Battery voltage 2013 Freightliner Cascadia

If you have the hvac blower, mirror heat, wipers etc on at low idle it usually shows mid 13s, 14.1-2 with everything off

 

Blower speed should be on high.

The idea is with everything running. The alternator should have enough juice to power everything and still have a half volt left to charge battery.

That's yesterday's world though.
Today's alternators are higher output. 13.5 volts in today's world would probably be the new sign.

In your case. It's clearly time for a new alternator. A good alternator will read 14 volts. Even with all accessories running.

In the future. Buy one of these. Newer trucks don't have volt meters so I run one of these. It shows voltage. And when the phone is plugged in. It alternates between vehicle voltage and cell phone amps.
There are many models to choose from so take your pick. These are going to be my new charger buys. No more buying just a plain charger.

https://www.amazon.com/Jebsens-Char...+phone+charger+with+voltsmeter,aps,155&sr=8-8