Even with a 300 mA draw the battery should last for more than 3 days. I would guess with that load it should last over a week (if decently charged). How do you know that is the current draw?

How old is the battery? I am guessing you might just have an old battery ready for the grave, or the drain current is much larger than you think. Causes listed below are good candidates for that.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra