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I can say with near 100% certainty at this point that the problem is directly related to how well the power source maintains a solid 5.0 volts when connected to your phone. A few 1/10ths of a volt more is tolerable, but less is definitely not. So why is this "voltage" problem happening? Well unless you're a fairly "techie" person, you can almost skip this next paragraph because its a case where knowing "why" is about as helpful as a sick patient asking "why" he/she has the disease causing their pain. Unless you're a doctor with some "fixes", knowing "why" doesn't help fix anything. But for some i t might.
The "why" of it comes down one of two causes. First, the voltage coming out of various USB based 5V sources ( even from some decent chargers) may be slightly "off", and second, even when the voltage is perfectly correct, it may be a lot lower at the other end of the charging cable! To test it both of these conditions, you literally have to sacrifice a cable by stripping some of the cable covering '''near the phone side''', then striping small segments of insulation from a few of the inside wires (there are usually 4 in any USB cable), ultimately isolating the wires containing the 5V and 0V common. If they are color coded RED should be positive (+) and BLACK Negative (-), but sadly all of them aren't color coded. Now once this is done, you'd have to carefully measure that voltage with a multi meter, preferably a digital one (DVM). Without the phone connected, you should measure anywhere from 5.0 to perhaps 5.3V tops. One I have here that often gives me the problem was found to output 5.25V. Great! In fact if a charger didn't put out good voltage with no phone plugged in, then its total garbage and is always going to be a problem! So assuming you have decent voltage at "no load" (no phone)", the next thing to do is plug in the phone while watching that voltage. Oh now its not so good right? The one I had dipped down to about 4.3 volts momentarily and recovered to about 4.45. (If you look on faster measuring device like an oscilloscope, you'll likely see the voltage dip even worse!) Why? Two reasons. First, the voltage regulation in the supply is not so good (poor circuitry) and Second, some voltage is always lost because of the relatively high current being drawn over VERY thin wires. So both the charger AND the cable that came with it are each partially at fault. Trust me... if you were to use a technicians bench power supply and adjusted it so that at the point you are measuring (near the phone side of the wire) is actually 5Volts, not only will you NEVER see "device not supported", but your phone will charge a lot quicker!
So is 4.5V enough? Well consider that the Lithium cell in your phone, at full charge, should reach 4.2 volts. Realizing this, you can see where 4.5V is barely adequate, and something around 4.3, even for a short instant, is probably going to trigger an alert inside the 'smarts" of the "smart" phone. At best it will charge slower than it should, and at worst it will make the dumb phone think "hey this isn't a charger... it can't be... the voltage is WRONG too low! So I guess it must be some other device, and since I can't talk to it, its not a supported device!). It then internally disables the charging circuit, because wanting to protect it self from the opposite of charging... DISCHARGING. Could apple fix this? I'm sure they are aware. But they are treading a fine line!
Now its already been mentioned that a dirty or poor contact can cause the problem. If you think of an old fashioned flashlight (not so much the LED kind), you probably have seen where as the battery gets weaker, the light output of the flashlight can sometimes be restored a little by banging the flashlight in your hand (the great American 'fix-all" :-) ). Well this is the same reason why it is often helpful to keep the contacts clean. But electrically, the best thing to do is often difficult. If you're dealing with a newer I-phone which has a standard USB plug on both ends, you may be able to keep your inexpensive charger and obtain a better cable. Look for the words "high current" or current ratings of "2 amps". If its an older I-phone with the "apple only" connector, finding a better cable will be a challenge. Especially with the way search engines "ignore" search terms it can't specifically find. The problem is that a higher current cable means thicker wires, and everybody wants a thin "spaghetti" like wire. If you can find one with a thicker wire, there's a good chance you'll have a fix. And definitely look for the shortest wore possible, since the voltage drop decreases with shorter wire length.
If you're really technically capable, you should be able to find some much thicker USB cables meant for something other than an I-phone. If you keep the USB side and cut off the other side, it is definitely possible cut off the phone side allowing a couple inches of wire, and splicing the two together. Of course you'd have to solder, re-insulate, and cover the whole splice with heat shrink tubing, and this method is obviously not for anyone who doesn't know which end of a soldering iron to hold. But doing this will likely bring up the voltage to the phone by as much as a volt, and will definitely solve your problem. Again, if you're a DIY electrical person, you can build a variable DC power supply good to about 1.5 amps or better, with a USB port, and then following the measurement procedures mentioned earlier, set it to whatever voltage is necessary (with the same cable) to get the voltage at the phone end up to 5.0V. You're final charger will probably not be as small or convenient as the "itsy bitsy" ones you can buy dirt cheap, but it will work well and charge your phone a heck of a lot faster!
Unless such a project is within your grasp, you're going to have to pay a little more for a charger that has good reviews and has been shown to be actually rated for high current and fast charging, has specs to prove it, and has specs by a believable source (You know how much outright BS is published by "no-name" companies from 'certain" countries). This and following all reasonable procedures o keep the contacts clean in your i-phone port will help. Sometimes just plugging them together and apart several times will help. Good luck everyone!
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