DescentBB

I've been detaching my laptop battery and putting it in my closet, for some reason. I'm assuming it shouldn't be completely charged when I do this, nor completely dead. Like, maybe close to 50%?

I'd leave it in the laptop.

Lithium-ion batteries store best at around 40% charge, but they will have to be recharged once every couple months and they will still lose capacity over time (although less than if they were stored at 100% charge). Letting them discharge completely can kill them if the voltage drops below a certain level because a safety circuit will trip and break the connections (otherwise the cells might explode if you tried to charge them). A closet is generally a good location, although the basement might work even better if it is usually a little cooler. Some people looking for really long storage life will keep them in the fridge but the safety and effectiveness of doing so is debatable.

The main thing to avoid is storing them someplace hot, excessive heat (like inside a parked car during daylight) can damage or rapidly degrade a lithium-ion battery.

Exercising batteries occasionally helps them have a long life, especially ones that aren't used for long periods and do better than the ones constantly cooking on a charger.