Partial-State-of-Charge E Dickinson, Axion Power International Inc., New Castle, PA, USA & 2009 Elsevier B.V. All rights reserved. Lead–Acid Battery: Partial-State-of- Charge The 150 years history of the lead–acid battery has seen technological improvements in numerous areas, in- cluding grid alloy, jar material, and manufacturing methods. The most notable improvement is the valve- regulated lead–acid (VRLA) battery, both absorbent glass mat and gel electrolyte versions. The majority of these improvements have focused on conventional float and deep-cycle applications. However, there are a growing number of new applications demanding an optimized, purpose-built lead–acid battery. Applications such as remote area power supply (RAPS) systems, motive power battery opportunity charging, and micro and mild hybrid electric vehicles (HEVs) are stressing the existing lead– acid battery design in ways these advances have not ad- dressed. The key difference these applications share compared to traditional float and cycling applications is the battery’s state of charge (SoC) during operation. Partial-State-of-Charge State of charge refers to the ampere hour (Ah) output available at any point during a charge/discharge cycle, represented as a percentage of the battery Ah label rating. Discharging a battery to 80%, a common lower limit, or 80% depth-of-discharge (DoD), leaves 20% of the rated Ah available, hence a 20% SoC. The lack of a predictable charge/discharge pattern is the biggest challenge to the systematic investigation and design of an optimized battery for partial-state-of-charge (PSoC) applications. In the case of a RAPS system, natural renewable energy (solar, wind, or hydroelectric energy sources available in remote locations where typical power grids are not present) is stored in lead–acid batteries for later delivery during peak demand. Simu- lated testing requires assumptions about the available energy input (weather conditions, conversion efficiency, and so on) and the expected usage output. Likewise, the same is true for opportunity charging motive power batteries, where a single battery is used to power a forklift rather than the exchange of two or three batteries in various states of use, charge, or cooling. In this case, the SoC pattern can be based on historical usage and the available breaks throughout the shift used to charge the battery, variables subject to change over the life of the battery. In the case of micro and mild HEV operations, which use regenerative braking to recharge the battery, the charge pattern depends on braking frequency, i.e., driving habits, routes, and other variable conditions. In the context of these emerging applications, the SoC is less often, or in some cases never, completely returned to its original state, hence the partial in PSoC. This difference is illustrated in Figure 1, which depicts a traditional deep-cycle mode of operation and a PSoC mode of operation. Deep-cycle operation typically re- quires an additional 10% Ah above that removed to reach 95–100% SoC, a necessity given the inefficiencies of the charging process above the gassing potential (B2.4 V per cell, where hydrogen gas is generated). Partial state-of- charge operation, on the contrary, has infrequent full charges (equalization charges), if any at all. The upper and lower limits of the PSoC window (i.e., top-of-charge voltage (ToCV) and end-of-discharge voltage) vary with application. This window may be described as shallow (small DoD) or deep (large DoD), and narrow (low overall change in SoC) or wide (large change in SoC). The rate at which the battery is cycled and the ability of the battery to accept charge have a Deep cycle 0 20 40 60 80 100 120 0123 Cycle number SoC (%) Partial-state-of-charge 0 20 40 60 80 100 120 0123 C y cle number SoC (%) SoC window Figure 1 The state of charge (SoC) % limits of both traditional deep-cycle operation (top) and partial-state-of-charge (PSoC) operation (bottom). 452 . number SoC (%) Partial-state-of-charge 0 20 40 60 80 100 120 0123 C y cle number SoC (%) SoC window Figure 1 The state of charge (SoC) % limits of both traditional deep-cycle operation (top) and partial-state-of-charge. traditional float and cycling applications is the battery’s state of charge (SoC) during operation. Partial-State-of-Charge State of charge refers to the ampere hour (Ah) output available at any point. is the biggest challenge to the systematic investigation and design of an optimized battery for partial-state-of-charge (PSoC) applications. In the case of a RAPS system, natural renewable energy