How to determine when to replace battery that has always been charged / maintained?

Even on a maintainer, it will eventually die a slow death. I had a Die Hard Silver series that lasted 16 years on a maintainer. One day, I left for the store. When I went to leave for home, wouldn't even energize the solenoid....nothing.
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Every engine start can be considered a load test, comparable against itself.


This^ 5 digit , 3 wire voltmeter claims to sample voltage 5 times a second.
While it might not catch the absolute minimum voltage the battery is able to maintain when turning the starter, if one were to watch it each engine start, but especially ovrenight cold engine starts, one will see it fall lower as the battery ages and the colder it is, or when it is undercharged.
A good healthy battery will likely hold 11v+ starting an overnight cold engine in mild ambients.
A weak battery will likely fall to the 9v range.
A battery at death's door will fall to 8.

Some vehicles hate voltage falling this low during cranking. It's certainly harder on the starter.

The third yellow wire is for voltage sense.
Ideally, both ground(black) and sense+ wire( yellow) would go right to battery itself, the red wire to a source that turns it on with the ignition.

Battery life Nazis will likely benefit by insulating the battery from radiant, and convective heat from engine/radiator. Depends on the vehicle as to how effectively this can be accomplished, or if it is worth it the effort, but the battery which stays 10C cooler on average, will likely last 2x as long, all other factors being equal.

Trunk mounted AGMs usually report some of the best lifespans, and AGMs are not really happy when they are not regularly topped up to true full.

But keep em cool, and fully charged, and they will still fail in their duty, at some point.
 
I just replace my batteries at 5 years. I do not want my wife (in her 70's) to be stuck somewhere because of a dead battery.
Same, I would rather replace it on my own terms in my driveway with access to my tools instead of being at a store or parking lot and it won’t start and then I have to call her to bring some tools and then go through the hassle etc..
 
When the car does not start or it is hard to start! A Battery should last between 3 to 5 years or so!
Had an 11 yo Costcp in wifes car. Worked well but she got paranoid with an 11 yo battery. I replaced it with another Costco so I did not have to hear about it. Just kept water level up and cleaned terminals on it and block regularly. Had a 12 yo Delco that was getting weak so didnt wait for it to go. It was kind enough to warn me. LOL. For some reason, all my batteries last a long time. Dont want to jinx myself saying this
 
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