The design engineers determined it "NEEDED" 550CCA for a fresh battery and then calculating the typical loss of CCA over the useable life of the battery and the wear and tear on the starter/engine etc. This would also cover the expected temperature range that is normal for the car. Now, if you do not live in a cold climate and don't expect to see below 32 deg F, then ignore CCA and look at CA. CCA is rated at 0deg F while CA is rated at 32deg F. A rough difference or derating is that CCA is ~80% of CA give or take. The only way to know how many amps your car "needs" is to measure it under whatever conditions it will see in day to day service.
Your CCA question is now clear. The answer is not answerable.
Well put!
As far as determining how a battery is doing, for years I had two methods.
First was checking the voltage after it was well-charged, either by a longer driving run with major load items, like headlights and window grid defroster OFF. But before measuring, knock off the Surface-charge effect by turning on headlights for a few minutes. Then turn lights OFF, wait a couple minutes, and read the fully-charged battery voltage. Often (not always!) an aging battery's fully-charged voltage will droop lower and lower as time goes by.
Second, and I believe most reliable, doing a hygrometer check, after doing the routine I just listed. Note absolute, and cell-to-cell variation of Specific Gravity of each cell's electrolyte. My experience is that most batteries die due to one or two cells crapping out, and the remaining cells would be okay. But since all 6 cells in a 12 volt battery are in series, any one cell holds the battery's fate.
Many batteries used to be "Maintenance Free", but you could still pry off the two 3-cell caps and test each cell. But even those seem to be disappearing, leading to truly "Sealed" batteries, so a hygrometer is of no use.
So I bought earlier this year on Amazon, this battery tester. Used by one of the auto repair guys I watch on Youtube, and respect.
https://www.amazon.com/Schumacher-B...22314&sprefix=schumacher+bt175,aps,126&sr=8-1
The price went up $5 - 6 since I bought mine. Can set it for various battery technologies, like Spiral AGM, flat-plate AGM, common flooded battery, Gel Cell. It will tell you if the battery is presently too discharged to run its proper test (charge it, then test), has different test method/outputs, CCA is what most of us would use.
As to how far under, percent-wise, I would go to decide to replace, I might go down to 80-85%, but I live in a warmer climate. It doesn't take much power to start an engine here, if there is no underlying engine/electrical problem. If I still lived in the midwest, very cold, slogging through snow and ice with headlights on and defrosters on, engine at low RPM (therefore lower charging capabilities), very cold mornings, etc. I'd replace it a lot sooner, I wouldn't be taking a chance!
Contrary to some experiences by others here, I have found that batteries in my environment last a lot longer than they did in the cold north!
Oh, I should mention... for batteries that I could pry off the vent caps, I WOULD add distilled water occasionally if cells were down. Add just to the bottom of the well sides, where the visual reflection of the electrolyte suddenly changes from flat, to concave (due to the surface tension created when the electrolyte level reaches and touches the bottom of the well tube). Maybe that's why my batteries last a lot longer?