The battery that came with my MCS when i bought it used was weak and I am hard on them with stereo systems, so I wanted to put the biggest battery in that I could. I opted for a Group 34R Sears DieHard Advanced Gold AGM. Does not have vent tube openings, but it is an AGM battery and they are sealed, so no need for the vent tube. Here it is, sorry for the phone pics:

Please ignore my 1/0 ground, 1/0 power and circuit breaker for my stereo

 

Cool!

 

How did it fit through the pass through hole? Any suggestions for others wanting a stronger battery?

 

You have to remove the plastic trim to replace the battery. There is actually room for a battery that is a bit wider. Also, if you want to use a battery without reverse terminals, there is enough slack in the cables to make that work as well.

 

I replaced my Clubman's stock battery with this 34R DieHard and it fits perfect. I suspect their computer does not approve it for the Mini because while it it has no vent pipe, if it should ever vent, it will do so right into the passenger cabin as the air intake is located in the battery compartment. $139 from Sears. Paid for it online and they sent me a note a few minutes later it was ready to pick up. Sears will install for $20 but I wanted to fiddle with it so I could be satisfied it fit without fiddling.

I also have the IBS but the swap has worked without any warning lights. My stock battery was being controlled by IBS in that I had to reset my date/time a few times before it sank in that the battery was on the way out. A "Hey, time to check your battery!" warning would have been more helpful.

 

I put the same battery in last year....perfect fit!