The majority of ATVs operate on a 12-volt battery architecture system (at rest). This is the traditional voltage system that is used by most vehicles within the United States.
The number 12 volt, however, is only a generalization. The number is generally higher than that and may range, depending upon which battery type your ATV is carrying.
Conventional, or flooded, ATV batteries use a voltage range of 12.6 to 12.8 volts on a fully charged battery that's at rest. An AGM battery, may have a voltage range that starts at 12.8 volts but can reach as high as 13.1 volts.