Project
| # | Title | Team Members | TA | Documents | Sponsor |
|---|---|---|---|---|---|
| 7 | Automated Thermal Battery Cell Tester |
Daniel Songer John Stimpfl Joon Lee |
Stasiu Chyczewski | design_document1.pdf final_paper1.pdf photo1.JPG photo2.png presentation1.pptx proposal1.pdf |
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| # Joon Lee, Daniel Songer, and John Stimpfl – wonjunl2, dsonger2, and stimpfl2 ## Automated Battery Thermal Cell Tester ### Problem Lithium-ion batteries are being increasingly employed in industries such as EVs and solar power systems. The biggest drawback with this technology is that lithium-ion batteries have a tendency to start extremely dangerous and destructive chemical fires. Generally, the cause of this can be traced to either a manufacturing defect or improper use of the battery. The latter cause is often avoidable and in many cases is due to a lack of insight and knowledge of how a battery is currently behaving versus the expectation. One specific example is the battery management system (BMS), which uses different sensors to manage these large battery packs. But most BMS’s on the market today do not work across all chemistries because they lack the knowledge of the characteristics of the cell’s chemistry that is being managed, leading to very serious implications. On the contrary, a battery chemistry that has well-defined characteristics can be optimized for performance as well as allowing for safe and dependable operation. One problem with this is that defining these characteristics is a bit difficult for those less technically inclined; characteristics that can be unique to each battery cell depending on the chemistry, temperature, and stage of life of the battery. Therefore, the issue we would like to address is the lack of testing equipment available to people working with lithium-ion batteries. ### Solution We are proposing an automated cell-tester with a thermal chamber for a complete definition of battery characteristics at all temperatures. The chamber would house a single cell of a given chemistry and run that cell through the necessary tests to accurately and succinctly communicate the cell’s characteristics to the end user. Specifically, we want to know the overall capacity (in Ah), the state-of-charge to open circuit voltage relation (SoC-OCV curve) of the cell, and the equivalent circuit model (using two time constants) of the cell. Ideally, we want to have the user be able to select which tests they would like to run, press “play”, walk away, and return hours later to a very understandable representation of the battery characteristics at the given temperatures. ### Solution Components **Battery Charger/Discharger** The device will need a good amount of power electronics including IC chargers, power resistors, fuses, and whatever circuitry necessary to isolate the battery completely from the grid and the chip. The power resistors are what the controller will use to demand current while the IC will do coulomb counting and voltage reads. Given that we are testing lithium-ion cells, this circuitry will ensure the battery is run under safe operating conditions. **USB Interface** The device will include a USB interface for communicating with the user’s computer. **Applet** Although we are leaning towards using Lab View, we are also considering using a Java applet to convey information to the user as well as allow for control of the device (definitely open to suggestion). This will also allow us to store data from the device’s tests on the local machine. **Thermal Chamber** The device includes a thermal chamber which ought to range at least between 0 degrees Celsius and 40 degrees Celsius as a minimum due to the drastically changing characteristics of these cells under extreme temperatures. This will include a heating coil, as well as some form of cooling element which has not yet been decided upon (because freezer/compressor solutions are not ideal for space or weight). **Power Supply** We will include a power supply that attaches to the AC wall port. This will provide the necessary voltage and current for charging the battery. This will also include a power supply as well as circuitry for safely driving current to an isolated battery cell. ### Criterion for Success We will be running three specific tests (note: this can be done for any temperature selected by the user so long as it is within our range) with each their own criterion. We will compare our results with different common chemistries used in the market to determine the accuracy of our device (note: C stands for the capacity of the battery e.g. a rate of C/10 implies 3 Amps for a 30 Ah cell). **Tests**: 1. **Capacity Test** is a test to measure exact capacity of the cell. This test would include charging the battery to full, and discharge fully at a rate of C/5. Coulomb counting will allow us to determine the capacity of the cell, within a 1% error, given that we have a 1% error on our Coulomb counting IC. 2. **Constant Charge/Discharge Test** is relatively straightforward. It charges and discharges the battery at a constant rate of C/10 while reading the voltage as often as possible and reporting this to the applet. Given that we ought to know the capacity with coulomb counting, we can infer the SoC-OCV curve. Again, we will be looking for a 1% error as we are dependent on the Coulomb counter here. 3. **HPPC Test** is the most involved test and will require some back end work in order to implement. This test is used to find the RC polarization characteristics that all lithium-ion batteries exhibit. To do this, the controller will drain 3xC from the battery until 10% is removed (SoC = 90%), then charge for 20 seconds at C/10, then let the battery rest for an hour or two, then repeats this until the battery reaches 0%. This is a really tough thing to test and there isn’t much likelihood we will have the measurements precise enough for 1% of error given that even our equivalent circuit model isn’t perfect, therefore we would like to see a 5% error here. |
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