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.
Yes, that's not real off, it's the bootloader which doesn't have the full stack of stuff for charger ID and treatment hence it's a bit more conservative.
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.
That'd be the overvoltage protection circuit...
Once charging had begun with iPhone ON the input voltage could rise to 5.60 volts before charging would shut down.
Again the OVP, but the ~5.5v threshold looks different when current is flowing (ie, there's a drop between where you're measuring it and where the PMU is measuring it).
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).
This is because it's not a linear charger, it's a DCDC that bucks down to just above the current battery voltage, then LDOs into the battery (to keep the battery rail clean). Hence you get a multiplier effect that will reduce the input current as the voltage rises (it'll be vaguely constant power, as you saw with the 1A brick/charging.
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
That would indeed be nice, but typically battery vendors don't like you charging at a higher rate that 0.7C (this is ~1A charge current maximum with a 1420mAh cell). Due to various tolerances in the system, the typical battery maximum current is around ~0.63C, though individuals units could be +/-10%.
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