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

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

Answer

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.