@stuffe came up with a couple of correct solutions based on traditional wisdom, but failed to realize the depth to which Apple will go to obfuscate problems from the user.  It appears that there is a software setpoint for current (or power since the voltage is supposed to be the fixed) below which this state is triggered.

The message: "This accessory may not be supported", is due to insufficient power.

I am sure there is/was an Apple engineer could tell you exactly what current threshold triggers it (probable reasoning is adverse affect on battery life), but that is really irrelevant. Here are some things that you can do to correct it.


Clean connections.  Crud on connectors act as an insulator, reducing
the amount of current flow.
If its a usb cable, plug it in to something with a higher current (amps/milliamps) or power (watts/milliwatts) rating.*
Turn off, power saving features that may reduce power to usb ports. (if it stops after X minutes this is probably the reason)
Make sure your usb adapter outputs sufficient Volts (indicated by it only charging to XX% before you get the message)
Use a usb port closer to your laptop's the power supply (They often
handle more current than easier to access ones)
If you have one, you can try a powered usb hub (good ones are better
with high powered devices)
If none of that works, try a known working cable from a someone.
If that doesn't work you may have an issue with the connector in the
phone itself.


Usb power adapters should have the design voltage and current printed on them; however, this is the "design" rating.  Not all of them actually perform to spec in real life and it is not uncommon for components to weaken or fail over time, but still have some output.



Footnote:

I have an ASUS laptop running Linux and it has an easy to access usb port on the right  hand side near the front that doesn't output quite enough power.  Combined with a bit of connector corrosion, the only way it would charge was if I held it down fairly tightly on the charging base.  As soon as I would let go, I would get "This accessory may not be supported".  Simply switching it to the usb port near my power connector (in a less convenient location) was sufficient.  This tells me that it likely has nothing to do with any "device" specifics and since I have no iApps installed, those are ruled out too.  The only difference is the power output.  I will probably eventually need to clean my connections, but for now I'm good.

TODO: Write a polite letter to Apple requesting error messages that actually mean something.... It's probably in errno.h and strerror.h depending on how far they have gotten from their BSD roots (just add something like ELOWPWR and "This accessory has insufficient power for charging." respectively).

TODO: Get a cheap charger from the Dollar Tree, expose the wire and use a potentiometer and ammeter to determine an approximate current threshold that triggers the message inoperable charging state.

iPhone dock connector problems, with and without accessories
For all of my questions I have received great answers. I have implemented the answers in code and they work. I now feel compelled to close the questions because I don't believe there are more or better answers out there.

Should you close your questions that you feel have been answered sufficiently? Would allowing more people to answer dilute the existing, good answers?

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