Calculate phone's battery capacity I found this product (check the product here) on Amazon. Can this product be able to calculate the power of my phone's battery if connected to the USB port of my phone? I want to check the original capacity apart from what the manufacturer or any file on Android says. 

Will the phone's USB port output the required power for this function?

I tried 3C Battery Monitor, the app says it's 1580mAh, but its 2100mAh in websites and on the battery (it's a phone by a well-known manufacturer, but I don't want to say it here because I don't want the answers to be focussed to the phone make or the model).
You could try the accubattery app, it is known to give a somewhat accurate reading. It also gives other quite interesting information about your battery.