Batteries self-discharge. Electricity doesn’t like being all bottled up. The battery has barriers inside to keep a bunch of electrons on one side and hardly any on the other. But they seep through eventually and the charge equalizes, killing the battery.
Then when lead acid batteries run low, crystals form on the inside which degrades them and keeps them from charging up again. It happens pretty fast. You should always keep car batteries, and any lead acid batteries, charged to prevent this.
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