Have I got a bad alternator?

[stevie888]

Modern battery testers are very sophisticated and give an in depth report on the battery state far beyond "it's flat", so worth getting the battery checked out before deciding to replace anything.

I had a head scratcher a couple of years ago when the battery was flat and wouldn't hold charge. Turned out it was my battery charger that was defective which was a result as a good quality relacement was a lot cheaper (and easier) than the battery or alternator.

 

[Harry Bloomfield]

Modern battery testers are very sophisticated and give an in depth report on the battery state far beyong "it's flat", so worth getting the battery checked out before deciding to replace anything.

I agree, and must stress that voltage alone, and reading from a table of condition, can often very misleading - as I was reminded just this week.....

Stair lift with two LA AGM 12v batteries in it - though not a matched pair, stopped dead, halfway up. The good battery read 13.1v, the failed one 12.35v after a resting for a while. The table posted earlier, would suggest the 12.35v should mean a near 70% state of charge, however - apply even a small load to it, and the 12.35v drops away, to almost nothing. I have come across the same, numerous times.

 

[Johntheo5]

Yes, indicative at best, here is a (VCDS) log from a 2019 Polo (petrol), 6 year 5 month old with only 21,615 miles, all very short runs of a few miles/day, original 59A EFB battery, the minus 4.99A is from the DRLs (daylight running lights) which are on once the ignition is switched on, required to communicate with the diagnostic cable.
The 12.4V "battery voltage at rest" does ~ equate to 71% SOC. the internal resistance is the one I would watch, not sure what the "not normed" one at 5.4mOhms means but if the 7.8mOhms is used then the voltage drop on a assumed 300A starting load is, 300*7.8/1000, 2.34V, giving a starting voltage of say 12.1-2.34, 9.76V, not sure what voltage is required to pull in the starter solenoid, maybe 9V ish?.
This model does have a smart Alternator which charges at just short of 15.0V on overrun but normally is around 13.0/13.2V with a occasional burst of 14.8V while driving normally.
I will be quite happy if I get 8 years+ from the original battery.
I have never had to use a trickle charger on any car in over 60 years of driving, 3 of which were in the family for over 21 years.

IDE01834 Battery voltage 12.015 V
IDE01836 Battery current -4.992 A
IDE01837 Temperature of battery sensor 6 °C
IDE01839 Battery charge level 71 %
IDE01841 Battery internal resistance 7.8 mOhm
IDE01842 Usable battery charge 23 Ah
IDE01843 Battery voltage at rest 12.4 V
IDE01948 Battery internal resistance not normed 5.4 mOhm