I put batteries in panniers when I started, but at 36V I had to put
two on one side and one on the other. It was unbalanced and made
riding tougher even tho the center of gravity is lower.
Now I have one LiFePO4 pack up high, and it's perfect.
(It would be cool to balance two 24V packs when I go to 48V, but
G.M. doesn't have high AH 24V packs. (Hint Hint. ) )
Anyway, that would be the way to go, mount two of them
low like exhaust pipes on a motorcycle.
One thing to worry about, most racks don't hold heavy weights well.
I am thinking of a battery suspension rack instead of just baskets.