Quote:
Originally Posted by JWBFX4
Well just to add a little to this.
I have had a good bit of electrical problems a while back (not on truck) and after talking and researching a lot about batteries, everything says that 12.6v is a fully charged battery. Anything above that is considered a "hot charge" Possible the 12.9 volts that your battery maintainer says is where it automatically kicks off to make sure it charged the battery enough 
I was always told if the battery is atleast 12.2 volts its in good cranking range anything under that needs charging and if it in the low to mid 11's very possibly bad battery. |
Welcome to the discussion!
I doubt there's any "hard and fast" figure. I'd certainly consider 12.6 V in a resting state battery to be fully charged. And, my manual states that age will cause all readings to drop, but the 1.5 V difference between fully charged and discharged is supposed to be fairly constant.
My experience with batteries seems to support this. And yes, 12.2 V should give you sufficient cranking voltage under normal circumstances. But, it might seem "weak" at that voltage.
Your "hot charge" is what you see on a battery right after disconnecting a charger. There's a "surface charge" that has to dissipate and it takes a few hours to do this if there's no draw. I think it MAY be a capacitive effect.
But, when you have a charger on the circuit delivering over 13 V, it keeps increasing the battery voltage to match. At some point, the charge voltage has to drop off or the battery will be overcharged (boiled). My BatteryMinder says this is 14 V. I know in the maintenance mode, both "maintainers" hold about a 13.5 V state, and according to the manuals, deliver very little Amperage.
Once you disconnect the chargers though, the surface charge (the "hot charge" of 13+ V) bleeds off and the battery settles into its "resting" state. I've honestly never seen 12.9 V here, but 12.7 V seems about right.
One of my trailer batteries suddenly went bad recently, while I was charging it through the onboard, regulated charger. At near full charge, I was used to seeing it draw about 34 Watts at 120 Volts. Suddenly, it spiked to about 96 Watts. I checked the batteries, and one was hot and "gassing". I pulled it out and immediately tested the voltage - it was around 11.2 V. My guess is, it had a shorted cell.
- Jack