I replaced my cars battery last year with the correct size AGM battery all seems fine until a few months ago I stopped for fuel, when I came to restart the engine it struggled to turn over and the fault light came on so I stopped and turned off, I turned it back on and it started with no fault lights.

It did the very same thing this morning again at a fuel station, this time it stared on the 3rd attempt and the engine fault light stayed on, the car drove normally when I arrived home I turned the engine off and plugged in my OBD diagnostic tester only for the fault light have reset.

So tested the battery 12.5 v and kept monitoring the voltage it went down to 12.3 v within 30 minutes not enough to start the car, left it another hour came back battery back up to 12.5v and the engine started

12.5v appears to be low for an AGM battery, apparently I should be seeing 12.7v +, now before I go out and buy another battery as anyone else experienced this issue?

Previous Mog Plus 4 Fiat Twin Cam.
Plus Six