A battery has two ratings:

The reserve capacity is an estimate of the maximum length of time it is possible to travel at night with a minimal electrical load and no generator output. Expressed in minutes, it is the approximate time the fully charged battery can support a 25 Ampere load without the battery terminal voltage dropping below 10.5 Volts at 27°C (80°F).

The CCA is expressed at a battery temperature of -18°C (0°F). The current rating is the minimum amperage which must be maintained by the battery for 30 seconds at this temperature, while meeting a minimum voltage requirement of 7.2 Volts. This rating is a measure of CCA.