Charging

Rules for charging are on DMG 66. Note that encumbered creatures cannot charge. Charging is a type E action in the combat procedure.

Outdoors

MV bonus when charging outdoors is only 33.33%, compared to the 100% bonus when indoors. Quadrupeds get a 50% bonus outdoors instead.1) DMG 66.

From within melee

Can be done, but has the same downsides as disengage-retreat – enemies in melee each get a free rear+stunned attack against the charger, since the charger is breaking melee recklessly.

Without intent to strike

A charge can be performed to obtain the once-per-turn extra movement even if the charger will not end up engaged or attack at the end of the movement.