You really need a hydrometer to test the electrolyte, a volt meter will not give a proper answer to how much sulfuric acid is in solution. Once the sulfur starts bonding to the lead inside the battery a meter will give misleading results just like you've experienced.

A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)