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.

Failing due to engagement

If a combatant declares a charge (E) but ends up unable to charge due to becoming engaged in melee before the charge, the charge does not happen at all (no attack on this round from charge, no +2, no AC penalty).

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.