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I have a Nexus One adapter, a Kindle adapter and a 12v Cigarette lighter to USB adapter, which provide 1 amp, 0.85 amps and 0.75 amps respectively. What happens when I use each of them for the Kindle or the Nexus One and is there a chance something may be damaged?

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If all three adaptors provide a 5v USB-type supply and the connectors on the adaptors will all fit the device to be powered then the issue is whether the adaptor you use can supply enough current for the device to which it is connected.

The Amperage values you quote are the maximum that those adaptors can supply, so there is no problem in using, for example, a 0.85A adaptor with a device that draws a maximum current of 0.75A, but if things are the other way round and a device needs a current of, say 1A and you use an adaptor that can only supply a maximum of 0.75A then one of several things may happen:

  1. The adaptor may detect an overload and shut down
  2. The adaptor will supply as much current as it can and the device it is powering will struggle to work properly or charge its batteries
  3. The adaptor will be overloaded and will overheat and/or blow its fuse (if it has one).
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I hadn't thought of this originally, but surely both devices are able to charge from a computer USB port supplying only 0.5 amps? Are they able to use the data connectivity to negotiate more current? – Ben Heley Dec 28 '10 at 19:27
Hi Ben, it depends on the device and computer port - a device can theoretically draw more than 500mA if the hardware spec of the port allows/supports it and the device has the relevant value resistors connected to the USB data lines to signal its power requirements. There was a discussion over here about all this:… – Linker3000 Dec 28 '10 at 23:22
It looks to me as though the answer lies in the USB-1.8A section of the accepted answer to the question you've linked. I hadn't thought of it as battery charging, but I can't think of a situation where that isn't what the power is being used for. – Ben Heley Dec 29 '10 at 2:30
I might be wrong but given that the 'battery charging specification' exists, it must be the case that most devices are able determine the amperage of the power adapter. – Ben Heley Dec 29 '10 at 2:34
up vote 0 down vote accepted

There is a standard for USB-based battery chargers that allows a device to detect the potential output power (amperage) of an adapter. Assuming both devices are within the specification, this ensures that it is very unlikely any damage will occur.

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