Rick Brandt wrote:

I have had one case though where the plugs from two chargers were the
same (they would both fit my phone), but only one would actually charge
it. The other caused the display to read "unrecognized charger" or
something to that affect.

Ah, the old Motorola "Unauthorized Charger" display.

A standard Motorola USB charger puts ~1.4vdc onto the x pin (the 5th
pin) of the mini USB plug. A Motorola phone looks for that voltage and
will display "Unauthorized Charger" if it doesn't see it. So you can't
charge your phone from a USB port using a standard USB to Mini USB cable
(though if you have Motorola Phone Tools running on the computer it
will work).

I don't know if Motorola did this to sell more of their own chargers (if
so it didn't work because the after-market chargers quickly figured out
the necessary circuit) or for some other reason.

Motorola mini USB chargers will charge non-Motorola phones, but
non-Motorola compatible mini-USB chargers will not charge Motorola phones.

My son had a similar experience with a Garmin gps unit. He was telling
us he couldn't use it while the charger was plugged in (the charger did
charge the battery properly).

The GPS units have larger batteries and higher current chargers. There's
enough current to charge the battery, but not to charge the battery and
operate the unit at the same time.

So "compatible plug" != "compatible charger"

Unfortunately that's true, though it certainly would be possible to
build a high-current mini-USB charger with Motorola compatibility. It
would just cost more to build.