If the battery is down on voltage that means a cell has gone bad or is in the process of going bad....then it just gets worse. 12.4 is still pretty good. Mainly if its showing like 12.2 or less then its time to replace. One of the other cars in my sig had a 9 year old energizer battery that worked great up until one day, when it randomly died and wouldn't even crank at all. Voltage prior to that had been about 12.25 or so...and it was for quite a long time before the failure.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)