Seiko Instruments Inc. (SII) has applied precision process cultivated by watch manuafacturing technology and started the production of silver oxide battery in 1975. We have a line-up of high quality silver oxide batteries for watches small to large, quartz to digital. Since silver oxide batteries are small batteries with large electrical capacity and stable voltage characteristics, they are mainly used in wrist watches. Silver oxide battery needed a small amount of mercury in order to prevent performance degradation and leakage. While environmental concern became an issue worldwide, SII started the research of mercury-free battery in 1991 in order to decrease the use of environmentally harmful substances in batteries. In 2006, SII finally succeeded in developing mercury-free silver oxide battery that has equal or better characteristics than the present silver oxide battery. Particularly, we have been able to significantly improve leakage resistance and discharge characteristics in low temperature compared to the present silver oxide battery.