(Bear with me for a bit, because I'm going to be stating some facts)
The Engineer's Charge bar holds up to 5 charges. Charges are generated through dealing damage and through the effects of certain skills. The following skills have effects that generates charges:
- Based on number of enemies hit (Dynamo Field, Shield Bash)
- From killing enemies (Charge Domination)
- Per hit (Supercharge)
There are also certain skills which do not damage Charges, despite dealing damage. They are:
- All summons (Spider Mines, Gun Bot, Sledge Bot, Immobilization Copter)
- Ember Hammer
Charges are used for the following:
- Passively improving the effects of certain skills (Shield Bash, Ember Reach, Ember Hammer)
- Expended to improve the effects of certain skills (Force Field, Shock Grenade, Flame Hammer)
- Expended to enable further effects (Tremor's Damage)
- Additional passive effect when charge is expended (Charge Reconstitution)
There are also skills that have no direct interaction with Charges (other than dealing damage).
[ To be updated at a further date ]
Flame Hammer Engineers use up charges fairly quickly, but other Engineers usually stockpile Charges for Force Field. In general, I feel that there is a lack of Charge generating and expending skills; the Charge bar does not feel as important as it should to the Engineer.
I'd like to see more skills using and expending charges, and managing charges to be a central and unique aspect of Engineer gameplay.