Hmm... Dunno if this has been done before. I'm experimenting with the use of abandoned sounds/models left in by valve, and am currently working on a system for a mod (all entity based) to tell the time depending on how far you are through the levels, and if the power level drops to a certain level, will strip the player of his HEV 'til he finds a charger, and reactivates it. I've been doing it today in spirit, and it has gone quite well