The crystals seem to differentiate based on function. I never noticed any mention of needing a power crystal to utilize a weapon crystal. However since they can be drained completely, as demonstrated by Ferrus, I assume they store energy and release it. So perhaps they need to be recharged, similar to reloading a modern firearm, but there is no mention of a power crystal along with a weapons crystal in a gauntlet setting. We know that contact with a lumen or weapon crystal is required but there is no mention of needing a power crystal.