Dear Jeepers,
I hope you'd be able to help me on this one. 8 months ago I purchased a brand new Optima Yellowtop AGM battery for my Jeep Wrangler Sahara 2010 to replace my old one. Occasionally I'd measure the voltage on the battery to make sure it's OK and I get readings between 12.5 and 13.2 Volts. Today I measured it before stating up the engine and I got a 15.1 reading. When I started the car I measured 17.2 Volts. It's a cold day but the temps are in the 40's F. I read online that this is an abnormally high voltage. What would you recommend to me to do? I don't know if this helps but a while ago I disconnected the grounding wire strap that goes from the front hood to the frame to repair it and still haven't reconnected it (but I'm not sure if that specific strap has anything to do with the high voltage).

 

So the battery wasn't charging (engine off) and you measured 15.1 volts at the terminals? I'd check the way of measurement first. Is your measuring device working properly?

 

I decided to check the battery voltage on my other car (Renault Megane 1999 with a wet battery), and I got a similar reading (> 15V). Then I checked 2 days later when the weather was warmer (without driving the cars) and the voltage seemed to be around 13.1-13.6V for both. From this I assume that the outside temperature does affect the car battery voltage and in the cold weather it would be normal to see 15V.