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?
Best Answer
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.