC power bricks are made to allow for this. Since QC is using the data pins for 2-way communications it can't advertise that it is able to provide more power to devices looking for USB-BC and BrickID power bricks.

Apple iPhones can charge faster from non-Apple power bricks but only if the power brick uses USB-BC/BrickID, or the power brick uses USB-PD on a USB-C connector. Phones using QC for power won't take more than 5 watts from a USB-BC/BrickID charger because they can't "speak the same language".

I realize that given the age of the original question that at the time QC 4 was likely not on the market, at least not widely available. QC 4 chargers may provide faster charging with iPhones by figuring out a way to get USB-BC/BrickID to "play nice" with QC. A QC 4 power brick with USB-C will fast charge an iPhone because QC 4 includes USB-PD as part of the means for the power brick to advertise it's capability.

Nearly every USB-A power brick will be compatible with charging nearly all phones, including iPhones, at 5 watts. That's just the lowest common denominator. Phones should not even attempt to draw more power than that if they can't first determine the power brick capability as that's not safe. Phones that did draw more power than 5 watts without first checking for a compatible power brick should be long gone by now, either by being recalled for being a fire hazard, or by simply going out of style.


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 ) Nov 23, 2016 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, 2016 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 Nov 24, 2016 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 Nov 24, 2016 at 17:29

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