I have a 1999 Mustang w/ only 54, 000 miles on it.. It sits for the most part, but during these times the battery continuously drains. I have checked for lights being left on, radio issues, trunk lights, and under hood lights. I have had a shade tree mechanic look at it but w/ negative results. Ideas??????

 

Get a battery tender.

 

I use a trickle charger on my 67 mustang fastback and the battery always maintains full charge.

 

There are a few tests that can be done. if you own a DVM ( digital volt meter) it will make things easy and fast. How old is the battery? the battery may not be holding a charge due to age so keep this in mind. A fully charged battery should be at 12.6 volts a reading of 12.0 or lower and it is dead. 12.4 volts and it would be around 75%.

set teh dvm to read milliamps ( make sure the the meter has a 10amp fuse to protect it if there is a high drain) open the hood and leave the car off and alone for about an hour, the pull off one of the cables and connect the meter in sieries with the batter ypost and clamp and take a reading. should be about 20ish ma normal. if it is high then pull fueses/relays one at a time until the reading drops ( don't pull the pcm until the other circuits are checked) once you have found the circuit causing the problem then check/test the related items on the circuit.

somtimes a faulty diode in the alt can cause the battery to drain, a bad diode can leak current back. a quick way to test this, make sure the battery us fully charged. disconnect the alternator overnight and see if the battery has run down.

hope this helps.