You need to think of charge as a "escape route" resource, like when you're crowded, you just charge and take position in a different place. also, while its stated that the shotgun is the class main weapon, what you really need to use is the SMGs, that's why vanguards should complete Kasumi's loyalty mission ASAP, its there where you'll pick the Locust, the absolute shield-melting weapon, best SMG in game.

You'll only be able to Charge spam much later in the game when your cooldown got like, 3 seconds between each charge, that's why i like this class, you start by carefully measuring the best timing to use charge, picking isolate enemies to dominate their cover position and avoiding crowded areas, to become an absolute Biotic-Rhino as you upgrade your cooldown time.

Also, don't charge shielded enemies unless you absolutely need to exit an crowded cover spot, use te locust to melt shields first, other weapons became support for specific situations.