Lithium-ion batteries have higher energy content and power density than Nickel-metal hydride (NiMH) batteries, but require carefully management for durability and safety. Unlike NiMH batteries, which are controlled on a battery unit basis, each lithium-ion cell generates a different voltage. Typically, the complex controllers required to equalize individual cell voltages are large and costly. We have developed a low-cost battery monitoring unit that performs the same function with a proprietary cell-voltage equalizing system. This new unit also offers various innovative technologies, such as detecting overcharge and over-discharge, fault diagnosis and the measurement of the batteries internal resistance to monitor degradation.