If the battery surface Voltage was only 12.36 Volts when you picked up the battery, it is not fully charge and/or is bad.

A fully charged battery should have a 12.6 Volt surface Voltage, it sounds like your battery is at least 50% discharged assuming it does not have a problem.

A large battery may take 6-8+ hours to charge. AGM batteries sometimes have longer charging time.

Typically charging Voltage should be between 13.5-14.5 Volts. My E70 runs right around 14.0 for the most part.

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2008 4.8i Black with Tabacco. Black Y-spoke wheels and shadowline trim.