Large-Volume Battery Calorimeter
With funding from the U.S. Department of Energy and input from battery and vehicle industries, NREL designed and fabricated the Large-Volume Battery Calorimeter (LVBC) to support the development of batteries for energy-saving vehicles. The feasibility of electric-drive vehicles—hybrids (HEVs), plug-in hybrids (PHEVs), and all-electric vehicles (EVs)—is currently limited by the high cost of lithium-ion batteries as well as their service life and performance in a wide range of climates. One important factor that impacts life and performance is the operating temperature of batteries, which is why proper temperature regulation of battery systems in these vehicles is critical.
The next-generation battery systems in electric-drive vehicles need to operate at maximum efficiency, performing at optimal temperatures in a wide range of driving conditions and through numerous charging cycles. NREL's Large-Volume Battery Calorimeter is the only isothermal calorimeter capable of measuring the efficiency and heat generation of the large batteries used in these advanced vehicles. The calorimeter chamber measures 40 cm x 60 cm x 40 cm—the largest of its kind in the world for battery applications—and has the ability to test cells, modules, sub-packs, and some full-size battery packs. The calorimeter is also uniquely able to test liquid-cooled batteries, such as those found in the Ford Electric Focus, the Chevy Volt, and the Tesla Roadster.
NREL's Large-Volume Battery Calorimeter differentiates between the heat generated from the cells in a module, the inter-connects between cells, the battery management system, the thermal management system, and the connection points to the vehicle. Understanding where the heat is generated aids in the design of the battery pack as well as the thermal management system.
The figure below shows a cross-section of the calorimeter and highlights many of its unique design features, including:
- Large-volume test chamber
- Precision-control isothermal bath surrounding all six sides of the test chamber
- Busbars for accurate testing of high-power batteries
- Ability to test air- and liquid-cooled batteries.
How the Calorimeter Works
A battery pack is placed within the test chamber, the electrical connections are made, and when necessary, the liquid-cooling lines to the battery are connected to an external pump and isothermal bath. Spring-mounted clamps secure the lid of the test chamber and maintain a seal regardless of temperature changes. The isothermal bath fluid is pumped into its stainless holding tank to submerge the entire test chamber on all six sides.
The isothermal bath is then brought to the desired test temperature, which can range from -40°C to 100°C. Due to the large thermal mass of the calorimeter—with roughly 1,980 pounds of material to cool—the cooling system employs a chiller sufficient to cool a 10,000-ft2 building. The heating capability of the system is sufficient to heat a 3,000-ft2 home.
The system uses a number of proportional-integral-derivative controllers to taper off the cooling as the calorimeter approaches the target temperature and to maintain it within ± .001°C. A commercial battery tester, featuring an integrated power supply and programmable electric load, then charges and discharges the battery pack, typically either to simulate real-world use across a full range of drive cycles or to test the battery pack at the limits of its performance specifications. The battery cycler can simulate any voltage, power, or current profile versus time to test the energy storage system under real-world conditions without having the vehicle present.
As the battery pack charges and discharges, it generates heat that flows from the energy storage system through the heat-flux sensors to the isothermal bath. The calorimeter measures this heat flow to a precision of ±15 mW. Once electrical cycling of the battery is complete, the calorimeter continues to collect data as the battery cools.
Automakers can use these data to predict battery performance and design thermal management systems capable of maintaining a battery pack within the desired temperature range. Regulating the operating temperature of a battery pack is essential because it affects performance (power and capacity), charge acceptance (during regenerative braking), lifespan, safety, and vehicle operating and maintenance expenses.
Using data from the LVBC may improve the life and operating temperature of batteries while optimizing the complexity of the battery management system, resulting in lower-cost batteries and enabling better market penetration of electric-drive vehicles.
For more information, refer to the Large-Volume Battery Calorimeter fact sheet.