Ultra-low voltage is the key to ultra-low power consumption. Hence, there is a drive to lower the operating voltage of wireless sensor System-on-Chips as much as possible. State-of-the-art components already operate below 1V and some below 0.5V. However, commercial rechargeable lithium-ion batteries show a high nominal voltage in the range of 3.4-4.1V. Were such batteries paired with ultra-low-voltage systems, cost effective power-conversion design (without off-chip components) becomes difficult and energy will in all likelihood be lost in the conversion. While sub-2V lithium-ion batteries exist, sub-1V water-based electrolyte batteries with high energy density should also be developed. Shown here is experimental low voltage lithium battery with an ultra-low-voltage DC-DC converter / processor test system. Simulations on the DC-DC show from 12% to 74% efficiency improvement depending on the configuration. Measurements on the processor show 83% energy-delay product and 39% energy / operation improvement.