To revive leisure batteries that are failing to hold a charge I use Epson salts. Mix 5 spoonfulls to half litre of boiling water to mix the salts then when cool add to the 6 cells in an open cell lead acid 12v battery making sure there is enough space to do so. If the problem was sulpher on the plates this will loosen it and revive the battery. Of course if its a dead cell then a new battery will be required. Epson salts do the job like the battery pills Halfords sell and at 1% of the cost of an advanced charger with revive function like the excellent Ctek units. Many batteries are thrown away simply because they have sulphated which happens automatically at a voltage under 12.2v. There is no need to remove this mixture later just leave it in there it does no harm and mixes with the electrolyte