510331-001 Water Me Sensor Vent Cap Style - Replace a standard vent cap with one of these and it will monitors the battery for a low water level. A flashing green light lets you know the water level is fine and no light at all lets you know that the water level is low and needs water added.
Warning: This product may expose you to chemicals including lead which are known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov