Conviction of Power question

If you are at minimum charges from Conviction with power, are you able to expend charges on things like Immortal Call and gain the benefit?

I was likewise thinking of trying Cold Snap with the Winter's Bounty gem to always have power charges feeding elimination of the cooldown, but I'm not sure if it could actually spend the charges if I was at the 4 minimum. Anyone have thoughts or try this out?

You can't expend minimum charges, the game treats minimum charges the same as being at 0 charges in consideration of abilities that consume them.

What happens if you equip something that reduces your maximum charges below your minimum?

I think you just have up to your maximum (haven't tested this but since the conviction of power node on heiro specifically raises the max by one along with the 4 minimum I'd assume you can't have above your max)