I am in southeast Pa. Gets chilly. I don't drive my truck very much but I do make sure I drive it once a week. I started to notice it wasn't preheating before starting up, but would start fine but then the screen turned on but nothing worked except for the climate. Once driven a while all worked as normal.
Got home and put a Noco Genius10 charger on it. Left it a few days, checked the voltage on batts, it said 12.3v! But again the truck started right up and again no preheat, but everything worked, it was 50 degrees outside, the first time it was 30 degrees. Voltage in the truck (on dash) first said 11v then I revved it a little it went to 14v and stayed there..
Let it run a bit, shut it down reconnected charger. Voltage ended up going back to 12.3v after sitting on charger overnight. When it reads 12.3v the charger is slow flashing green meaning maintenance mode apparently, in charge mode it will go to 14+v then down to 13+v then back to 12.3v.

Sooooo, battery issue or charger issue?

 

I'd suspect a battery issues, especially if the battery is 3 or more years old. I'm not familiar with the interface on the Noco Genius10, but if it has a voltage display and that's what you're going off of for the 14+, 13+, 12.3, etc., it's probably periodically checking the no load, no charge voltage on the battery to determine state of charge. So it stops providing current to the battery, monitors the voltage (hence the 13 down to 12.3V), determines the battery is not fully charged yet, and goes back to charging. This is pretty typical for modern smart chargers. The fact that the battery is never holding more than 12.3V would suggest to me a battery that is just waiting to leave you stranded.

-Rod

 

Voltage fluctuation after start up in the cold is normal with these diesels, the grid heater will cycle, sometimes even triggering battery save mode while its running, there's no issue there.

I'm not sure the level of the charger your using but batteries charge with a higher voltage that resting, if you have on say a 10amp charger, you should be reading 13-14v at the battery, a 20amp plus charger could read over 15v. This is why boosters are very dangerous on modern vehicles they can easily spike 16+ volts in the system and damage sensitive electronics

If your heater is not cycling I would start with some type of data reader on the OBD and see if you can read both engine coolant temp and air intake temp, if these sensors are providing false readings warm enough the heater will not run. and 30 degrees is not cold enough to cause starting issues with or without the heater

 

I forgot to mention the batteries are 1 yr old, die hard gold. Also I'm reading the voltage with a meter, it doesn't show on the charger.

 

It did do exactly that stated in your first sentence. The charger is 10amp. I will see if I can read the temps with my OBD.