Massive growth around the link cable terminal on the right-hand battery. It was only when I replaced the cable that I noticed a crack in the casing by this terminal. Probably the cause, even though it has run for another 12 months without developing again and only a very slight reduction in electrolyte level.
The new armoured cable going to the terminal at the top left. The extended main battery cable from the cut-off switch now goes to the further terminal, the battery having been rotated 180 degrees. As all the cables are black I was bothered about incorrect connection right back when I fitted twin 6v batteries. I happened to have a length of red cable from somewhere, so I cut the insulation off that in a spiral and used two lengths to wrap round the main 12v cable ...
... and the +ve end of the link cable.
