*3.2. Test Protocols*

Four lead-acid battery modules are subjected to two peak shaving duty cycles and two frequency regulation duty cycles. The duty cycles are based on the "Protocol for Uniformly Measuring and Expressing the Performance of Energy Storage Systems" developed by energy storage industry stakeholders, led by Pacific Northwest National Laboratory and Sandia National Laboratory [29]. The tests are ongoing for over one year. The test data correspond to 28 months for one duty cycle and 18 months for three other duty cycles. Testing was performed using an N.H. Research Inc. (NHR, Irvine, CA, USA) battery pack test system (9200 series, model 4912), with a maximum DC voltage of 120 V and current of 200 amperes (A), with an 8 kW limit for charge and 12 kW limit for discharge [30]. Figure 1 shows the experimental setup for the BESS, where the lead acid battery module is the DC storage block, and the NHR battery tester performs the role of the grid and site controller. At the beginning of each test, the battery module is charged at a constant current (CC) of 17.5 A (10-h rate) until module voltage reaches 14.1 V (2.35 V/cell), followed by a constant voltage (CV) charge until current drops to 1 A. After charge, the battery is subjected to a

rest period of three hours. Reference performance tests are done at the beginning of the test regime to establish a baseline and repeated periodically. These tests consist of a capacity test and a pulse resistance test to determine battery degradation during operation. The reference performance capacity test is conducted every month, corresponding to 58 peak shaving duty cycles or 30 frequency regulation duty cycles. A pulse resistance test is done every two months, after 116 peak shaving cycles and 60 frequency regulation cycles to measure internal resistance.

**Figure 1.** Experimental setup of the BESS, comprising battery module DC storage, and battery tester representing the grid and site controller.

#### 3.2.1. Reference Performance Capacity Test

The battery is initially charged at a CC of 17.24 A (10-h rate) until the module voltage reaches 14.1 V, followed by a CV charge at 14.1 V until the charge current drops to 1 A. After the initial charge, the battery is discharged at the C5 rate of 31.34 A until the module voltage drops to 11.1 V (1.85 V/cell). Note that during this test, the end of discharge voltage is kept at 1.85 V/cell to avoid deep discharge-related degradation during the capacity test, since the main goal was to estimate duty cycle-related degradation. The battery is not subjected to rest after discharge to minimize duration at a low state of charge, where irreversible sulfation may occur. The CC/CV charge is repeated, with termination condition during CV charge corresponding to either the current decreasing to 1 A or charge capacity (Ah) reaching 103% of previous discharge Ah capacity. Note that the battery module is charged at a higher rate (5-h or C5 rate) for 1 h to minimize time spent at low SOC, followed by charge at the C10 rate. After charge, a 3 h rest period is incorporated. The discharge and charge steps are repeated for a total of two discharges, with the battery capacity calculated as the average of these two discharges. Capacity retention is calculated as the ratio of Ah capacity measured during reference performance test as the battery ages to the initial or baseline measured Ah capacity of the battery.

#### 3.2.2. DC Current Pulse Test to Measure Internal Resistance

The lead acid battery modules' internal resistance can be measured using DC pulse current or electrical impedance spectroscopy (EIS). EIS employs multiple frequency sine waves to measure resistance over a wide range of frequencies [31]. However, EIS is not a viable option for a large battery module due to hardware limitations related to voltage and current. Per the battery cycler specifications [30], the current change time is <5 ms, while the internal resistance is reported every 10-millisecond (ms) time interval. However, it took 30–100 ms for the applied pulse current to stabilize, hence ohmic resistance is estimated at 100 ms, where there is a significant contribution expected from charge transfer.

The internal resistance of the battery module comprises ohmic resistance, charge transfer, and mass transport resistance. For AC impedance tests, ohmic resistance is estimated at 10–100 kHz [32], which corresponds to a duration of 10–100 μs, while charge transfer resistance is evaluated at 1–10 Hz (0.1–1 s), and mass transfer resistance or diffusion resistance to ion transport to and from the electrode pores is measured at 0.001–0.1 Hz (10–1000 s).

It has been shown that the pulse width at a fixed pulse current should be such that the ΔSOC is ≤0.1% [33]. This condition is met by using C1 or 1-h rate pulse of 115 A for 6 s, with a ΔSOC of 0.11%. The discharge and charge pulses are applied at every 10% SOC change from 100% SOC to 0% SOC, with only discharge pulse applied at 100% SOC and charge pulse at 0% SOC, with a 30-min rest period imposed after each pulse to allow the battery voltage to relax and the temperature to equilibrate. The battery's internal resistance is calculated from the voltage change during the pulse.

Total internal resistance, Rtotal = <sup>Δ</sup><sup>V</sup> <sup>Δ</sup><sup>I</sup> , ΔV = (VF − VI) and ΔI = (IP − II).

Where, VI is the initial voltage before pulse is applied, VF is the final voltage at the end of the pulse and II is the initial current, which is 0, and IP is the pulse current. Figure 2 shows a discharge pulse current and voltage profile of the battery.

**Figure 2.** Measurement of internal resistance by DC pulse current.

In this work, the 100 ms data is used to calculate ohmic resistance, while the 2-s data is used to calculate the sum of ohmic (Ro) and charge transfer resistance (Rct), and the 6-s data is used to calculate total resistance (Rtotal) [34]. Mass transport resistance (Rm) is the difference in 6-s and 2-s data. Corresponding times for calculation of resistance from the voltage relaxation after charge or discharge are 0.1 s, 2 s, and 1800 s.

#### 3.2.3. Peak Shaving Duty Cycle Test

Two battery modules are subjected to peak shaving duty cycles. A peak shaving (PS) duty cycle consists of a discharge at constant power for a duration ranging from 1–4 h during the daily on-peak period followed by a recharge during the off-peak period. Table 2 shows the peak shaving duty cycle operating parameters. The depth of discharge (DOD) of the battery module is 50% based on the initial Ah-capacity measurement. Discharge is done at two power levels (730 watts (W) and 246 W) to analyze the effect of power on performance and degradation. At the end of 58 peak shaving cycles, the battery is fully discharged, followed by a reference performance test.

For the 730 W, the discharge time is 1.2 h, with the highest current corresponding to the 2-h rate, and is referred to as PS 2-h. The corresponding numbers are 3.8 h and 8-h rate for discharge at 246 W, referred to as PS 8-h. The charge back power is kept the same for both modules at 265 W. The charge power is calculated by multiplying the voltage at the low end of SOC with the C10 charge current. The batteries are charged at a constant power of 265 W until the voltage is 14.1 V, followed by a CV charge until current decreases to 1 A. The total duration of both the peak shaving duty cycles is 12 to 14 h. The rest time for peak shaving is 2 h to accommodate close to 2 cycles per day.

**Table 2.** Peak Shaving Characteristics.

