In general, many electrical problems are intermittent. And many times there is doubt about whether it is a true electrical problem vs. a weak/old/problematic battery problem. How many times on these (or any automotive) forums do you see an intermittent electrical problem (starter, alternator, literally anything else) that results in an unneeded battery replacement as the first step?

And I actually once had a tough intermittent battery problem on my old 911. Very simple electrical system on that car, and I would have about 1/10 failure to start, randomly. Hooking up jumper cables would make it start instantly (no need to charge). Battery, alternator, everything I could measure tested fine. Removed and cleaned all ground straps, etc. In the end it was basically process of elimination and I swapped in a new battery. It was not until about 6 months of no failures that I was sure I had fixed the problem.

So in that case, it was an intermittent internal battery problem. Due to its rare occurence, one would need to be lucky to ever detect it. But it was a real problem when it happened, as you can imagine.

That experience led me toward the setup where I try to have interchangeable batteries so at the first puzzling observation, I can just swap batteries and see what changes. My X5 can swap with my Cayenne, and my 3 Japanese cars can swap. The 911 is not directly interchangeable, but I could get it done if needed.

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2001 X5 3.0i, 203k miles, AT, owned since 2014