From Unofficial Zero Manual
The C rating of a battery is current relative to the amp-hour rating.
- "1C" refers to a current rate that would charge or discharge the battery in 1 hour if applied constantly.
- "4C" refers to a current rate that would charge or discharge the battery in 1/4 hour (15 minutes) if applied constantly.
Because a battery cell's voltage varies across its range of charge states, the way current is applied or available will differ through that range.