Batteries produce electricity through a chemical reaction. When it is colder, that reaction creates less electricity.

If you fully charge a battery, freeze it, then warm it back up, it will still be close to fully charged.

It’ll impact the usable charge because the colder temps will decrease the ability of the battery to produce high currents. Effectively setting the floor for a “useful” charge level higher. Below that useful level, current supply is too low to power whatever it is you’re powering.

It’s pretty well known (in industries that use batteries) that temperature affects the capacity of batteries.