Plug in hybrid electric vehicles (PHEV) and electric vehicles (EV) are using large lithium ion battery packs to store energy for powering electric traction motors. These batteries, or Rechargeable Energy Storage Systems (RESS), have a narrow temperature operating range and require thermal management systems to properly condition the batteries for use in automotive applications. This paper will focus on energy optimization of a RESS cooling system. The battery thermal management system for the General Motors Chevrolet Volt has three distinct modes for battery cooling: active cooling, passive cooling, and bypass. Testing was conducted on each individual thermal cooling mode to optimize, through control models, the energy efficiency of the system with the goal of maximizing electric vehicle range. While the three modes were intended to be mutually exclusive, optimization of the individual modes led to scenarios where all three modes could possibly be utilized depending on driving habits, ambient conditions, and power available. This paper will present the strategies that were used to determine the most energy efficient thermal management system controls for cooling the RESS. This cooling optimization strategy can be applied to current and future battery thermal management systems.