what is normal voltage drop when measuring between the negative cable on the consumer battery. mine is about 1.5 but my battery keeps dying.

 

the 2005 to 2008 221's use 37 amps with the ignition turned on ! Even a brand new fully charged 26kg consumer battery in the boot will be at a critically low voltage within 30 minutes without the car running
At 18 months the rear will no longer cope for more than 20 minutes. If you like a fight peel back the label and fight the screw top lids of (double oring) and put 120ml of distilled water in each cell and it'll be good for another 10 years

 

how much when off and locked

 

it should cope sitting doing nothing for 3 weeks at a time with OK batteries, if the early pre 2007 cars with two batteries one at the front to start it and a big one in the boot behind the rear passengers seats (get to it via the boot/trunk), its supposed to cope with the stupid inefficient electronics (but it can't really). The one at the front will likely get it going with extras on the dash even if the back is down past 11v