> a spare sitting on a shelf

Does that work for batteries? I feel like unused batteries tend to become unusable batteries.

Sorry, my wording was misleading there. A spare Pixel 5 on the shelf so if something happens to this one I have an immediate replacement. I pop it onto a charger every couple months to prevent the battery from going too low.

I've had a "spare" smartphone kept in a drawer for a year and when I needed it it was impossible to charge and I've never been able to wake it up.

Would be better in a drawer in the refrigerator. Calendar aging for batteries is mostly about the temperature and storage SoC, which should be in the 30-50% level.