Battery Hydrometer Hydb 94-A5
Even maintenance free batteries need some attention every now and again.
The balance of electrolytes and water in the cells of the battery will eventually become too low, and you will need to top off the cell with distilled water.
While some batteries come with a hydrometer built in, that only reads the state of one cell.
You have to use a battery hydrometer and test each cell to really know the condition of your battery.