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Parasitic Draw Test
1) Remove the negative side battery cable from the negative battery terminal.
2) Connect the black wire to the com input on the multimeter and the red wire to the 10A or 20A input on the multimeter. the meter needs to be able to read at least a 2 or 3 amps for this test to work. Connecting the red wire to the mA input on the multimeter won't work and could damage the meter.
3) Attach a multimeter(set the dial on the multimeter to measure Amps as per multimeters instructions) between the negative cable and the negative battery post. Wait a few seconds to several minutes for the car to go into sleep mode - i.e. when you make the contact with the ammeter, the cars computer systems "wake up". After a bit of time they will go back to "sleep".
4) If the ammeter is reading over 25-50 milliamps, something is using too much battery power.
5) Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings)last. Perform the same steps for relays found in the fuse panel. Sometimes relay contacts can fail to release causing a drain. Be sure to observe the ammeter after pulling each fuse or relay.
6) Watch for the ammeter to drop to acceptable drain. The fuse that reduces the drain is the draw. Consult the owners' manual or service manual to find what circuits are on that fuse.
7) Check each device (circuit) on that fuse. Stop each lamp, heater, etc. to find the drain.
8) Repeat steps 1&2 to test your repair. The ammeter will tell you exact numbers.
9) You can also try unhooking the big wire from your alternator. The alternator can sometimes have a shorted diode that can cause amps to flow through the alternator's power cable and through the shorted diode and into the case and through the bolts and back to the negative battery terminal. This will drain a battery in a hurry. Make sure to read the ammeter before and after unplugging your Alternator.
