A vent plug for an automatic battery watering system features a passive, vibration-resistant diamond valve assembly that prevents overwatering of lead-acid batteries in industrial vehicles. The vent plug includes a head portion with an inlet for receiving water from a reservoir and a float portion extending into the battery electrolyte. A buoyant float is mechanically coupled to an actuator that selectively engages a spring-biased diamond valve element located in a water chamber. When the electrolyte level drops, the float lowers the actuator, urging the diamond valve element downward to open a precision orifice, allowing metered water flow under hydrostatic pressure. As the electrolyte level rises, the float lifts the actuator, permitting the spring, buoyancy, and hydrostatic forces to reseal the orifice. Monte Carlo analysis confirms near-perfect reliability with negligible weep risk under vibration. The design provides precise, independent fluid replenishment to each battery cell without float valves or active controls, extending battery life while traveling with the battery assembly.