5 May
2012
5 May
'12
11:41 a.m.
Pete Turnbull wrote:
For example, an iPhone can pull far more current than
USB guarantees
(well over an amp), so it tests first. The proper charger has a couple
of voltage dividers across the power such that the data +/- lines are
held at specific voltages even at light load. If the voltage on each
is within a specific (fairly narrow) range the iPhone knows it can
pull at least an amp. A different range tells it 500mA. Else nothing.
The iPad is obnoxious about this. If it can't get the extra current it
wants, it tells the user that it won't charge. It lies, though:
actually it does charge, but slowly.
This had me in a panic when I first found out about it after having
flown somewhere. I assumed that I could charge it from my laptop, and
didn't bring the iPad charger. The first evening the battery was low,
so I plugged it into the laptop, and it told me that it wouldn't
charge. I thought I was out of luck for the whole trip, but it actually
did manage to charge overnight.
They really should change the message to say that it will take a long
time to charge, rather than claiming that it won't charge at all.