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Life extension through charge equalization of lead-acid batteries

Authors:

Abstract

Charge equalization is an important part of the charge process for series-connected battery cells. This paper reviews battery behavior and performance related to the equalization problem, in the context of valve-regulated lead-acid batteries. As established in prior work, equalization precision on the order of 10 mV/cell is required for a successful process. Equalization processes that can achieve this precision do indeed extend the life of battery packs. Active equalization speeds the process and supports exchange of a single failed cell or monoblock Passive equalization (conventional overcharge) is too slow in most contexts for strings longer than about 12 cells. Active equalization provides clear life advantages over other approaches. The results are consistent with the hypothesis that a properly designed active equalization process can provide battery cycle life for a series string that matches the cycle operating results for an individual cell.
INTELEC 2002 Paper 32.1
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Abstract—Charge equalization is an important part of the
charge process for series-connected battery cells. This paper
reviews battery behavior and performance related to the
equalization problem, in the context of vavle-regulated lead-acid
batteries. As established in prior work, equalization precision on
the order of 10 mV/cell is required for a successful process.
Equalization processes that can achieve this precision do indeed
extend the life of battery packs. Active equalization speeds the
process and supports exchange of a single failed cell or
monoblock Passive equalization (conventional overcharge) is too
slow in most contexts for strings longer than about 12 cells.
Active equalization provides clear life advantages over other
approaches. The results are consistent with the hypothesis that a
properly designed active equalization process can provide
battery cycle life for a series string that matches the cycle
operating results for an individual cell.
Index Terms— Battery equalization, charge equalization,
battery management, charge balancing
I. I
NTRODUCTION
Batteries are nearly always used in series combinations of
multiple cells. When a series string of cells is charged as a
group, a single current is imposed on all the cells. However, if
the voltages begin to differ, the result is a charge imbalance
that can lead ultimately to battery failure. In any useful series
battery charge process, some type of “charge balancing” or
“equalization” must take place to restore balance or at least
prevent it from growing.
The need for equalization is well established. In most
conventional battery charging practice, equalization is
addressed either by driving the charge to a sufficient potential
to assure some degree of overcharge for all cells, or with a
separate higher-voltage charging step intended to reach the
weakest cells. In recent years, electric and hybrid vehicle
applications, which tend to use very long series strings and
which push battery performance to extremes, have brought
charge equalization into wider view. In [1], results with lead-
acid battery packs in the General Motors EV1 production
electric car are presented. The paper focuses most of its
attention on the charge equalization problem. In [2], a team
from Optima Battery (now part of Johnson Controls) reports
The authors are with the Grainger Center for Electric Machinery and
Electromechanics, University of Illinois, Department of Electrical and
Computer Engineering, 1406 W. Green St, Urbana, IL 61801 USA (e-mail
machines@ece.uiuc.edu).
that “Pack imbalance is perhaps the most serious issue with
large series string packs.”
Although much of the research with respect to equalization
has considered long series strings (100 or more cells in series),
problems of equalization extend down to short strings of just a
few cells. In [3], a conventional block of six lead-acid cells
was tested with a voltage-limited charge process, in which the
maximum charge potential is held below 2.35 V for any
individual cell. When a 12-hour charge was used in this case,
performance degraded from cycle to cycle. Only when the
charge process was extended to 16 hr was there enough time
for equalization to bring about recovery. Even then,
approximately one week would have been needed to fully
restore charge balance. The equalization process represented
by this charge strategy reflects a “passive equalization”
approach: ensure that strong cells are subjected to overcharge
(limited by voltage in [3]) until weak cells regain full charge.
The process is slow even for familiar 12 V monoblocks.
Several key questions must be considered with respect to
equalization:
1. Many equalization processes match cell voltages. The
real need is to match cell state-of-charge (SOC). Is voltage an
accurate surrogate for SOC?
2. How accurately must the cells be matched to make the
process useful?
3. What are the benefits of equalization?
4. Will equalization extend battery life and reduce costs?
These questions are addressed in this paper, primarily in the
context of modern valve-regulated lead-acid (VRLA)
batteries.
II. V
OLTAGE BALANCING
An equalization process is intended to match the SOC
among cells in a string. Since the charge current matches in a
series string, the implication is that voltage should match.
However, the relationship between voltage and SOC is not
necessarily trivial. Fig. 1 shows a graph (from [4]) of cell
voltage vs. SOC for a lead-acid cell. The relationship is based
on measuring open-circuit voltage after about 30 min of rest.
It shows a simple linear relationship. The total voltage change
over a 0 to 100% SOC change is about 0.2 V, corresponding
to 2 mV for every 1% change in SOC. Two key inferences
can be made from this graph:
1. For lead-acid cells in steady state, voltage provides
useful information about SOC.
Life Extension Through Charge Equalization of
Lead-Acid Batteries
Philip T. Krein, Fellow, IEEE, Robert Balog, Student Member, IEEE
INTELEC 2002 Paper 32.1
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2. Cell-to-cell voltage matching on the order of 10 mV
corresponds (in steady state) to SOC match on the order of
5%.
State of Charge(%)
1.9
1.95
2
2.05
