Originally Posted by Doug Huffman

Major battery manufacturers/retailers often have battery selection dialogues that will take your vehicle description and return the proper battery.

Yes, I believe that some modules will flake-out, that's why BMW says NOT to disconnect the battery for troubleshooting charging issues, the evidence that the modules might retain will be lost.

It is the Twenty-first Century, not much is as it was.

Originally Posted by PAX5

Yes, take a picture of it and post it.

It should look like this: black case with a BMW sticker. The most important parameter of the battery that you should match is the Amp Hour (Ah) rating. It should be about 90Ah.

Problem is that most, if not all batteries that you can find in the retail stores, do not list the Ah number. Only if you have the store manager call the manufacturer, can they sometimes provide you with the Ah number.

You can calculate an approximate Ah number based off of the Reserve Capacity rating:

Amp/Hours = (Reserve Capacity / 2) plus 16.

But this is ONLY an approximation. If you want something more precise, you have to use Peukert's law (https://en.wikipedia.org/wiki/Peukert%27s_law).

If you used the above rule on the RC rating of the battery in the picture below, you'd get 96Ah, yet the label says it is 90Ah ...

After you install your new battery and register it, the alternator is programmed to charge the battery at the Ah capacity of the old battery.

If you install a battery with higher Ah rating, you need to code (program) the car and tell it that you have a battery of higher Ah rating. e70 X5's build prior to 10/2011 have the following set points only:

KLASSE_BATTERIE:
46ah
55ah
70ah
80ah
90ah
110ah
40ah_agm
60ah_agm
70ah_agm
80ah_agm
90ah_agm

In other words, you cannot install a battery with an Ah rating grater than 90. If you do, the battery will be constantly undercharged, and you'll compain that you have a "bad" battery ...

GL