Lead Battery-angle

[GordMay]

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.