I think the confusion here is the trouble that a lot of people have with LEDs:
You can't just run them like a lamp, you need a current-limiting resistor.

The forward voltage of most backlights is 4.2-4.7V, so if you need to provide them with 100mA of drive, you need (5-4.5) = .5V/100mA = 5 ohms in series.
You can power the same backlight from 24V, but then you would need (24-4.5 = 19.5/100mA = 195 ohms in series. Please note that in the latter case, the resistor would dissipate close to 2watts.

A few LCDs have the resistors "on board", but most require external current-limiting resistors.