Android - Why is charging from computer using USB slower than using an outlet?

A computer USB port is limited to 500 milliamperes (or 900 mA for a USB 3.0 port). The dedicated wall charger, on the other hand, is usually able to provide 1000 mA or more.

Let's take the case where a connected phone uses 200 mA. Then, the USB port would have 300 mA left to charge the battery while the wall USB charger would have 800 mA left. A typical smart phone battery has a capacity of about 1500 mAh (milliamperes-hour) nowadays. This means that in a perfectly efficient world, a current of 1500 mA would charge the battery in 1 hour. So the same battery would then charge in 5 hours over the USB 2.0 port and in about 1h50 with the wall charger.

Of course in reality power losses would also affect charging times but these numbers are quite close to what I get with my actual phone.

Charging from a USB port of a computer is slower than charging using a charger because the voltage and amperage provided by the computer is considerably lower than in the case of using a normal charger, so the mobile is using “slow charge mode”

I don’t know which is the charging speed ration between USB and normal charger in the case of a SGS, but in my case (HTC Desire) with a normal charger I need approximately 2 to 3 hours for a full charge, but with a USB connection that time only charges 15 to 35 percent.

The reason your android device draws less power from your USB connection than from a wall adapter is because of the USB specification. This can be side-stepped by shorting the data wires in in the USB cable, which will switch the phone or tablet into wall mode where it can draw the full available current. The amount of current available will vary considerably from PC to PC.

The reason the specification exists in the first place is that the voltage to the computer's USB ports is usually supplied via the motherboard by a single loop connection which services all of the USB ports at once. Being only a few microns thick, this circuit is not able to deliver the same current as a dedicated charging wire. It is highly improbable that you will 'burn out your motherboard' by trying to draw the full available current through this circuit (although it is theoretically possible if your computer was a total P.O.S. to begin with). What is more likely to happen is that you will exceed the manufacturer's design specification, thereby denying sufficient power to any other USB devices you have connected, and/or other internal circuits which can cause the PC to crash. This is the very same reason it is recommended you use powered USB hubs.