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The UL 1974 standard51,52 covers the sorting and grading processes of battery packs, modules, and cells as well as electrochemical capacitors that were originally configured and used for other purposes, such as EV propulsion65,66,67, vehicle auxiliary power68,69,70, and light electric rail applications71,72,73. Furthermore, the focused purposes intend for a repurposed application, such as for use in energy storage systems74,75,76 and other applications for battery packs, modules, cells, and electrochemical capacitors. This standard also covers application-specific requirements for repurposed battery systems and battery systems utilizing repurposed modules, cells, and other components. (This standard does not include the process for remanufactured batteries, also referred to as refurbished or rebuilt batteries.)
The battery module can be decomposed into cells and used components according to UL 1974. The used components of the battery systems, such as the battery enclosure, battery management system (BMS), thermal management systems, and other auxiliary systems, should not be considered for repurposing if they have already been used longer than the calendar expiration date specified by the original manufacturer. The cells preparing for repurposing will undergo the performance test for sorting. UL 1974 suggests that the following test procedures shall be conducted by the repurposed manufacturer as part of the routine analysis of the incoming battery assembly:
1.
Incoming open circuit voltage (OCV) measurements (Sec. 19.2 of UL 1974)
2.
Incoming high voltage isolation check (Sec. 19.3 of UL 1974)
3.
Capacity check (Sec. 19.4 of UL 1974)
4.
Internal resistance check (Sec. 19.5 of UL 1974)
5.
Check of BMS controls and protection components (Sec. 19.6 of UL 1974)
6.
Discharge/charge cycle test (Sec. 19.7 of UL 1974)
7.
Self-discharge (Sec. 19.8 of UL 1974)
The charge and discharge profile measurement according to Sec. 19 of UL 1974 is divided into two primary procedures. The first procedure with detailed steps containing Secs. 19.2 and 19.4 of UL 1974 are listed in Table 1. The second procedure with detailed steps containing Secs. 19.5, 19.7, and 19.8 of UL 1974 are listed in Table 2. The key parameters in the procedures are described as follows.
Table 1 Test procedure 1 of charge and discharge profile measurement.Full size table
Table 2 Test procedure 2 of charge and discharge profile measurement.Full size table
In the incoming open circuit voltage (OCV) measurements (P1S1 in Table 1), the OCVs of cells (\(OC{V}_{ini}\)) are measured. The measured OCVs shall be compared to the minimum voltage limit acceptable for the cell specified by the repurposed manufacturer, e.g., \(2.5{\rm{V}}\le OC{V}_{ini}\le 3.5{\rm{V}}\) for LFP battery cell in this work. In addition, the OCVs are measured for a period of time (\({t}_{rest}=1\) minute) to further check the stability of the OCV. The incoming high voltage isolation check is ignored, since the battery module is decomposed into cells. The insulation breakdown check of the battery system becomes unnecessary. Three charge steps with small current rates (P1S2–P1S4 in Table 1) are added into the procedure for slow and safe charging. The cell is charged in standard CC-CV mode with constant current \({I}_{const}={C}_{R}Ca{p}_{N}\), threshold voltage \({V}_{thres}=3.5\) V, and cutoff current \({I}_{cut}=({C}_{R}-0.005{{\rm{h}}}^{-1})Ca{p}_{N}\), where \({C}_{R}\equiv I/Ca{p}_{N}\), also called C-rate, is the current I per unit of nominal ampere hour capacity \(Ca{p}_{N}\). The chosen C-rates in P1S2, P1S3, and P1S4 are 0.05 h−1, 0.1 h−1, and 0.2 h−1, respectively. The charge current is gradually increased to avoid abnormal voltage raising. (The details of standard charge and discharge processes are stated in the following subsection).
The capacity check of the battery cell according to the instructions of Sec. 19.4 of UL 1974 is designed as follows (P1S5–P1S10 in Table 1). The cell is fully charged by the standard CC-CV charge process under conditions \({I}_{const}=0.5Ca{p}_{N}/{\rm{h}}\) (i.e., \({C}_{R}=0.5\)), \({V}_{thres}=3.5\) V, \({I}_{cut}=0.05Ca{p}_{N}/{\rm{h}}\). Then, the cell is fully discharged by the standard CC discharge process under conditions \({C}_{R}=0.5\) and discharge cutoff voltage \({V}_{cut}=2.5\) V. The discharge ampere hour capacity \(Ca{p}_{D}\) is obtained after the full discharge process. At last, the cell is fully charged again for the next test, and the charge ampere hour capacity \(Ca{p}_{C}\) is also obtained, where the charge (discharge) ampere hour capacity is calculated by integrating the current I over the full charge time tc (the full discharge time td), i.e., \(Ca{p}_{C}(Ca{p}_{D})={\int }_{0}^{{t}_{c}({t}_{d})}|I(\tau )|d\tau \). The rest time between the charge and discharge processes is one hour.
