The battery cooling air, like radiator cooling air (in a perfect world!) exiting
through the belly pan should be ducted/directed such that it is moving as
parallel to the pan as possible. BMW has done some work on this. Here's a
picture:
IIRC, taking the easy way out and just shooting the air straight down acts like
a barrier to the air moving under the car. This acts like additional frontal area
and also greatly disturbs the flow under the car.
Off topic: Look at the size of those front tire deflectors, 2 or 3 times the size
of the little OEM things you can see on lots of cars.