If you cannot view the fluid (water) level in your battery, then you need this tool.
Practical Sailor calls the WetCheck "A safe and accurate method of checking the electrolyte level of your lead acid batteries. We use them on our boat".
Prevent Dry Cells - The #1 cause of battery failure
It is hazardous to peer into battery cells, and when you do, it is often difficult to determine the correct water level, 1/4 to 1/2 inch above the top of the plates.