Large-Volume Battery Calorimeter (Text Version)
NREL's Energy Storage Lab has designed and constructed a state-of-the-art calorimeter that measures heat generation from energy storage devices and power electronics.
Overheating can reduce the life of energy storage devices, such as battery packs, so thermal management is very important. Data from the new calorimeter helps researchers design more effective battery thermal management systems for hybrid electric, plug-in hybrid electric, fuel cell, and electric vehicles.
NREL already had a calorimeter that measures heat generation from hybrid electric vehicle battery modules and cells when industry identified the need for a larger calorimeter. Industry members determined that a new calorimeter was needed to test larger modules and battery packs for plug-in hybrid electric vehicles and other electric vehicle applications.
At about the same time, NREL identified the need to measure heat generation from modules that are liquid cooled. That capability was missing in our module calorimeter.
This animation shows the critical components of the new calorimeter's design.
It consists of a heating and cooling system for keeping the bath surrounding the calorimeter test chamber isothermal—in other words, at a uniform and constant temperature.
The external enclosure of the calorimeter consists of a lid and stirring motors for circulating the isothermal bath fluid, and an outer casing that houses the insulation surrounding the isothermal bath.
The bath uses 160 gallons of thermal fluid, which can be drained to gain access to the test chamber. The isothermal bath has an operational temperature range from –30 degrees Celsius to +100 degrees Celsius, and it is controlled to within one thousandth of a degree Celsius.
The test chamber is the heart of the calorimeter. When the chamber is removed, we can see the supporting structure along with the heating and cooling coils for the isothermal bath.
Twenty-four clamps tie the lid to the test chamber, as shown here. The lid prevents the bath liquid from entering into the test chamber.
A closer look at the pipe above the calorimeter test chamber reveals a burst disc. The burst disc prevents the accidental rupture of a cell from damaging the inside of the calorimeter. If the burst disc should be activated, it is placed above the bath liquid level to prevent the fluid from entering the test chamber. When the lid is removed, we can see the cavity of the test chamber. Within the cavity, the battery is connected to liquid cooling lines and to one of two sets of busbars on the walls of the test chamber cavity.
Between the internal and external walls of the test chamber are two hundred and eight heat-flux gauges connected in series to measure the heat produced by the energy storage devices. The flux gauges can measure heat signals as low as 15 milliwatts and as high as 4000 watts.
To show how the calorimeter works, we will place an energy storage device back in the test chamber, and connect it to a programmable power supply to charge and or discharge the device.
When measurement signals are sent to the control computer, the computer calculates 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 electrical power to the battery is turned off, heat generation stops and the battery cools back down to the temperature of the surrounding bath.
The calorimeter thus provides key data about heat generation and efficiency. NREL researchers then use that data to help the transportation industry improve batteries and thermal management systems for advanced vehicles.