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Quick Overview and Recommendations:

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Individual cell testing will take about 2 weeks to complete, and can be done concurrently with module prototyping in order to not push timelines back.

I strongly advise testing individual cells for all parameters that we can fit into our timeline.

If we can get access to a capacity testing machine on campus or from Keysight, that would be amazing. In the reports below, there was rarely a strong correlation between capacity and any other parameter, and there were always outliers in capacity.

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The goal of cell testing:

The more data and the more informed we are about the status of our pack, the better, safer, and more reliable our pack will be.

Any imbalances present in the pack at the start will only get worse as the pack is cycled. Within series modules, this variance can be overcome with cell balancing, but within parallel cells there is no way to overcome this. Due to the nature of li-ion cells being a manufactured component, there is always some degree of tolerance from the manufacturer. This page will discuss the parameters that can vary, how they affect pack imbalance, and what testing we should be doing to eliminate it.

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What parameters can be measured, and do they cause pack imbalance?

As-Received OCV - Assuming the cells were manufactured at the same point on the voltage curve, the as-received OCV is an indicator of the amount of self-discharge current of the cell. See the self-discharge method.

Self-Discharge - (https://literature.cdn.keysight.com/litweb/pdf/5992-2517EN.pdf?id=2911018). Keysight has developed a self-discharge measuring unit, which incorporates a super-high precision DC voltage source and a super high precision current measurement device (accurate to the tens of uA). The bucket method described in the document provides a quick (1 hour) test of the self-discharge current. A single cells having a significantly higher self-discharge current will cause the module to have a larger self-discharge current. Over long periods of time (such as sitting idle in the bay for a month), this self discharge current will cause an imbalance in SoC of the cells and lead to decreased pack capacity. Over the course of a week long competition, Self-Discharge will not cause any significant difference in SoC.

Cell Weight - The weight of a cell is an indicator of how much material is inside the cell. According to this paper (https://www.nature.com/articles/srep35051), the weight of a cell correlates loosely with the capacity of the cell, before factory screening occurs, as tested on 5,473 Boston Power Li-ion cells (Swing 5300 5.3Ah). See capacity section for the effect of capacity on imbalance. Another study (https://www.hnei.hawaii.edu/sites/www.hnei.hawaii.edu/files/Initial%20Conditioning%20Characterization%20Test.pdf) did not strictly test the correlation between capacity and rate but, however the capacity ration vs weight was 'barely correlated' with a 'r' value of 0.47.

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Capacity - The difference in capacity between cells in parallel groups causes current spikes in these cells at the end of charge or discharge cycles. In order to maintain a stable cell voltage, the cells will discharge at different rates. This will create different voltage drop across the cells due to IR and thus different OCV. When the current from the pack stops being drawn, then the higher capacity cell with charge the lower capacity cell until the voltage is again equal. This rapid change in current direction and extra cycling on these cells decreases the pack life (https://www.sciencedirect.com/science/article/pii/S0378775316313921?via%3Dihub).

The above figure is based on simulations, however it shows the reverse charging current well.

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The second set of graphs show Initial Conditioning and Characterization Tests performed on 100 Panasonic NCR18650B cells, and a similar amount of outliers and standard deviation is noted. The Panasonic cells have been around for a long time. An email from Steve McMullen, ASC Inspector, notes that "Their testing was also with very mature technology Panasonic 18650 3.2AH cells that have been manufactured in the millions if not billions. The newer cells don’t have as much credibility. I believe your team is using LG CHEM INR 18650 MJI which is a variant of the technology to trade off Rate for Life and Weight for Capacity, so the cell is already working in realm different than the Panasonics". We should be aware that the MJ1 cells we are using could have more variation than the data from the panasonic NCR18650B and other LiFePO4 tests noted below.(https://www.hnei.hawaii.edu/sites/www.hnei.hawaii.edu/files/Initial%20Conditioning%20Characterization%20Test.pdf - red/blue/black graphs).

