The yellow cap is used with Basement Watchdog wet cell standby batteries (a discontinued product although some may still be available in retail stores) to accommodate the fluid sensor. It is to be placed in the second cell from the positive post and the sensor is inserted into the off-center hole in the cap.

Note: Some Basement Watchdog batteries have only 2 long rectangular caps with a hole already present in the top of the battery case. If you have this type of battery, the yellow cap will not be needed but keep it in case you need it in the future. If you have misplaced your yellow cap, please call Glentronics at 800-991-0466, option 3, for a free replacement.