When your battery charges, the electrolyte heats up and some of the water evaporates. During a process called electrolysis, the water breaks down into hydrogen and oxygen gases that dissipate. 

If the electrolyte level is too low, the plates in the battery cells are exposed and will suffer damage. In addition, the sulphuric acid will be more concentrated. This means you need to replace the electrolyte. This is how you do it.

The more you use your battery, the more the capacity of different cells may fluctuate. One cell might be fully charged while another is only half-charged. If this happens, your battery will not charge fully.