[BrianG]

Most of the time, when a device uses a 3-prong plug on a 120v circuit, it is because the device has a metal casing; the ground connection is tied to the ground plug so that if the "hot" wire were to internally come loose and touch the case, the case would float hot which would trip the breaker. Incidentally, the white "neutral" wire on house wiring is electrically the same point as the ground wire - they both go to their respective bus bars at the panel, which are tied to ground.

Some devices are more flexible and can run on an input voltage from 120v to 240v. If on 240v the two main prongs are both "hot" while the ground is the only ground and still tied to the case for protection. There might be even more to it, but it's been a while since I've even looked at an NEC book.

Mistercrash; I would say that if you want faster charge times, invest in a LiFe setup that has close to the same voltage along with a BMS circuit to make sure the cells stay in balance. Then, you can charge MUCH higher, but you'll likely need a real 240v feed in your garage to keep input current manageable. Charging a ~50v 40Ah pack @ even 2C will require ~4700w (with charger efficiency figured in), which is almost 40A at 120V, but "only" 20A @ 240v.