TL;DR? Watch this video which not only talks about iPhones and charging but also hot air rework.
Some inexpensive USB toys (say a USB fan or mini soda cooler) don't have any data transfer, they just suck the power from the USB port to run. In this case, they do not use or connect to the Data pins (they are left to 'float').
Now technically USB ports can provide up to 500mA of current output, and technically every device is supposed to perform a basic data transfer to the computer (called enumeration) where it says "hey, I'm about to drag 500mA out of the computer, just so you know" and the computer can say "go ahead" or "no can do" (this is called the power negotiation).
However, we've found that every device that does not require to do any data transfer (say, USB fans or USB battery chargers) don't bother to warn the computer and just go ahead and grab up to (or even more!) than 500mA from the USB port. As long as they aren't going thru an unpowered hub, this seems to be just fine. All computers have a resettable fuse on the USB port so that if more than 1000mA is drawn, the power will be disconnected. This protects against short circuits in your $5 USB fan that would take down the entire computer!
Knowing the above, we designed the first Mintyboost to not have anything on the data lines - we assumed that nearly every charger and device would just ignore those pins as they tend not to be used
For example, in the CAD file of the first Mintyboost, the USB connector is the top square. The four ovalish pins in row as the USB wires. Pin #1 and #4 are used for power (blue lines are connected) but there are no traces on the middle two data pins - they are floating.
When we first released the v1.0 of the MintyBoost oh so long ago, we quickly got feedback from people who owned all sorts of Apple brand gadgets. It turned out that older devices worked fine but some of the newer ones, such as the iPod Mini, were not charging. Hmm! Time to experiment!
We figured there was something simple that would make the Apple device charge, and it definately had to do with the data lines (the power lines are fixed at ground and 5V). We thought "is there a enumeration chip inside every charger?" but since that's expensive and kind of overkilly we decided instead to read up on the USB protocol (go Jan!). In particular, in her fantastic book there's a part about the low level signaling states. Since you want to get the iPod charging, but NOT make it try to enumerate, we figured that we should see if there was some sort of special state you could put the data lines into that would say "no computer is attached but there is power". Turns out there is! It's called the SEI and occurs when BOTH data lines are at 3V.
Now to test. We stole an iPod from a friend and cut open a cable so we could mess with the data lines. We tried 3 options each - connected to ground, connected to 3V and not connected (float). At the same time we measured the current draw going thru the power line. We found the following:
OK, so yes when the pins are floating no charging occurs. The next thing to note is that whether the pins are pulled up or down effects the current draw. Since this was the first Mintyboost, using the MAX756, we wanted to use the lower 100mA rate so one pin pulled down and another pulled up. This would be more efficient for the battery use and keep the chip from getting hot (it could provide 250mA but didn't like it much).
Thus was born Mintyboost v1.1!
This worked ok for about a year, then people starting getting the newer iPods and noticed that the Mintyboost didn't work anymore.
This worked for a bit until the iPhones came out. With enormous batteries, the iPhone was not happy charging at 250mA - it wanted 500mA or even 1000mA to charge! We sought out an upgrade to the MAX756 and found the LT1302 which could provide 500mA no problem.
Apple stopped being as 'lax' with the charging interface and started being very picky about having the official chargers. We still doubted that there was an enumeration chip inside each charger - too expensive and complex. So there must be something else going on in those data lines.
Time to sacrifice an official Apple iPhone 3Gs charger!
This time both voltages on the data lines are = 49.9K / (49.9K+ 75K) * 5.0V = 2.0V
We did some experimenting (see the video up top) and determined that in fact the different voltages/resistances did effect the charging rates! Using the 2.8V&2.0V setup resulted in a 1 Amp charge rate and the 2.0V&2.0V setup resulted in a 500mA charge rate.
This made us very happy, because 500mA is within the capability of the MintyBoost chip. We redesigned the PCB to allow us to have 4 resistors on the datalines and put two 75K and two 49.9K resistors in each kit. So far we have had no problems charging any of the latest Apple devices. Hooray!