I found an interesting claim (I don't know how accurate it is) that most 2 amp chargers only supply 2A to iPhones/iPads. Other devices are pinned or cabled differently, and the chargers only supply .5A to non-Apple devices.
Here's the link.
Here's the review with the charger claim:
This product has two major issues:
1. While it's description claims it has a 2 Amp port and a 1 Amp port, the reality is that it has bridged the two ports and it can only source 2 Amps total. It is not capable of delivering 3 Amps combined like the description implicitly claims and could never simultaneously charge an iPad and a smart phone. (UPDATE: The technical details section now indicates more clearly the 2A total, which is appreciated.)
2. The 2 Amp port appears to be using the Apple protocol for USB power, so that port will only provide 0.5 Amps to most other devices.
The upside of this is the 1 Amp port somewhat over delivers, and can source 2 Amps if nothing else is connected and 1.5 Amps if the 2 Amp port is sourcing 0.5 Amps for a non-Apple device. Thus this product can support one-high power non-Apple device (in the 1 Amp port) and one lower power device (in the 2 Amp port).
Now some more background/details...
First, a little background on USB charging for those who don't know:
The USB power spec is for 0.5 Amps at 5 Volts... or 2.5 Watts. This was great up until the last couple years when devices have gotten really power hungry, particularly smartphones and tablets and to a lesser extent dedicated GPS's. Some of these devices use over 2 Amps, particularly the tablets like the iPad (or in my case the HP Touchpad).
Manufacturers of these devices therefore had a dilemma. If they had their devices pull more than 0.5 Amps, they risked damaging the power source, which could be a computer, that was only prepared to source 0.5 Amps. Thus the manufacturers have used tricks to determine whether their device is connected to an unknown source, at which point they purposely only draw 0.5 Amps, or to the dedicated charger that was provided with the device, where they can draw all the power they need.
There seem to be two common tricks used. The first is to short the two data-wires together in the charger. This is what most non-Apple devices do. Since a computer or older device wouldn't have done this, the device can assume it is safe to draw all the power it needs.
Apple seems to have taken a different approach, one that I don't fully understand but know can be seen by the fact that the data lines are neither open nor shorted when the charger is plugged in, and is more sophisticated and probably superior, because I suspect it allows the device to know exactly how much power it can use.
While Apple taking a better approach might be nice in concept, it's created chaos in the USB charger product category. What is the non-specific charger to do? They can't support both.
This device chose to go the Apple route for the 2 Amp port. I don't own any high-power Apple devices, so I can't say for sure, but I suspect this unit performs well with Apple devices. Using electronic equipment I was able to draw 2 Amps when putting a fixed load on this USB charging port. However, when I connect any of the 3 high-power devices I have (HP Touchpad (2 Amps), Motorola Triumph Android phone (0.85 Amps) and the LG Optimus Slider (0.7 Amps)), none of them drew all the current they could have, because they were expecting a shorted set of data-lines.
Luckily for my purposes, the 1 Amp port on this device uses the data-line shorting method. As such, it was able to deliver all the power my Triumph and Optimus wanted. However, for some reason, even when it was the only device connected, it would not fully charge the HP Touchpad at the 2 Amps it could have, because for one reason or another, the HP Touchpad rejected it as a high power charger.
To be honest, this surprised me because it was able to source the 2 Amps the Touchpad claims it needs and the HP Touchpad charger has the data-lines shorted. So I'm not sure why this didn't work.
Thus this device gets 3 stars (UDPATE: 4 stars) because it can source the current it claims (minus the somewhat deceptive 2A + 1A notation (UPDATE: Which is now more clear in the technical details)), would likely support an iPad like it claims, but falls short in other regards noted elsewhere.
For non-Apple users, or even more so mixed users (say an iPad and an Android phone?) this device is superior to the other device I purchased, the "Kensington K33497US PowerBolt Duo Car Charger" (http://www.amazon.com/gp/product/B003PU01M4/ref=oh_o02_s00_i00_details
) because on that device, NEITHER port works with non-Apple devices (although it performed better power-wise, see my review over there for details).
I've ordered two additional devices:
- "Ultra Compact High Output Dual USB Car Charger - 2.1A Output" (http://www.amazon.com/gp/product/B00470C35E/ref=oh_o01_s00_i00_details
- "Bracketron Universal Dual USB Car Adapter (UGC-298-BL)" (http://www.amazon.com/gp/product/B0056VNVV8/ref=oh_o00_s00_i00_details
Hopefully one of the two of them will be able to deliver to their specs for my devices.
UPDATE: I've increased my review to 4 stars for two reasons:
1. The changed technical details text makes it more clear its total power capacity.
2. Of the 4 models I purchased, it's one of the two I kept and it's the only one I kept in unmodified form. That's got to count for something.