By all means, it should be possible to increase the apportioned value to drain from and lower the value consumed every term (I'm not sure the rate at which that energy would be consumed, but I mean to decrease that), but that would merely make it last a long time, not forever, and I would not be willing to change the engine itself so it can actually last forever. Aside from which, that would mean you could never get rid of it. I suppose an alternative would be to give it a small icon, like a shield or a forest miracle, so you can dispel it later.