**1. Introduction**

Enabling a more sustainable energy supply system requires the integration of renewable energy resources as well as energy storage systems (ESS) with the power grid. For a robust and resilient grid, ESS can provide various grid services such as load leveling, frequency regulation, energy management, backup power, voltage support, and grid stabilization [1,2]. To validate and identify the challenges related to the extensive use of ESS in the power grid, ESS is tested for reliability and safety as part of the "Grid Energy Storage Strategy" released by the U.S. Department of Energy (DOE) [3]. The lessons learned from these tests are expected to increase the deployment of ESS.

Many different energy storage technologies are being tested, evaluated, and deployed globally. Grid-scale lead-acid batteries were deployed as far back as the 1980s [4] and are a cost-effective and reliable option for different grid services [5–8]. Materials sustainability is one of the most important aspects of lead acid batteries since nearly a 100% recycling rate is achievable [2]. However, for the larger battery size required for grid scale battery energy storage systems (BESS), there are several challenges that need to be addressed.

The charge-discharge reactions for lead acid battery at the positive and negative electrodes are given in Equations (1) and (2) respectively, while Equation (3) shows the overall cell reaction [9,10].

$$\text{PbSO}\_4(\text{s}) + 2\text{H}\_2\text{O} \overset{\text{charging}}{\underset{\text{discharging}}{\rightleftharpoons}} \text{PbO}\_2(\text{s}) + \text{H}\_2\text{SO}\_4 + 2\text{H}^+ + 2\text{e}^- \tag{1}$$

$$\text{PbSO}\_4(\text{s}) + 2\text{H}^+ + 2\text{e}^- \overset{\text{charging}}{\underset{\text{discharging}}{\rightleftharpoons}} \text{Pb(s) + H\_2SO\_4} \tag{2}$$

$$2\text{PbSO}\_4(\text{s}) + 2\text{H}\_2\text{O} \overset{\text{charging}}{\underset{\text{discharging}}{\rightleftharpoons}} \text{PbO}\_2(\text{s}) + \text{Pb(s)} + 2\text{H}\_2\text{SO}\_4\tag{3}$$

**Citation:** Shamim, N.; Viswanathan, V.V.; Thomsen, E.C.; Li, G.; Reed, D.M.; Sprenkle, V.L. Valve Regulated Lead Acid Battery Evaluation under Peak Shaving and Frequency Regulation Duty Cycles. *Energies* **2022**, *15*, 3389. https://doi.org/ 10.3390/en15093389

Academic Editors: Alon Kuperman and Alessandro Lampasi

Received: 8 April 2022 Accepted: 3 May 2022 Published: 6 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The discharged species for both electrodes is lead sulfate (PbSO4), while the charged species is lead dioxide (PbO2) for the positive electrode and spongy lead (Pb) for the negative electrode. The formation of lead sulfate at both electrodes during discharge results in electrolyte dilution due to sulfuric acid consumption, with protons migrating from the negative to the positive electrode. Lead dioxide in the positive electrode is reduced while spongy lead in the negative electrode is oxidized to lead sulfate. The reverse process takes place during charge, with electrolyte specific gravity increasing due to sulfuric acid generation.

In a valve regulated lead acid battery (VRLA), the electrolyte is absorbed in a glass mat separator, also known as absorbed glass mat (AGM) separator, or absorbed into a gel, with no excess free-flowing electrolyte, unlike its flooded counterpart.

VRLA battery capacity is typically positive electrode limited, generating oxygen at the positive on overcharge (Equation (4)) before the negative electrode is fully charged. Gas channels are formed either as connected porosity in the AGM separator or as microcracks in the gel [11]. This creates a passage for oxygen gas to the negative, where it is electrochemically reduced in the presence of hydrogen ions to form water (Equation (5)). In addition to oxygen generation, there is an ongoing parasitic reaction related to positive grid corrosion during charge, which is accelerated during overcharge in the presence of oxygen. The depolarization of the negative electrode by the oxygen recombination reaction increases the positive electrode potential during constant voltage charge mode, further accelerating grid corrosion. This parasitic grid corrosion reaction has to be balanced at the negative electrode. Since the negative electrode is in excess, the balancing reaction is expected to be charging of the negative electrode active material from lead sulfate to spongy lead. If the oxygen generation rate exceeds the recombination rate at the negative, this excess oxygen generation also has to be balanced by charging of the negative electrode active material from lead sulfate to spongy lead. Ideally, there should be sufficient excess negative active material to account for positive grid corrosion and incomplete oxygen recombination, such that at the end of design life, hydrogen generation at the negative is avoided, thus preventing water loss. The reactions for oxygen generation at the positive during overcharge, followed by recombination with lead at the negative, and subsequent conversion to lead sulfate are given below in Equation (4) at the positive and Equation (5) at the negative, which is the reverse of Equation (4), thus keeping the positive and negative electrode state-of-charge (SOC) fixed [12]:

$$\text{H}\_2\text{O} \rightarrow \frac{1}{2}\text{O}\_2(\text{g}) + 2\text{H}^+ + 2\text{e}^- \tag{4}$$

$$\frac{1}{2}\text{O}\_2(\text{g}) + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}\_2\text{O} \tag{5}$$

Common failure modes of VRLA batteries are discussed in Section 2, along with cell design to optimize performance and life. Degradation modes are proposed to explain the internal resistance increase and capacity degradation trends under peak shaving and frequency regulation duty cycles for VRLA battery modules in Section 4. Suggestions for post-test disassembly and analysis of individual components to validate the proposed failure mechanism for each duty cycle are provided in Section 4.4. Mapping has been done between the proposed degradation modes for each duty cycle with corresponding degradation modes for hybrid electric vehicles in Section 4.7.

#### **2. Failure Modes for Lead Acid Batteries**

The lead acid battery failure modes comprise the following, with electrolyte stratification and dry-out limited to VRLA [13–16]

