You may want to consider using a good-quality battery maintenance device to keep the battery properly maintained during storage, preferably units that are microprocessor-controlled and have specific settings for AGM batteries. This is the method that OPTIMA® batteries recommends:
When your vehicle goes into storage, connect the maintainer – with the battery either in or out of the vehicle. This will ensure a full charge when you're ready to use your battery again. (Always remember to follow manufacturer instructions.)
Some battery maintainers, like the OPTIMA Chargers Digital 1200 12V Performance Battery Charger and Maintainer, also include ring terminal leads. These will offer easier access to maintain the battery both during storage and in between uses. Hook up the leads to the battery connections and extend them so they're safely accessible from under a bumper or wheel well. The leads can remain there indefinitely so you'll never have to open the hood to keep the battery fully charged.
You'll extend the life of your battery and ensure your vehicle is ready to use with these simple battery storage tips. Just remember – check the battery's voltage before your first drive no matter what storage method you use.