I haven't, probably because power chair batteries are so close to the ground that anything that would cover them would impede driving, it would seem. There is a lot of wiring as well, that is not as exposed from rain coming from above, that within a shower, would be at risk. "Water" even in the absence of flooding is listed in the Permobil warranty, for example, as an exclusionary event.
I wish it were possible but I can't think how. But if anyone invents something that keeps the battery and wiring dry, there is a market!