Great comments so far. My plan is to also leave my battery at around 30% to 40% charged between uses. I've been putting the battery on the rapid charger until the 50% begins to blink then pull it off soon afterwards. One more important note re battery longevity: Store your batteries in a cool location whenever they're not in use. I believe below 60F is optimal but below 80F or so will do fine. I'll be keeping my batteries in my garage fridge which is kept at around 45F.
We have final word: unused batteries will automatically kick into self-storage after 30 days, whether or not they’re on the charger. Also, You may feel warmth from the button on the top of the battery- that’s normal. Know that these are Lithium-Ion batteries that are designed to hold their charge for along time, so if your battery doesn’t kick into self-storage within 30 days, do not be alarmed. It may take longer than that to actually trigger the self-storage discharge.