In backup power sources for telecommunications equipment, it is desirable that the lithium-ion (Li-ion) cells have large capacity to minimize installation size of backup systems and to improve reliability of them by reducing a number of cells and accompanying protective devices. On the other hand, large-capacity Li-ion cell is difficult to manage safety problems. Therefore, safety improvement must be done to replace lead-acid batteries in conventional backup power sources. Improvement of lifetime has also to be achieved under the float-charging condition. We have been developing large-capacity Li-ion cells of which safety are improved by adding phosphazene flame retardant. In this work, 200-Ahs cell is fabricated and several safety tests under the assumption of telecommunications backup use are carried out. The results show no explosion, ignition, or thermal runaway. Forced thermal runaway test of a cell in the module which involves 13 cells are also carried out. There is no explosion, ignition, or thermal runaway in other 12 cells. In addition, 36-kW Li-ion battery system consisted of the 200-Ahs cells and battery management system (BMS). The system was operated for about 400 days in a state of floating charge. More than 95% of initial capacity is obtained.