Lead acid batteries have had the same basic design for nearly a hundred years, with plates of lead and zinc sandwiched into a sulphuric acid bath. The electrolyte reaction can store vast amounts of electrical energy but can be self-corroding or destroyed by heat. Repairing these defective cells can be done by the average backyard technician in about a half-hour.
Check the acid level and ratio. Most lead acid batteries will have a cap for each cell, and commonly there are six cells. Prying off the plates or caps over these cells with a screwdriver will give you access to the cell compartment. A common battery acid tester is a dropper with small coloured balls inside of it. When a sample of battery fluid is pulled into the dropper, the number and colour of floating balls indicates the acid composition.
Replenish the acid to the proper level and ratio. The physical level of how much fluid is in the battery can be seen through the top holes, one per cell. If a cell is low, it may indicate in issue with the battery. The battery can be "topped off" with fresh acid mix--but never plain water, as this may damage the cells.
Replace the battery plates as an assembly or individually. The metal plates inside the lead acid battery are prone to corrosion and other factors that prevent them from charging correctly. Transmutation of the zinc plates, in particular, can lead to failure. By taking the top of the dry battery completely off (it may be sealed with a locking mechanism or heat-shrink plastic), the plates can be accessed. Some models will have the plates connected to the top panel, and they slide out when the panel is removed. Others may have static plates, accessed from the side of the battery casing. These plates can be disassembled to repair, but it is easier to replace the entire bank of plates because without scientific equipment, it would be difficult to determine which plate was at fault, if any.
Charge the refurbished battery on a 1-amp trickle charger. The battery will be "full," or fully charged, in about 30 hours. Test the acid level often, and replenish the fluid when low.
Adding a mix of 35% to 40% sulphuric acid to the battery may increase capacitance in the short term, but it may cause higher corrosion rates and premature failure of the battery.
Disconnect the battery before working on it. Use safety equipment when working with sulphuric acid.