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I'm sure this is a stupid question but I can't find the answer.

iPhones have roughly a 6-7 Watt Hour battery according to multiple sources online.

They can also use a 5W or 10W charger (1Amp x 5V - or 2A x 5V) for charging - which I have observed that they reliably draw for the first few hours of charging (then of course, slow down over about 80% charge).

Even a 10W charger takes 3 or 4 hours to get to 80% from no charge. Doesn't that mean it should have a 30 or 40Watt Hour battery? What am I missing?

thx.

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    \$\begingroup\$ The current that the iPhone uses for itself in ON state. \$\endgroup\$ – Turbo J Oct 23 '16 at 12:23
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There are at least three things you are not considering:

  1. As Turbo pointed out in a comment, the phone is using some power while the battery is being charged. Not all the power from the charger goes to charging the battery.

  2. Batteries aren't 100% efficient. More energy needs to be put into them when charging than what you get out when discharging. The circuitry around the battery that manages the charging and discharging also has some losses.

  3. Just because a battery is rated for 10 Wh doesn't literally mean that's a good way to charge it, even if it were 100% efficient. Wh is a unit of energy, and doesn't imply charging time and rate. You could just as well express the energy in Ws, but attempting to charge a cell phone battery in one second would cause pyrotechnics.

    Depending on the battery, it's life may be extended by charging from empty to full over more time than one hour. Note that it takes more than one hour to run down a fully charged battery, even with everything on.

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  • \$\begingroup\$ What you are pointing out is, as usual, correct. I would add that cables play an important role too. If the charger is original and the cable is original (1m or 3ft) I expect the voltage at the charger inside the phone to be about 4.7V@2A or something. And I seriously doubt OP claim about the two amp measurements. \$\endgroup\$ – Vladimir Cravero Oct 23 '16 at 12:56
  • \$\begingroup\$ How efficient ballpark - is a 10W charger into an iPhone Li-ion battery likely to be. 90%? 50%? iPhones come with 5W chargers - and Apple say it's fine to also charge them with 10W iPad chargers - generally this doesn't generate too much heat so shouldn't adversely affect the battery too much. Quick Charge 2.0 used on some Android phones allows for up to 18W charging. \$\endgroup\$ – niico Oct 23 '16 at 13:02
  • \$\begingroup\$ Regarding measuring current - I use a volt meter at the charger end (2 different ones actually - giving the same result) - and can confirm 2A 5V (happy to send photos). You are also right cables do affect the number of amps drawn quite a lot - I select cables that are capable of 2A. (cable variation seems quite wild and random - and users generally are totally ignorant of this fact). \$\endgroup\$ – niico Oct 23 '16 at 13:04
  • \$\begingroup\$ My aim here isn't really to stuff as much power into a battery as possible as quickly as possible - but to understand why there seems to be such a large difference between the Watt Hours rating of the battery - and how long it takes to charge on a 10 Watt charger. \$\endgroup\$ – niico Oct 23 '16 at 13:06
  • \$\begingroup\$ The impression I am getting is that there are a large number of different factors which all contribute - and all must be taken into account? \$\endgroup\$ – niico Oct 23 '16 at 13:07
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Ballpark efficiency fort Li-ion batteries are according to wikipedia 80-90% https://en.wikipedia.org/wiki/Lithium-ion_battery#cite_note-PHEV1-4

You might be interested in testing charge time with phone turned off, or in power save mode. WiFi and blue-tooth tend to consume a lot of power.

Cables do vary a lot, design varies according to standards as well. Basically there are two concerns, current causing heat resulting in fire (more/thicker copper for better cooling), and isolation so that intended voltage does not jump through cable and cause short circuit.

I would do a resistance check on the cable, including the cable connectors (they are usually not made of copper and cause losses). I would also clean all connectors properly with isopropanol or similar.

During charging the phone will/should monitor the heat and limit charging in order to prevent fire.

Does the technical specs say anything about charge time?

As a post note I'd like to add that charge time tends to increase with battery age/usage.

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