system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a battery decrease without being used?
gpt
All batteries slowly lose charge over time.  This is because all batteries use a chemical reaction to store energy.  Over time this reaction breaks down inside the battery.  

Without getting too deep into the chemistry the reactions in batteries are non-spontaneous, meaning they won't just happen in the world on their own.  They require energy (the charging) to happen.  When this energy is not there it will try and get back to the way it was before the energy was there.

Each battery has a different self discharge rate different depending on the type. Here are a few in "normal" temperature range.  The higher the temperature the greater the rate.

Primary lithium-metal
10% in 5 years

Alkaline
2–3% per year (7-10 years shelf life)

Lead-acid
5% per month

Nickel-based
10–15% in 24 hours, then 10-15% per month

Lithium-ion
5% in 24hours, then 1–2% per month (plus 3% for safety circuit)