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
Trang 1E 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
Cycle number
Partial-state-of-charge
0 20 40 60 80 100 120
Cycle number
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