Due to a sailboat’s inclination to heel, athwartships may be the better orientation for batteries.
Electrolyte doesn’t flow between cells (across the plates), but will flow (if inclined) from one end of a cell to the other. Because the plates run from side to side, across the width of the typical battery
, the electrolyte is only free to spill from side to side.
Were the battery
mounted fore & aft, the electrolyte would be able to spill out, or flow towards the sides, when heeled, exposing parts
of the plates.