Mini gorilla from powertraveller and the specific ipad adapter plus an original apple usb cable, lasts longer than 135 litres of avgas. :)
I have an inexpensive USB charger from Maplins which promises 2A plugged into the cigar lighter socket of my 12v Grumman Tiger. Into this goes a 2m non-Apple cable bought on Amazon. At the end of a 4.5 hour flight from London to the south of France, my iPad 3, fully charged on departure, is down to about 75% (and feeling a bit hot)
As a follow up to this, I flew for 8 hours yesterday (2 x 4 hours roughly) and running SkyDemon without any external power, the iPad battery went down to about 45% after 4 hours. On the way back, I plugged it into a USB cable on the cigarette lighter and the battery level was only going down by about 1% every 10 minutes, so not quite sustaining the battery level. Using this the battery level was going down by about 1% every 40 minutes.
So, not really scientific in any way because the times are not precise, but I concluded that an iPad not running any software would maintain it's charge running off the cigarette lighter but is not enough with SkyDemon running (understandably), and an external battery booster like the product above almost sustains the battery while running SkyDemon, and would of course charge the battery if it wasn't running anything major.
I carry a 12v 7 amp/hr sealed lead acid battery to which I have duck-taped a female cigar lighter socket, and into which I have installed an amazon cigar lighter plug with USB outlets into which the standard ipad power cable fits. It does the job splendidly and takes a lifetime to drain the charging battery which sits nicely in the side pocket of my Cessna with only a short cable run to the ipad. The cigar lighter plug has two usb outlets only one of which (2A I think) will actually charge at full rate. The charging arrangement does not have to be used in flight (my ipad seems to consume about 25% per hour at medium brightness and I never do 4 hour flights.) I simply recharge at destination if there is no conventional power source.
I received my and it works very well with the iPad. The box says the output is "5V 1A & 5V 2A", with a capacity of "12000mAh/3.7V). That doesn't mean much to me, but it charged my iPad a quick as being plugged into a socket. I have yet to try plugging the iPad 2M cable into the plane cigarette lighter socket to see if it will at least maintain the current charge while in flight.
[edited to remove the comma immediately after the URL :) - Peter]
Is there a spec on the lowest voltage an Ipad will accept at its connector?
There are countless non-brand chargers but nearly all of them are 5.0V. No wonder there are so many problems.
It's easy enough to construct a low resistance USB charging cable. Just cut off the ends of any USB cable and rejoin them with a length of something really thick. Then you need to put in four resistors (for Apple) or a short circuit (for everybody else, I think) on the two data pins.
These Lindy USB extension cables work fine up to 2M and will charge an iPad at full rate from the 10W charger.
Your problem could be a combination of problems. Some chargers output 5.0V exactly, whereas the Apple chargers output 5.1V for the smaller ones and 5.2V for the 10W and 12W ones.
If you've got a marginal power source (maybe just exactly 5V or even a shade under), coupled with a skinny USB extension cable, you could end up quite far from 5V at the device.
Someone else explained that its resistance along the length of the cable
I don't have a low-res ohmmeter here but let's say the USB cable is 0.5 ohm (plausible for 1m, both conductors) then 2A will drop 1V, so the Ipad will see just 4V. It probably won't think that's enough...
The USB cable which came with my Lenovo Tablet is a short thick coiled job, about 0.5m.
So not a very large sample, but suggests that there is something too it though.
I would bet the USB ports which actually charge an Ipad are crudely implemented and have no current limit on them, or the limit is something generous like 1A - enough to protect the motherboard from a short circuit in the cable.
USB port power can be implemented with a variety of chips, and some of them are pricey, so you won't use them unless you need to.
Or if a PC has say 8 USB ports (i.e. 4A total) they just power them all from a 5A-limited regulator 
Then, how much the Ipad will draw depends on its interpretation of the voltage on the two data pins.
I have just been speaking to somebody who knows more and he says that a driver is able to access the data pins directly. It cannot set up a DC voltage on them but it can modulate them with some sort of duty cycle, chosen to fool the Ipad A-D converter into seeing the right kind of DC voltage. This is quite bizzare but it may be what that software is doing, to enable an Ipad to draw up to the full 2A from the PC USB port.
That is weird... faulty cables?
There were three different cables that I tried. Someone else explained that its resistance along the length of the cable, and it just becomes too much for Apple's detections. I don't know enough about electronics to know if that's true. That's your area of expertise ;)
I can't see how a Windows app can alter the current a PC USB port can deliver - unless the PC is specially designed to support that.
I was sceptical too. I tried it on my work pc (maybe 2 years old) and it worked perfectly. I tried it on my laptop (6 years old) and it worked perfectly too. It's not just indicating charging, but actually charging. So not a very large sample, but suggests that there is something too it though.
I have recently run ForeFlight on my iPad 2 with a BedElf GPS on some long(ish) x-country flights in the US. While I never ran out of juice, it would appear that the iPad in this configuration (i.e. where the BadEld also draws current from the iPad battery) would last about 3.5 to 4 hours. There is, however, one great thing about the BadElf, i.e it has a through-charging port that accepts a mini-USB plug. All you therefore need is a car charger for the iPad with a mini-USB on the other end (my iPad car charger has the Apple connector). This time can, of course, be greatly extended if you just shut it off for phases of flight where you don't need it.
One thing to watch out for in real life is the overheat shut-off. On one of the flights I left the iPad in the sun on the dashboard for perhaps 15 minutes and it shut off. Came back to life after cooling down or a little while in the back, but something to be aware of. No big deal in my situation as I had ForeFlight running on the iPhone also and, of course, had the panel-mounted 430 (plus VOR), but could be disconcerting if it happened at an awkward moment.
All the above said, the combo iPad / iPhone / ForeFlight is absolutely fabulous - a real pity there are no European Jepp charts available for it. I also think there probably won't be, as Jepp are in the process of creating their own product.