The battery cells require capacity sorting before the next procedure. The obtained discharge ampere hour capacity of the repurposed battery cell is usually small than the nominal ampere hour capacity, i.e., \(Ca{p}_{D}\le Ca{p}_{N}\). The battery cell shall be sorted into various groups (\(Ca{p}_{RX}\le Ca{p}_{D} < Ca{p}_{R(X+\Delta X)}\)) according to the value of CapD, where \(Ca{p}_{RX}=(X/100)Ca{p}_{N}\) is the remaining ampere hour capacity and \(X\in {\mathbb{R}}\) is a positive real number. For example, when the battery cell is in the \(Ca{p}_{R80}\le Ca{p}_{D} < Ca{p}_{R85}\) capacity group, its discharge capacity is greater than or equal to 80% of \(Ca{p}_{N}\) and less-than 85% of \(Ca{p}_{N}\). In this work, X = 100, 95, 90, 85, 80, …, 10, 5, 0, and \(\Delta X=5\) is used to cover all capacity range without gap and overlap. It should be noted that the current rate (\({C}_{R}\)) in procedure 2 is based on the remaining ampere hour capacity (\(Ca{p}_{RX}\)) instead of the nominal ampere hour capacity (\(Ca{p}_{N}\)) in procedure 1.
The internal resistance check following the instruction of Sec. 19.5 of UL 1974 is listed in P2S2–P2S9 in Table 2. After the one-minute rest (P2S1), full charge at \({C}_{R}=0.5{{\rm{h}}}^{-1}\) (P2S2), and one-hour rest (P2S3), the internal resistance is measured under CC-mode discharge by two-tier direct current (DC) load method at two different states of charge. State of charge (SOC) as an indicator for the remaining capacity ratio of the battery is defined as \({\rm{SOC}}(t)={\rm{SOC}}({t}_{0})-Ca{p}^{-1}{\int }_{{t}_{0}}^{t}I(\tau )d\tau \), where \({\rm{SOC}}({t}_{0})\) is the previous SOC of the battery, \(Cap\) is the ampere hour capacity of the fully charged battery, and \(I(\tau )\) is the current with positive (negative) value for discharge (charge)77,78. The \(Cap\) could be chosen as the nominal ampere hour capacity (\(Ca{p}_{N}\)), the latest capacity, or the capacity at a given time for a specific purpose. (The details of the two-tier DC load method are described in the following subsection.) The battery cell is discharged to SOC = 85% under the current rate \({C}_{R}=0.2{{\rm{h}}}^{-1}\) (P2S4), as well as the voltage \({V}_{85,1}\) and the current \({I}_{85,1}\) are recorded at the end of this step. Then, the discharge current rate is changed to \({C}_{R}=1{{\rm{h}}}^{-1}\) (P2S5), and \({V}_{85,2}\) and \({I}_{85,2}\) are measured at the end of this step, where the time duration of the first tier \({t}_{1}=10{t}_{2}\) should smaller than the step time ts and the time duration of the second tier t2 = 100 seconds is equal to the step time. The internal resistance \({R}_{85}\) at \({\rm{SOC}}=85 \% \) can be calculated by Eq. 1. After the cell is discharged to \({\rm{SOC}}=20 \% \) under \({C}_{R}=0.5{{\rm{h}}}^{-1}\) (P2S6) and rest for one hour (P2S7), the cell is discharged under \({C}_{R}=0.2{{\rm{h}}}^{-1}\) for \({t}_{s}={t}_{1}\) (P2S8), obtaining the voltage \({V}_{20,1}\) and the current \({I}_{20,1}\) at the end of this step. Then, \({C}_{R}\) is changed to \(1{{\rm{h}}}^{-1}\) for \({t}_{s}={t}_{2}\) (P2S9). \({V}_{20,2}\) and \({I}_{20,2}\) are measured at the end of this step, and the internal resistance \({R}_{20}\) at \({\rm{SOC}}=20 \% \) can be calculated.
