Using magnesium and either iron, tungsten, or molybdenum, the researchers were able to build a small battery that delivered a constant output for at least 24 hours. Warm salt water broke the battery down over weeks Sitting in a salt-water solution at 98.6 degrees Fahrenheit, it took a four-cell battery 11 days to break apart; That battery weighed about 3.5 grams, with cells measuring 3 cm by 1.3 cm and placed 4mm apart.