Thermal Properties of Microstrain Gauges Used for Protection of Lithium-Ion Cells of Different Designs
- Wednesday, 24 August 2011
Commercial uses include lithium-ion batteries used in a human-rated environment, such as in automobile applications.
The purpose of this innovation is to use microstrain gauges to monitor minute changes in temperature along with material properties of the metal cans and pouches used in the construction of lithium-ion cells. The sensitivity of the microstrain gauges to extremely small changes in temperatures internal to the cells makes them a valuable asset in controlling the hazards in lithiumion cells. The test program on lithium-ion cells included various cell configurations, including the pouch type configurations.
The thermal properties of microstrain gauges have been found to contribute significantly as safety monitors in lithiumion cells that are designed even with hard metal cases. Although the metal cans do not undergo changes in material property, even under worst-case unsafe conditions, the small changes in thermal properties observed during charge and discharge of the cell provide an observable change in resistance of the strain gauge. Under abusive or unsafe conditions, the change in the resistance is large. This large change is observed as a significant change in slope, and this can be used to prevent cells from going into a thermal runaway condition. For flexible metal cans or pouch-type lithium-ion cells, combinations of changes in material properties along with thermal changes can be used as an indication for the initiation of an unsafe condition.
Lithium-ion cells have a very high energy density, no memory effect, and almost 100-percent efficiency of charge and discharge. However, due to the presence of a flammable electrolyte, along with the very high energy density and the capability of releasing oxygen from the cathode, these cells can go into a hazardous condition of venting, fire, and thermal runaway. Commercial lithium-ion cells have current and voltage monitoring devices that are used to control the charge and discharge of the batteries. Some lithiumion cells have internal protective devices, but when used in multi-cell configurations, these protective devices either do not protect or are themselves a hazard to the cell due to their limitations. These devices do not help in cases where the cells develop high impedance that suddenly causes them to go into a thermal runaway condition. Temperature monitoring typically helps with tracking the performance of a battery. But normal thermistors or thermal sensors do not provide the accuracy needed for this and cannot track a change in internal cell temperatures until it is too late to stop a thermal runaway.
The microstrain gauges under study have shown remarkable changes in resistance with changes in temperature that show a very close tracking to the current used to charge and discharge the lithium-ion cells. As the cells are charged, there is a very slight increase in temperature at the end of charge, and the same during the discharge process. Although normal thermistors do not show a big change in temperature, the strain gauges have been able to track with great accuracy the thermal changes in the cells during these processes. Although strain gauges have been used to track pressures internal to cells in battery chemistries that use pressure vessels such as the Ni-hydrogen cells, they have not been used to track resistance changes due to temperatures. Existing thermal sensors do not have the sensitivity to be able to track small changes in internal temperatures of the cells, so monitoring systems cannot detect changes fast enough to be able to provide any protection. With lithium-ion cells, when the thermal sensors record an alarming temperature reading, it indicates that the cell’s internal temperatures have reached a point where no external controls can stop the thermal runaway. With the thermally sensitive new strain gauges, the changes in slope are so sensitive that this change can be used to stop the charge or remove the load on the lithium-ion cells before the event can spiral into an uncontrollable one.
Four different configurations of lithium-ion cells of were tested. Two cylindrical cells with metal containers (two different diameters), a prismatic metal container cell type, and a pouch cell type (aluminized plastic pouch) were used for the study. Several tests were performed on all our designs. The tests included normal charge and discharge cycling at two different charge and discharge rates at room temperature and at low temperature. The tests also included off-nominal conditions of overcharge, overdischarge, external short, heat-to-vent, and crush (simulated internal short). Overcharge tests and external short tests provide the most valuable data as these conditions produce the worst-case reactions in lithium-ion cells.
This work was done by Judith Jeevarajan of Johnson Space Center. MSC-24764-1
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