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

Question

And what is roughly the ratio of the charging speed in USB vs charger? My phone is Samsung Galaxy S, if that matters.

Answer 1

A computer USB port is often limited to 500ma (milliamperes). The USB charger, on the other hand, is usually able to provide 1000ma, sometimes even more.

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

Answer 2

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.

Answer 3

Consider that computers often have other things inside of them or connected to them that need the same charge. The power within a computer itself is always lower than direct power from a outlet from a wall. When power is distributed through a computer to your phone, that same amount of power is also being used throughout the whole computer slowing the voltage to your phone, and other things that are plugged up. Always remember, if you want direct power and a quicker charge, its always good to use to wall outlet instead of a computer or labtop.