W211 220cdi 06 facelift charging

[SHAMGIT]

Sons car on dash before startup it says its battery is showing 12.4.
When driving it stays just below that then after about 10 to 15 minuets it jumps straight up to 14v
Any ideas please .
thanks in antisipation.

 

[mioba]

The sensor in the alternator or the alternator itself (worn bushes) to hazard a guess

 

[onefortheroad]

14v is normal. Not sure why there would be a delay. Possibly the regulator in the boot ?