[Sohcahtoa82]

Yup, this is it, IMO.

On my OG Motorola Droid, I once tethered my laptop to it and downloaded a torrent.

In 15 minutes, I drained 25% of the battery. IIRC, it had a ~2,000 mAh battery. That meant I was pulling 2 amps from it. This was back in 2010, when most phone chargers and USB ports were still only 500 mA.

If I had relied on USB power, it wouldn't have been able to power it.

Oh, and yes, the phone got incredibly hot during this time. I thought I was going to burn my hand when I picked it up.