Just for your consideration:
The original location follows two main rules:
1) The point where shakes/impacts/forces coming from the irregularities of the road are minimum (halfway between both contact patches), because those are the enemies of the internals of any battery.
2) The point close to where the three axis of rotation of the bike cross, because centralization of mass = more agile bike.
https://www.ninjette.org/forums/showthread.php?t=208528