Battery thermal management is critical for high-power battery packs commonly used in electric vehicles (EV) and hybrid electric vehicles (HEV). To design battery thermal management systems properly, obtaining accurate heat generation data from battery modules is essential. To measure heat generation from full-size, multiple-cell battery modules, we developed and tested a custom-made calorimeter for large modules. Battery modules with dimensions up to 21 cm x 39 cm in cross section and 20 cm in height can be placed in the cavity of the calorimeter for measuring heat generation rates from 1 W to 100 W with the battery operating at -30(C to +60(C. The instrument is capable of measuring heat effects as small as 10 joules with accuracy of 5%. A state-of-the-art high power battery cycler is used to cycle the modules in the calorimeter. This paper provides a description of the calorimeter, calibration test results on its performance, and test results from a module simulating a HEV lead-acid battery.