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?