My 2012 K3500 HD dually/ Duramax LML takes about 3 to 4 minutes to finally reach a 14 volt reading on the ammeter gauge after I start it. On my previous 3 Duramax trucks (2001, 2003, 2005), attaining 14V was within seconds whereas this one stays at 12.5V and slowly increases over the 4 minutes or so. Anyone know if this normal for this model?