Several countries and cities announced last year that they would prohibit sales of petrol and diesel cars until around 2040 so that Electric Vehicle (EV) has a big change to spread all over the world. This situation raises a great expectation for EV being introduced under massive renewable energy because EV can provide frequency regulation reserve in power system under the intermittent renewable energy generation. Many EV and Vehicle-to-Grid (V2G) researchers have been studying EV batteries control methods for both EV users' convenience and contribution to frequency regulation, however, they are still insufficient regarding especially EV users' convenience. In this paper, a new decentralized control method of EV battery is proposed and its advantage is verified. It enables EVs not only to sufficiently retain EV users' convenience but also to contribute to frequency regulation.