you need to measure voltage with the engine running, does it die right away?
12.40 to 12.39 could also be explained by a drop in temperature, but you might have a drain.
an easy way to measure voltage is obc test #9 you have to unlock your obc first. look under 'information' at this site http://bmwe32.masscom.net/
edit- after reading your other thread.... if you are charging your battery or have just put a new one in i think the engine should run at least for a short while just off the battery no? if it's not doing that i would say you were on the right track and i would suspect something else in the ignition system like the wires maybe.
if you can narrow it down to the charging system i would check the battery and engine to ground cables.... if that's not it you are in the same boat as me and might want to look at shogun's alternator cable replacement here.
http://twrite.org/shogunnew/fixes/el...cable_between_