mini's battery died

How long should a automotive battery last? The common wisdom would tell you it depends, but not more than 5 years on average. My 2005 (bought in 2004) Mini Cooper S still has the factory battery after 12 years. I have never tax it so it finally gave up during a winter cold spell. I charged it up and went shopping and almost got stranded.
I begun researching for a proper replacement. I refuse to pay the inflated price for a "factory" replacement from a BMW dealer. Further, I just didn't want to pay anymore than I need to for a generic replacement. From my research it is group 47 or H5. I way hoping I can get one from Costco, but no luck, according to one major Mini forum. Costco ceased to sell it many years ago.

It was just a few days before Christmas and I wanted to find a replacement with the least of trouble.

the factory battery that failed

first I verify it is indeed the battery that has failed instead of the charging system, or it being drained by something in the Mini

the electronics in the Mini draws about 65 mA of quiescent current - passes common sense test

As the factory battery is quite compact, I thought there may be a chance that I can find a battery at Costco that might still fit. I prefer buying battery from Costco over other usual suspects for comparable price. I carefully note the dimension of the battery box in the trunk.

with the aid of a vice grip I lifted the battery out and carefully measured the dimensions

I also verify Walmart has the H5 (group 28) drop-in replacement in stock before I headed out for battery shopping. I planned my trip to stop by Costco first. Should there is no possible suitable battery at Costco I will just go next door to Walmart to buy the H5 drop-in.

Fortune smiled at me. Scanning the Costco's battery stock I found one larger battery with one dimension larger than the factory H5. It is group 29 (H6). It is wider than the factory H5 by nearly 1.25 inch. Fortunately I planned for this as I measured the battery box's dimensions. There is some chance the larger battery would fit, though a small chance.

Not wanting to come home without a battery that would fit for sure, I dropped by Walmart to pick up the H5 also. I would return it if the larger one from Costco could not be shoehorned in.They are within a few dollars in price so bigger is better. The larger battery is about 10 AH more capacity.

I came home with two batteries

Costco H6 (left), factory H5 (center), Walmart H5 (right)

 here is the battery box with it's odd shape on the right side

I broke one of the stainless steel wedge-bolts for the battery terminal while removing them - I hate to find out how much the dealer want for a replacement

the larger H6 from Costco dropped in like a glove

I swapped the good terminal clamp from the negative side to the positive side as it is most critical for safety for it not to come loose. I was planning buying one from Amazon for circa $12. Upon some thought, I decided to come up with a workaround to avoid buying one. One of my worm-screw stainless steel hose clamps should hold the terminal secure, I decided.

To be doubly safe I added a terminal cover to the negative side.

The larger battery adds a few pounds of weight but give me about 17% more capacity for about the same price. Typically dollar to dollar a Costco battery is better quality than one from other DIY automotive retailers.

A point of interest. While servicing the battery I notice the positive battery terminal has an explosive charge device design to disconnect the battery in case of a collision.