The Smart-UPS On-Line provides protection for servers, voice and data networks, medical laboratories, and light industrial applications with high density and double conversion. The Smart-UPS On-Line can be used as a rack or tower. The lithium-ion batteries allow for higher performance at elevated temperatures and a longer lifespan for the UPS. When mission-critical applications require runtime measured in hours rather than just minutes, the Smart-UPS On-Line can be configured with multiple battery modules to meet these extreme autonomy time requirements.