Be sure to add fluid to all 6 battery cells in your wet-cell battery and then place the sensor rod in the correct cell—the second cell from the positive (+) terminal of the battery. If the sensor rod is put into any other cell, it will not measure correctly and the “low water” alarm may go off. Make sure all 2 ½ inches of the sensor is inserted in the predrilled hole in the battery cap, or in the hole in the battery case, depending on which battery you own. An arrow on the top label points to the correct location. If you are using a different brand of battery, the fluid sensor must be attached to the positive (+) terminal of the battery. Note: You should never drill a hole into the battery yourself.