Due to increasing power and safety requirements in battery electric vehicles, modern battery systems require a well-reasoned thermal management solution. One method to design such systems is a direct immersed cooling system. This system takes a fluid into direct contact with the batteries which are cooled or heated. Like this, not only an optimal temperature control but also an enhanced operational safety can be accomplished. Since thermodynamic processes depend on a lot of parameters, close to reality testing is required to obtain reliable results and differentiate different fluids regarding their cooling abilities. APL has developed and built up a test rig, the BatteRig, which tests how much energy a fluid can take from real cells utilizing direct immersed cooling. A fluid, in both its fresh and artificially aged state, is defined as the test specimen. In the evaluation, the measured temperatures are analyzed first. Subsequently, a physics-based approach is derived to determine the amount of heat dissipated. Using this method, the aged fluid which mimics ten years in duty is compared with the fresh fluid. The results are compared and show that there is no significant difference in the performance of a fresh and artificially aged fluid.