A feasibility study was conducted to find if a lithium-ion battery would safely shut down in the event of a short circuit. The MonoDAQ-U-X’s configurable front end enabled us to easily control and precisely log the data of a lithium battery destructive safety test.
The subject of our experiment was a commercial 18650 lithium-ion cell. We were interested in the cell’s voltage, discharge current, and surface temperature in a short circuit event.
The cells voltage (4.2V when full) can be measured directly with a buffered voltage input on the U-X. More information on voltage input configurations can be found in the manual.
Shorting the cell causes a large current to flow. Consequentially to get an accurate voltage reading we need to measure the voltage right at the cells contacts and not downstream where resistive voltage drops would skew the measurement.
The best way to ensure an accurate voltage measurement is connecting a thin wire to each of the cells contacts away from the main current path. In practice this was achieved by spot-welding two nickel contacts to the cell.
Importantly, the contacts need to be as close to the edge as possible. The cell will be placed in the black holder in the background. So that when the brass current carrying contacts are screwed in to secure the cell they don’t touch the nickel strip and skew the voltage measurement.
Since we will be shorting the cell we expect a current far exceeding the MonoDAQ-U-X’s internal shunt capabilities. In fact the current ended up reaching close to 100A. To measure such large currents an external shunt is necessary.
We built ours from four 10 mOhm shunts in parallel giving a shunt resistance of approximately 2.5 mOhm. While the resistors used were not high precision the MonoDAQ-U-X can conveniently be used to calibrate the resistor and that’s exactly what we did before the test.
The voltage drop over the shunt is measured with a differential voltage input with 1 V enabling us to measure short bursts of current up to 400A. The shunt must be designed so that it’s temperature increase during the test won’t significantly change its resistance.
The temperature measurement was straightforward. The end of a thermocouple was attached to the surface of the cell with Kapton tape.
The thermocouple was directly connected to the MonoDAQ-U-X. More details on connecting thermocouples can be found in the manual.
Shorting the cell
To short the cell, a MOSFET was used. In order to turn on the MOSFET, the MonoDAQ-U-X’s excitation pin is connected to the MOSFETs gate pin. Excitation was set to 12V to start the test.
If logic level MOSFETS are used they could be switched with the digital outputs instead of the excitation pin. This enables multiple MOSFETs to be controlled at the same time.
In our case, we only have a single channel to switch. That’s why a high excitation voltage was preferable. It achieves a lower MOSFET ON resistance compared to digital outputs which produce a maximum of 4 V.
To reduce the resistance even further two MOSFETs were used in parallel.
To have a visual indicator when the MOSFETs turned on a LED was also connected to the U-X excitation pin. This makes it easy to sync the video and logged data. The red LED can be seen in the photo from earlier showing the thermocouple
The entire test setup, the experiment and the recorded data are shown in the video: