I've got a spare battery for my cell phone. I'm wondering what the correct way to store it would be, so as to maximize the battery time. Should I recharge the battery to 100%, then leave it aside, or should I use about 20-30% of the battery before putting it aside?
This Battery University page recommends you store them at 40% charge.
The answer to this would be impacted by how often you swap the batteries, i.e. how long do you store them?
If you use a battery until it is dead, then put in the fresh one, while the now-used one is put in a charger, then I would not consider this storage at all, but merely normal use.
If you usually use just the one battery, and have a second for emergencies, then @Thomas answer makes more sense.