Originally Posted By: altman
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The iOS devices don't "carefully balance current draw against voltage sag"; they will try to take the amount of current specced by the adaptor (though a phone won't take over 1A), but *may* back off if they see undervoltage events. A lot depends on the exact phone as there are different current controls schemes in each generation. Some generations had digitally controlled current limits and others had analog limits that are more predictable.

... they don't charge when off - it'll always start the CPU if sufficient external power is available. If you plug an iPhone in then turn it off, it's only pretending to be off (it's actually gone off, then come back up in a fake-off mode)...
For completeness and future reference, further review of my old notes from my iPhone charging profiling (1 amp charger style resistor network on D+ and D-) with either an iPhone 3GS or iPhone 4 (not sure which I had at the time frown but I think it was the 3GS).

If the iPhone was plugged into power and then turned off the maximum charging current would limit itself to 500ma. If the iPhone was allowed to 'boot up' then the maximum charging current was approx. 870ma at 4.85 volts.

The minimum input voltage to start charging was measured as 4.50 volts.
If the initial input voltage was above 5.50 volts then the charging would not begin.

Once charging had begun with iPhone ON the input voltage could rise to 5.60 volts before charging would shut down.
WIth iPhone OFF the minimum charging voltage was 4.25 volts but would only draw 52ma. Current draw remained minimal until input voltage exceeded 4.45 volts.

Interestingly, as the input voltage was raised from 4.50 to 5.50 the input current would decrease from 496ma to 454ma (iPhone powered down).

Minimum input voltage for maximum current draw of 870ma was 4.85 volts with iPhone turned on. Further voltage increases maintained the wattage consumption at about 4200 milliwatts by moderating input current towards 756ma at 5.50 volts.

With the iPhone battery heavily discharged (large red battery symbol) the powered down iPhone would not immediately boot up when connected to the charger and would limit itself to under 500ma of charging current. Once the battery had partially charged the iPhone would start itself and then the charging current would rise further.

If D+ D- were left open then current draw limited to =< 100ma.
D+ D- resistor voltages applied after charging began, current limited to =< 500ma.
D+ D- resistors are connected right from the start and then disconnected the charging current can remain above 750ma.

I stopped collecting data once I had a good enough idea what the iPhone was doing with the D+ and D- signals from the charger and how charging voltage related to current draw.

Clearly the iPhone (3GS in this case, IIRC) is sensitive to both the semaphore voltages on D+ D- and to the actual charging voltage delivered to the 30-pin connector, and of course the available current.

It would be interesting to see if the iPhone 4S also limits itself to 1 amp maximum current draw even when connected to the high output iPad style 10 watt charger. A faster charging iPhone 4S would be nice to have smile