The discharge and charge cycle tests under normal and maximum loadings according to Sec. 19.7 of UL 1974 begin after the full discharge at \({C}_{R}=0.5{{\rm{h}}}^{-1}\) (P2S10) and the rest for one hour (P2S11). In the first cycle of charge and discharge, the cell is fully charged at \({C}_{R}=0.5{{\rm{h}}}^{-1}\) (P2S12) to get the charge ampere hour capacity of the 1st cycle \(Ca{p}_{C1}\), and then fully discharged at \({C}_{R}=0.5{{\rm{h}}}^{-1}\) (P2S14) to obtain the discharge ampere hour capacity under normal loading \(Ca{p}_{DN}\). In the second cycle, the cell is fully charged at \({C}_{R}=0.5{{\rm{h}}}^{-1}\) (P2S16) to get the charge ampere hour capacity of the 2nd cycle \(Ca{p}_{C2}\), and then fully discharged at \({C}_{R}=1{{\rm{h}}}^{-1}\) (P2S18) to obtain the discharge ampere hour capacity under maximum loading \(Ca{p}_{DM}\). The rest time between charge and discharge processes is one hour.
The self-discharge test as part of the determination of the state of health (Sec. 19.8 of UL 1974) is shown in P2S20–P2S23 in Table 2. The OCV of the fully charged cell shall be recorded at 5 minutes (\(OC{V}_{5m}\) in P2S21), 1 hour (\(OC{V}_{1h}\) in P2S22), and 24 hours (\(OC{V}_{24h}\) in P2S23) after charging (\(Ca{p}_{C3}\) in P2S20).
In this work, the voltage ranging from 2.5 to 3.5 V is adopted for safe working of the repurposed LFP battery cells (i.e., Vcut = 2.5 V and Vthres = 3.5 V), which is narrower than the safe working voltage range of new LFP battery cells (2–3.65 V). The voltage range can be adjusted according to the manufacturer’s design. In addition, the designed test procedures based on UL 1974 can be used for other types of Li-ion repurposed batteries.
It should be noted that not all battery cells are appropriate for repurposing. Before module disassembly, the OCV check is suggested for an effective judgement. For the modules with OCVs in the normal working range, their cells possess the potential for repurposing. For the modules with OCVs outside the normal working range, their cells should be recycled directly, saving the cost and time of the measurement.
There are four steps in the standard charge and discharge processes of Li-ion batteries. In the first step (as shown in the blue region I in Fig. 1), the battery is discharged under constant current \({I}_{c1}\), accompanied by a gradual voltage drop. As the voltage suddenly drops down to the cutoff voltage \({V}_{cut}\), the discharge process is terminated. The battery rests for the duration \({t}_{r1}\) in the second step, where no current passes through the battery and the voltage gradually rises to \({V}_{r1}\) (the yellow region II in Fig. 1). In the third step, the battery is charged under constant current \({I}_{c2}\) with a gradual voltage rise (the light red region III-1 in Fig. 1). When the voltage reaches the threshold value \({V}_{th}\), the battery keeps charging at the constant voltage \({V}_{th}\) by gradually lowering the charge current (the red region III-2 in Fig. 1). As the cutoff current \({I}_{cut}\) is reached, the charge process is completed. The battery rests for the duration \({t}_{r2}\) in the fourth step, where no current passes through the battery and the voltage gradually drops to \({V}_{r2}\) (the yellow region IV in Fig. 1). Based on the current and voltage constraints, the first and third steps are typically called the constant current (CC) discharge step and constant current-constant voltage (CC-CV) charge step, respectively.
Fig. 1Standard charge and discharge processes of Li-ion battery. Step I (CC discharge): The battery is discharged at constant current \({I}_{c1}\) until the voltage drops to the cutoff voltage \({V}_{cut}\). Step II: Rest for the duration \({t}_{r1}\) without the current pass. The voltage gradually rises to \({V}_{r1}\). Step III-1 (CC charge): The battery is charged at constant current \({I}_{c2}\) until the voltage rises to the threshold voltage \({V}_{th}\). Step III-2 (CV charge): The battery is charged by maintaining \({V}_{th}\) until the current reaches the cutoff current \({I}_{cut}\). Step IV: Rest for the duration \({t}_{r2}\) without the current pass. The voltage gradually drops to \({V}_{r2}\).
