Well, this was unexpected. They tested and tested, said the starter tests good. They replaced the battery and tried it a couple days of sitting and no issues. I picked it up today and so far it seemed fine.

As to the initial battery test, I was there, they hooked a battery test device up and it did something for a minute or two. The cables however were WAY too small to carry a full cranking load, so not sure exactly what it tests.

Time will tell if they got it right. It's a pretty trustworthy shop indeed if not honest they would have told me I needed a starter and a battery). Not that honest = always right of course.

Maybe it's time for a AAA subscription. :(

It does seem like the skinny cables can't handle the load but short time means they will just get warm.

They also don't pull the full current they use a prorated curve relative to the ambient temp. Eg. 500A @ 70F = 850 CCA.

Batteries can fail in many ways. Your acid test is repeatability when warm. Also, the acid test would be to repeat the hard start while hot, replace the battery and while still hot do a fast start.

I did drive it about 45 minutes in a round-about path home, stopped and started, it was about the same as starting at the shop. So not definitive as I didn't recreate the failure, but a good sign.

Still, always a concern when results are inconsistent - battery good, battery bad....

But as said above, the battery did not have long to live in a normal lifetime down here anyway, between being so far from the starter anyway, in a very high current car (e.g. when stopped), and in Florida heat, they don't last long.

I have had batteries tested that the store supposedly did a load test and said the battery was good but it was not. Many years ago I purchased a battery tester for that reason. I'm of the camp of trust but verify. I suspect the problem is most often the tech doing the test rather than a mystery battery issue. Testing a battery is a simple process with the right equipment. FYI, you can also verify the alternator wasn't a contributor by unlocking the cluster to see output in real time.