Given a nominal capacity of 3500mAh, results from a capacity assuming a range similar to the graphs above will give 3360-3640mAh, a difference of 280mAh from best to worst cell, 140mAh between the average and worst cells.

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Internal resistance mismatch between cells can lead to sudden capacity losses and a decrease in overall cycle life of around 40% (http://web.mit.edu/bazant/www/papers/pdf/Gogoana_2013_J_Power_Sources.pdf). While we are not concerned with the cycle life of our pack, sudden capacity losses must be avoided. The graph below shows that capacity suddenly drops by roughly 20% around the 100-150 cycle mark. The cells in these tests were LiFePO4 cells, and the internal resistance measured by a 15s 40A pulse, with a distribution shown above from 13.5 - 21.5 mOhm. This is a fairly large spread (50%) or IR and the currents that the pack were tested at are much larger than anything our cells will ever see. A high current test such as this decreases the time that the mismatch effects take to show up as the cells are performing at their peak characteristics.

The capacity loss shown in the figure happens after around 100 cycles, and is due to the mismatched cells being exposed to large charge/discharge currents as the ends of the charging and discharging cycles due to differing SoC-OCV curves cause by the mismatched resistance.

Internal resistance mismatch causes current mismatch on charge and discharge cycles. The current mismatch creates voltage drop difference and thus a difference in SoC between the cells in parallel groups. Thus there is an SoC mismatch between the cells when current is being drawn, which leads to internal current spikes when the external current flow stops (https://www.sciencedirect.com/science/article/pii/S0378775316313921?via%3Dihub),

A capacity drop in any one of our modules will affect the whole pack. If one cell in one module drops by 10% capacity due a an uncaught cell parameter outlier (IR or capacity), then we lose (3.5Ah * 10% * 3.7 * 36 = 47Wh) 47Wh of energy in our pack. That single cell will then also start to degrade quicker and the module will have to be replaced. Going by the efficiencies of other winning solar cars, 47Wh could add an additional 1-2 Km of range to the car (and avoid headaches from increases in variation after many cycles).

Thoughts on Temperature Variation:

Pack-Variation: As the temperature of a battery pack increases, the self-discharge rate also increases due to the increased rate of the chemical reactions inside the cell. An increase in 10 degrees Celsius will double the amount of self-discharge current. (Keysight link) The self-discharge will still be in the high-uA range (maybe low mA in the worst case), and thus not be an issue in module or pack balancing as the passive balancing circuit will take care of the pack-level variation in self-discharge. Assuming the temperature within the pack is only hotter when we are drawing current, the change in DC Internal Resistance will more than offset the change in self-discharge current.

Module-Variation: Temperature variation within a module, under extreme circumstances, can result in a positive feedback situation (until a certain delta OCV is reached). A decrease in internal resistance of the cell due to an increase in temperature will cause more current to be drawn from that cell, which will lower its OCV. Due to the close proximity of cells within a module, the temperature variation (unless caused by a cell with initially high IR) will be very small ans thus its effects negligible. With temperature variation, the useable capacity of the cell will increase as the internal resistance decreases and thus we have less power loss in the cells due to IR. (Show IR-temp graph and Capacity-Temp Graph)

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How long will testing take? - We will only conduct capacity testing if we can get access to the right cell cycling equipment. The University of Waterloo's logo is on Neware's site, so I am sure we have some high volume cell cycling machines on campus (just have to figure out where they are and if we can use them for a week). Setting up the testing of the other factors (weight, as-received OCV, DCIR, AC impedance), will take a few days, so we can schedule 1 week for cell test setup. Performing the cell tests will take a day, so we can schedule 2 days. After all the data is collected, it will take a few days to go through the data and group the cells based on lowest variation.

This testing can happen concurrently with module prototyping, so I do not believe the timelines will be shifted at all. This data and process will also prove to be invaluable when creating future packs.

The last thing we want to happen to our pack is to have our packs be at risk of reduced capacity due to failing cells or to have to replace a module during the middle of competition. Nothing can ever 100% guarantee that a spontaneous internal short can happen inside the packs, however proper testing and handling will significantly reduce the chances.

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