The preasure gauge is before the booster pump,I don't know if the normal 100 PSI pump is supposed to be used as a booster pump but I guess it reads the mains preasure,like I say when the battery is low the preasure seems to go up another 5-10 PSI on the gauge so when the battery is fuilly charged the reading is lower or is a coincidence the preasure goes up and down,the tank takes about 4 hours to fill on a gauge reading of 30 PSI