13V? Seems high. I could see 13V one second after taking a battery off a charger and not connected to anything. Sitting there after a few minutes it might be 12.8V if it's in really good condition. Connected, a bit lower. Ideal way to measure suspect battery voltage is to already have the hood open and check them a couple hours after leaving the truck alone. You don't want to do anything to wake up the modules like opening a door, unlocking the doors. Or disconnect one and isolate the vehicle leads and then check the voltage.