Posting here for more visibility and because the problem is likely identical across R52 and R53's.

Part 1:

My 2005 R52S has developed a pretty bad battery drain - it will be dead within 36 hours if left connected.

I have had the car at an independent MINI specialist and he has, at various times:

1 - pulled all fuses and relays
2 - replaced the alternator
3 - replaced the battery
4 - removed the iPod connector
5 - disconnected the EWS, BCM, speedometer and ignition modules
6 - checked all 3 ground straps
7 - checked the steering pump
8 - checked the HK amplifier

He also verified that there is no evidence of corrosion/water/dust/rodents, etc.

Part 2:

The saga continues.I took another stab at it this morning. I took the opposite approach: I disconnected every fuse and relay from under the hood and the cabin panel, then started adding them in one by one.

The end result was pulling fuses f4 and f24 in the cabin removed all of the large parasitic draw. I did this twice with the same result. Problem isolated - hooray! F4 is the "general module control unit" and F24 is the "immobilizer and 3 ultrasonic sensors" per a youtube video i watched. Here's where things get really weird, really fast.

Weird thing #1 - With those two fuses pulled, the car starts and runs with no error lights. How can it start with the immobilizer fuse pulled??

Weird thing #2 - with those 2 fuses pulled, the "you left your key in the ignition" chime sounds when the driver door is open, even though the key is in my pocket and I haven't started the car.

Weird thing #3 - with those 2 fuses pulled, I was able to lock/unlock the door with the remote ONCE. Subsequent attempts, the remote doesn't work, but the power locks work with the key put in the door/turn.

Weird thing #4 - the ultrasonic sensors have never functioned in my car since I bought it earlier this year - however, occasionally, I'll get the "reverse alarm" on for a few seconds when putting the car in reverse (manual transmission) - this behavior hasn't changed since the fuse pulling experiments, but thought I'd share since one of the fuses controls these sensors.

Note that the mechanic, as part of his diagnostics, physically disconnected the entire ignition module from the steering column and it had no effect on the parasitic draw. Also, the EWS/immobilizer was sent in alongside the ECU for diagnosis and tested good. The mechanic also, as part of his testing, disconnected the body control module which did not have an impact on the parasitic draw, and noted no corrosion/moisture/bad connections/signs of rodents or moisture.

Part 3:

The weirdness keeps giving.

This morning, I went out to the car with my voltmeter and the battery was already down to 12.02V after sitting about 11 hours (after a long drive). So, the drain is still present - even though it was totally gone TWICE yesterday, and those 2 fuses are still pulled.

Given that the immobilizer fuse is pulled and is still somehow getting electricity and the ignition thinks the key is in (chime sound) whenever the driver door is open (even when it is in my pocket) leads me to believe that the culprit is somewhere between the two. The mechanic pulled the entire ignition assembly from the steering column and the drain was still present. However, just to reiterate, the draw was down to .01A yesterday upon pulling those two fuses, and it did so twice.

12.4-12.5 immediately after driving and disconnected
12.2 after driving, connected
12.01 10 hours after driving

 

See if he is going to sleep - it takes 16 minutes. If it stays awake, battery will be drained. Check draw, and see if it drops.

 

The components go to sleep, but the drain continued unabated. I thought I had a breakthrough with pulling those two fuses, which, twice, brought the drain down to almost nothing. However, with those fuses still pulled, the battery still drained overnight.

 

How old and what type of battery is in the car?

 

I've tried 2 batteries, one new under a year old regular lead acid, the other brand new AGM.

 

Does your Mini have the Harmon Kardon stereo system ? Because the amplifier can cause a parasitic draw on the battery ,when going bad .

 

Already investigated above. Removed and no change. No sign of water, dirt, corrosion or rodents.

 

How to test for parasitic draw …
https://www.innova.com/blogs/fix-adv...parasitic-draw