I charged it and it dropped down to 12.26 overnight. I guess one of the cells has gone then?

Its a Kirkland battery. Still under warranty so I'll take it back to costco and get new one.

Just seems weird it all started after not running the car for a month. When I first started the car, the antenna was spinning which was really weird too. After we charged the battery it stopped though. I unplugged it just in case that was what was draining it, but still looks like the voltage slowly drops after charging.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k