Why battery voltage is not a good indicator of battery health

Because batteries are far harder than you think

Being the sort of geek I am, with access to an actual programmable load tester and TSD's python scripts, I'd actually load-tested the Trojan battery before I fitted it in May 2016 (it was already old then, maybe 10 years by the date label!), comparing the result when I finally removed it shows how performance degrades massively but the battery voltage at idle (no load) is almost identical.

The dead battery is managing about 3 hours at C/20 rate (5A for a 100Ah battery) compared to 18 hours it managed when fresh (but by no means new). And it's not managing the high-current spike at all - that's the first very low blue spike, at which point the test script gives up on that idea and just runs the C/20 tests.

Although the “good” battery starts a little higher (maybe 12.6v to the “bad” 12.4v), after a short period they're both floating up to around 12.4v at zero-load. Measuring the voltage, you would have absolutely no idea that one battery had about 100Ah capacity and the other had perhaps 25Ah.

Voltage tells us nothing much about how much capacity there is in a battery - you could make a perfectly healthy-looking 12v battery by stacking 8x AA batteries end-to-end, like this:

It will measure a healthy 12v but it's unlikely to start your car.

This is (sort of) what's happening in a dead or weak car battery - it can still measure 12v with no load, but it hasn't got the capacity left to actually sustain a large load like a starter motor.

If you're interested, here's the science behind the battery testing script, it's TSD's work so thanks to him for that.