Volts test at battery.

[b5blue]

  Had a discharged battery yesterday so being a new Red Top I charged it till full. (AGM setting on charger.) I let it sit 24 hours with - post disconnected and tested volts, read 12.86. Hooked up the - post and fired up the car fine and ran till ALT gauge centered. Volts now test at 13.35 so normal.
  Next checking things out with ignition off I disconnected - post, put my red meter probe on main ground wire and black probe on - post. I got a reading of 10.25? Ohms read 3.35 so I figure I'm reading things like Alt. field and FUEL gauge? 
Opinions? Had the car a long time but just thought to take readings this way and not sure what they mean. I get no arc touching the ground cable to - post and had left my new installed A/C on with the car off so not sure if that bled down the battery somehow. 

[green69rt]

Unless the battery has a shorted cell it will always read about 12.5 volts even if it's almost used up.  But that doesn't mean much, what matters is the load voltage and the time-to discharge of a full battery.  Hooking the battery up to your car will lead to all sorts of funny readings.  So...you need to take the battery to a shop and have it tested.   Then work from there.  I'm not really sure what you describe but it sounds like you're measuring the leakage to ground ( the diff between the battery post and the car chassis.  Might be worth while to take the battery out of the car and clean it off, set it on a insulator and do the same test, should read zero.  All things considered it sounds like a leak to ground which would drain the battery.

How long did it set to discharge?  For a few hours then this doesn't sound right,  a few days possible.  Also possible that you got a bad battery to start with.  Did the seller test it before installation?

Now after thinking about this I would have the battery checked, may be defective.

[b5blue]

Red Top is 2 months old. It pooped out overnight, 14 hours or so. I agree it's going to Advance Auto for a checkup readout. (I didn't have time.) A/C on shouldn't have functioned with ign. off the way I have it wired so possible brake lights on overnight. (?)

[Pete in NH]

Hi Neal,

If you meter has a current range  set it to the highest current range it has. Disconnect the negative battery terminal and put the positive meter lead on the cable end and the negative meter lead on the battery post. Any current reading on the meter would be a problem that is pulling your battery down. I have to wonder if it's something associated with the new A/C installation but, it could just as well be a light bulb left on somewhere like the trunk or glove compartment.