Can anyone tell me what the at idle Voltage should be for a 05 F350 6.0? It seems low on my trueck, but no knows for sure in my area(without taking it into a shop). My batteries don't seem to be getting enough charge to restore their health and the parts shop said it may be my alternator.

It checking, at idle I am getting between 13 and 13.5 volts with the AC off. However with it on(I live in Phoenix so that is 99% on) it is about 12.5 avg. The last few days I have had issues starting and I think its cause my batteries have a low charge, but I don't want to simply change em out if the problem is really a bad alternator.

My readings are from my Edge Insight, as well as the Checker Auto tester on my truck while running.

Any help would be appreciated,

-John.