MacBook Air M1 battery completely drained after not using it for a few months

I powered off my M1 MacBook Air a few (~3) months ago and I was surprised to find out that it had drained completely when I tried to power it on today. Why did this happen? If I recall correctly the battery level was around 50% when I kept it in storage, so what caused the complete drain? Will it harm the battery life? How should I store my MacBook without using it for a prolonged period of time and not harm the battery?