This is one of those threads that concerns what I do for a living - I am not a mechanic, but local garages bring cars to us when they have a "battery drain" problem. Police and other public safety vehicles come into our shop ALL THE TIME where the complaint is:
"Battery goes dead from time to time". or
"Battery goes dead on the weekends". (This last one is most common ... any drain of an amp or two over the weekend will usually take a battery down enough it can't start the car).
I am convinced that the ONLY way to find this is to use logical troubleshooting techniques. A voltmeter WILL tell you if the charging system is working, as you will see the voltage rise with the engine running.
But it won't tell you anything more. Voltmeters cannot tell you where current is flowing.
How we do it:
Pull the positive battery cable off and
insert between the battery and the cable an ammeter (see link below for the kind of meter I am referring to). You will need clip leads for this - and try to not let them come into contact with any grounded metal. If this sounds too scary, insert the meter in the GROUND CABLE as described.
IF the leads touch ground now, you won't blow any fuse or set anything on fire - you will just short your meter out and it won't see the current flowing -
but you will not damage the meter or anything else.
The drain you are looking for could be as little as 1/2 amp or less. Use a DVM or some other meter that will display currents down to .02 amp. Current drain of under .1 amp is probably normal. .1 amp is 100 milliamps. We often see drains of 20 to 70 mA (.02 to .07 amp) that are there to keep up memories in the various modules in the car.
But if you see anything over .1 amp, start pulling fuses until you find out where the current is going!
Once you find the fuse that causes the drain to go down to an acceptable value, you have found your problem.
Do not expect most garages to be capable of this - it's a simple test but most mechanics are MECHANICS - it is not up to them to understand what "voltage" and "current" actually mean.
So, get a REAL meter:
Fluke 27 II and 28 II Digital Multimeters
And BEWARE - these meters have fuses in them. IF, suddenly, you see no current flowing - verify that the fuse is good before you assume anything! On the low scale, the fuse will blow at something between 1/4 amp (250 milliamp) and 2 amps (2,000 milliamps).
DO NOT WASTE YOUR TIME TRYING TO USE A BIGASS AMMETER you have GOT to be able to see small currents. GET A DVM OR DMM like in the link! Look for a DC current scale of something no larger than .200 amp (200 mA). You will find several fuses that are responsible for 10 or 20 mA of current - no big deal. But I bet ONE fuse will be carrying .1 to 2 amps - that fuse is feeding the culprit of the problem!
Even Lowe's and Home Depot PROBABLY have DMM's with DC current scales of .2 amp!