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Similar question to here: Laptop power supplies, does current matter?

According to the answer, using a lower rated power supply/adapter could cause several problem such as melted power supply/cord. But what about using USB ports for portable devices?

AFAIK, My USB port ouput 5V/500mA (it can't even charge an iPad), but the charger that comes with my phone outputs 5V/1200mA, and I have been charging my phone via the USB port without any problem so far, except it takes a little longer to charge the device, but when i try to charge the phone with a portable power block that outputs 5V/800mA, it doesn't charge the device, why is this?

Does using a charger with different power rating can cause the battery to have shorter life?

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A significant difference between regular battery chargers and using a USB socket to charge USB devices, is the negotiation of power use thru the two USB data pins. This is called USB Enumeration.

The observations you describe include voltage and amps on the two power pins of the USB, but for USB connections there are also 2 data pins that are used in charging. On a PC, when you plug in a USB device, devices are supposed to use the data pins to negoiate their required power use. Simple USB devices (like USB toys) don't do this, and just attempt to grab as much power as possible from the power pins.

More sophisticated wall chargers with USB connections also implement USB enumeration, allowing them to provide more amps and to also identify the device its connected to.

An interesting example of this is documented on the design of a portable USB charger called the Minty Boost. During its different versioning evolution, it had to support new iDevices with different power requirements. The article gives some insight to how to debug black box USB power negoiations and has revelant links.

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so, is it safe to assume that it is better to use a charger with higher power rating in case it doesn't support USB enumeration? I am a complete idiot in electronics, but from what i read around here, it is safe to use a charger with higher power rating, but not the other way round? is there a kind of safe limit in terms of the difference that can be compromised? –  that_guy Jul 12 '12 at 12:40
Voltage is just pressure, so minor changes are not terribly important, but drastic changes can be devastating, but also can be nominal, depending on the amps. Amperage is essentially the volume of electrons passing through a point at a given time, it is a measure of a constant current. So the higher the amps, the more electrons, the faster a battery will charge. The difference between 5V and 6V with 500mA is not much, the difference between 5V and 6V with 5A is a bigger deal. The safe limit for differentials in value changes based on other factors, some that may be very specific to the device. –  MaQleod Jul 12 '12 at 15:46
@MaQleod, alright, lets see if i get this correct, so, if the device is charged with a lower amperage charger, it will charge slower, and if the charger is able to negotiate the power use, it is able to regulate the rate of the power the device is trying to pull, else, the device might try to pull power faster than the charger is able to supply which has the possibility of causing damage if the difference is too great... it is very much an oversimplification, but is that correct? –  that_guy Jul 12 '12 at 16:47
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