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Battery Module Conduction Calorimeter

Accurate heat generation data from batteries and ultracapacitors are essential to properly design thermal management systems. We use a customized battery module conduction calorimeter to measure heat generation at various rates, temperatures, SOCs, and heat capacities from full-size, multiple-cell battery modules and ultracapacitor stacks. State-of-the-art high-power cyclers cycle the modules in the calorimeter.

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Welcome to NREL's Energy Storage Laboratory, located in Golden, Colorado, where we use high quality equipment to conduct battery thermal management, modeling and analysis.

NREL's Energy Storage Lab has a state-of-the-art calorimeter (cal-oh-RIM-e-ter), which measures heat generation from energy storage devices. Data from the calorimeter helps researchers design thermal management systems for hybrid electric, electric, and fuel cell vehicles. This information enables the transportation industry to improve the cost and efficiency of advanced vehicles.

It tests energy storage devices, such as batteries, over various charge/discharge cycles and drive cycles, while measuring heat losses as low as 5 milliwatts.

The calorimeter consists of an isothermal bath and a cavity test chamber. The bath has an operational temperature range from -30 degrees Celsius to +60 degrees Celsius.

A battery is placed in the test chamber and connected to a power supply.

The schematic shows the test chamber's heat flux gauges, surrounded by isothermal bath fluid. The heat flux gauges measure the rate of heat generation from the battery. These gauges can detect heat generation rates from 5 milliwatts to 100 watts.

A computer measures the electrical power going into the battery as well as the heat being released from the battery.

The bottom (or red) graph shows the battery's power profile over time. The top (or green) graph tracks how much heat is released from the battery over time. When the electrical power to the battery is turned off, heat generation stops and the battery cools back down to the temperature of the surrounding bath.

This heat generation and efficiency data from the calorimeter allows NREL researchers to help the transportation industry improve batteries and thermal management systems for advanced vehicles.

Description

Photo of a researcher testing a battery in the calorimeter.

Battery Module Conduction Calorimeter.

This heat-conduction-type calorimeter is based in part on a commercially available isothermal calorimeter (CSC Model 4400 Isothermal Microcalorimeter). Heat conduction calorimeters sense heat flux between the sample and a heat sink, an enclosure that is fabricated with aluminum surrounded by an isothermal bath which contains the sample. If the sample is hotter or colder than the heat sink, heat flows between the heat sink and the sample. In practice, the thermal conductivity of the path between the sample and the heat sink is matched to the expected heat flow to minimize the temperature difference. The temperature of the heat sink is kept constant and the entire calorimeter shielded from its surroundings by a constant-temperature bath. The temperature control of the heat sink (together with proper matching of the thermal conductivity of the path between the sample or measurement cavity and the heat sink) renders a passive isothermal measurement condition.

Photo of a testing chamber.

Calorimeter test chamber.

The measuring unit of the calorimeter comprises a 39 cm L x 21 cm W x 20 cm H aluminum enclosure connected to a large aluminum heat sink via heat flow sensors (semiconductor thermoelectric devices) between the heat sink and the sample cavity. The bath temperature, which operates at -30°C to +60°C, is controlled with a stability of 0.001°C. For calibration purposes, the measuring unit also incorporates electrical heaters that allow for heat input at rates of 1-80 W. The measuring unit is designed so large-gauge leads, which must be connected to the sample battery for charging and discharging experiments, achieve thermal equilibrium. The large gauge leads generate a negligible amount of heat even at very large current. Further, they are attached to the isothermal aluminum enclosure, and have an insignificant impact on the accuracy of heat generation data obtained for battery modules. The battery temperature (internal or surface) in the calorimeter is measured with an accurate platinum resistance temperature device (RTD).

The measurement cavity can be either dry or filled with a dielectric, inert heat-transfer fluid. The air in the dry chamber can be stirred with small fans while the liquid-filled chamber is stirred with constant-speed stirrers to speed heat transfer from samples to the measurement chamber walls. The small amount of heat added for mixing is taken into account. The time response of the calorimeter is not affected by the stirring. However, the improved heat transfer between the sample and the calorimeter will significantly shorten the time required to reach a steady-state response for a battery at constant power. Even under the best circumstances, the time constant for a typical large battery module will be much longer than the response time for the calorimeter.

Read the following to learn more about this unique tool. "A Unique Calorimeter-Cycler for Evaluating High-Power Battery Modules".