Our evaluation of various characteristics of large-capacity 40Ah and 80Ah Lithium-ion (Li-ion) cells developed for industrial use shows that these cells exhibit reduced voltage characteristics and increased internal resistance in degraded conditions. We constructed a DC power supply system for telecommunications equipment using 80Ah Li-ion batteries incorporating a function to estimate the cell status from the voltage characteristics during an automatic discharge of the battery. The Li-ion batteries in this system are maintained by a floating charge method, and we miniaturized the battery unit. In a field trial test lasting approximately 2 years, this DC power supply system performed well inclusive of the automatic discharge. The specifications and characteristics of prototype 200Ah and 400Ah cells, which were assembled to expand the application areas of Li-ion cells, are shown. Because these cells have similar characteristics to those of current 40Ah and 80Ah cells, they can be widely used as a substitute for conventional valve-regulated lead-acid (VRLA) cells.