I have a AGM from 2017 that even after leaving it charging for 3+ days, as soon as its disconnected and I turn the lights on for 3 minutes to remove that "phantom" charge and the battery drops down to about 12.38v-12.4v.

Is that a typical sign the battery is beginning to degrade, or nothing to worry about?

Reason I ask is that after a few weeks I'm dropping down to 12.2v and then starting begins to be a little sluggish.

thoughts? thanks

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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