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The detailed charge and discharge processes might different for various manufacturers. Some differences are listed: (1) The order of charge and discharge steps could be exchanged. (2) The values of the discharge cutoff voltage \({V}_{cut}\), the charge threshold voltage \({V}_{th}\), and the charge cutoff current \({I}_{cut}\). (3) The value of the discharge constant current \(| {I}_{c1}| \) is not necessarily equal to the value of the charge constant current \(| {I}_{c2}| \). (4) The signs for discharge and charge constant currents \(({I}_{c1},{I}_{c2})\) might choose as (−, +), (+, −), or (+, +). (5) The rest duration \({t}_{r1}\) is not necessarily equal to \({t}_{r2}\). (6) The rest durations could set to zero, i.e., no step of rest.
Direct current internal resistance (DCIR) of batteries indicates the resistance of current flowing through the battery. The value of DCIR is not fixed and varies depending on multiple factors, such as battery materials, type and concentration of electrolyte, temperature, as well as depth of discharge. The variation of DCIR has a great influence on battery discharge performance, especially for high power batteries. In general, the better the battery, the lower the internal resistance. Therefore, most battery manufacturers identify DCIR as a primary indicator for evaluating battery quality.
Many techniques are applied to measure the DCIR of batteries, such as the tests conducted according to the IEC 61951-1 standard79, IEC 61960-3 standard80, and ISO 12405-4 standard81. In UL 1974, the two-tier DC load method is adopted, offering an alternative method by applying two sequential discharge loads of different currents and time durations. The battery first discharges at a lower constant current I1 for t1 seconds, dropping to a voltage V1, and then discharges at a higher constant current I2 for t2 seconds, dropping to a voltage V2 (as shown in Fig. 2). The DCIR, RDC, is obtained by the Ohm’s law as
$${R}_{DC}=\frac{\Delta V}{\Delta I}=\frac{{V}_{1}-{V}_{2}}{{I}_{2}-{I}_{1}}.$$
(1)
Fig. 2Two-tier DC load method for measuring the DCIR of batteries. The DC load test measures the battery’s internal resistance by reading the voltage drop. In the two-tier process, the DCIR is obtained by the Ohm’s law, dividing the voltage variation (\({V}_{1}-{V}_{2}\)) by the current variation (\({I}_{2}-{I}_{1}\)). The DC load test is the preferred method for evaluating the battery characteristic of DC power consumption.
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Some suggestions and comments from UL 1974: (1) The higher constant current is five times the lower one, i.e., I2 = 5I1. (2) Voltage and current during the discharge should be recorded at a rate not less than \(10/{t}_{2}\) sample per second, i.e., \(1/{t}_{rec}\ge 10/{t}_{2}\) (or \({t}_{rec}\le {t}_{2}/10\)), where \({t}_{rec}\) is the data recording time interval. (3) Evaluating the voltage signature under the two load conditions offers additional information about the battery (the values are strictly resistive and do not reveal SOC or capacity estimations). (4) The load test is the preferred method for batteries that power DC loads.
The charge and discharge performance of the batteries were evaluated using the battery test system (CTE-MCP-5082020A, Chen Tech Electric Mfg. Co., Ltd., Taiwan) as shown in Fig. 3. The data was logged every ten seconds (\({t}_{rec}=10\) sec) by the CTE-Will software (version 1.13tc). The environmental temperature was controlled at room temperature (25–32 °C). Output data was saved in the format of csv file, containing various information, including data point, step, step time (hh:mm:ss), voltage (V), current (A), power (W), temperature (°C), capacity (mAh), energy (Wh), total time (hh:mm:ss), and end status.
Fig. 3Battery test system. The CTE-MCP-5082020A battery test system (made by Chen Tech Electric Mfg. Co., Ltd., Taiwan) is used for evaluating the performance of the battery cells. There are 16 channels, and each of them can provide measurements of voltage, current, and temperature simultaneously.
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The first-life applications of these repurposed cells are power battery modules used in golf carts. The golf course is a relatively simple environment for design verification of the power battery. There are flat roads for continuous power output tests and some gentle slopes for the up and downhill tests. The power battery modules normally operate in two conditions: instant high power output (CR = 3–6 h−1) for motor start and continuous medium power output (CR = 1–3 h−1) for advancing the golf cart continuously. These battery modules have been used for 1–2 years, and then they reach the end-of-life (EOL).
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