2.1
2.15
Open Circuit Voltage(V)
0 10203040506070809010
0
Fig. 1. VRLA open-circuit voltage vs. state of charge (from [4]).
It is a challenge to interpret this behavior under dynamic
conditions. A reasonable assumption is that low currents will
reflect similar behavior, although high currents will distort the
voltage behavior and could lead to erroneous results.
Experimental life tests have been conducted to evaluate the
voltage-matching behavior of VRLA batteries under dynamic
conditions. Figs. 2 and 3 (from [5]) illustrate the results. The
figures present test results for two 12-cell VRLA strings tested
with and without an effective equalization process. With
equalization in action (Test pack 1), the cell-to-cell variation
is held to about 12 mV. Without equalization, the variation
begins to drift up after about 13 cycles. Fig. 3 shows the effect
on performance. The string capacity begins to drop as the cell-
to-cell variation rises above about 15 mV in cycle 15. A more
complete analysis is provided in [5].
0 5 10 15 20 25
0
5
10
15
20
25
30
Average per Cell Voltage Gradient (mV)
Cycle Number
Test pack 1
Test pack 2
Fig. 2. Cell voltage gradient for comparison test (from [5]).
5 10 15 20 25 30 3
5
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
4.8
5
Cycle Number
AH Capacity at 3 h Rate
Test pack 1
Test pack 2
0
Fig. 3. Cell capacity vs. cycle number for comparison test ([5]).
Based on the steady-state voltage behavior in Fig. 1 and the
dynamic analysis in [5], we conclude that voltage matching is
an effective way to match SOC for lead-acid batteries. This is
true provided the currents are limited (the equalization current
used in [5] is about 1% of the battery’s nominal C rate).
The conclusion: To be useful, an equalization process
should hold the cell-to-cell differences to about 10 mV or less.
This is consistent with other results from the literature. In [8],
for example, a large set of batteries is monitored cell-by-cell
for condition management. The graphs in [8] show that good
cells show balance to about 10 mV, while cells that deviate
from this lose capacity and must be changed out.
Most practical rechargeable batteries have a monotonic
relationship between voltage and SOC. The characteristic in
Fig. 1 is useful because it is linear and unambiguous. In other
cases, the result is more complicated. Lithium-ion cells, for
example, show significant voltage change over the 0% to
100% SOC range. The relationship is monotonic but
nonlinear. It would be expected that equalization would be
beneficial for Li-ion strings. This has been confirmed in the
literature [6,7]. Nickel chemistries show much flatter voltage-
SOC profiles, and require very precise voltage matching to
achieve good results.
III. W
HAT ARE THE BENEFITS OF EQUALIZATION?
Modern rechargeable battery cells of all chemistries are
specified by the manufacturer for hundreds or even thousands
of cycles. Series strings of cells in general do not perform up
to this level. The only difference between a single-cell and
series-cell applications is management of individual cell
voltages. This is the basis for many assertions in the literature
that charge imbalance is the primary failure mechanism in
batteries [1,2,8]. In principle, a perfect equalization process
would ensure that a series string performs just like a single
cell over time.
INTELEC 2002 Paper 32.1
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The need for equalization of VRLA batteries is clarified in
other ways in [5]. For example in one test, several strings of
12 lead-acid batteries were cycled without equalization. The
cells were rated for at least 400 cycles, but instead the strings
provided only 25 to 30 cycles before reaching end of life. The
charge profile did not provide the “overcharge” time (which
largely involves equalization time) specified by the
manufacturer.
The conventional equalization method is to provide a
“forced overcharge” interval after the main charge sequence.
The objective is to deliver full charge into the lowest cells.
The process can be termed “passive equalization,” since it
relies on the properties of the battery cells to restore matching.
Unfortunately, passive equalization works at the expense of
gassing and dryout of the highest cells. In addition, it is a slow
process. The forced overcharge equalization process is
routinely used with lead-acid batteries.
When this process is used, 6 V or 12 V monoblocks
become feasible. It is still true, however, that long-term
monoblock failures usually involve a single cell that has
weakened over time. For 48 V batteries, the results in [3]
imply that intervals of several weeks will be required for
equalization. Even if long intervals are available, the
overcharge exposure can lead to thermal runaway. At higher
voltage levels, results from [5] suggest that equalization time
increases as the square of the number of cells.
A more effective equalization process is needed. Even for
monoblock pairs at 24 V, an external active process to
supplement passive equalization can accelerate matching and
maintain cycle life. Figs. 4 – 7 (from [9]) compare passive and
active equalization for a 72 V battery pack. Fig. 4 shows the
voltages of the six 12 V monoblocks during charging. The
passive equalization process is slow, and the three-day
interval here shows no clear pattern. Active equalization,
shown in Fig. 5, brings the voltages together rapidly. The
operating results are emphasized in Figs. 6 and 7, which show
the standard deviation of voltages among the six monoblocks
in each string, corresponding to Figs. 4 and 5, respectively.
Notice that passive equalization is really not reducing the
voltage standard deviation, although perhaps it is starting to
fall after about 60 h.
Fig. 4. 72 V battery string, charged under passive equalization (from [9]).
Fig. 5. 72 V battery string, charged under active equalization (from [9]).
Fig. 6. Standard deviation of cell voltages, passive equalization (from [9])
INTELEC 2002 Paper 32.1
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Fig. 7. Standard deviation of cell voltages, active equalization (from [9]).
The process presented in [9] has several key advantages
over competing passive and active equalization technologies:
1. It is simple and direct, relying on a capacitor switching
approach to equalization.
2. The voltage match is exact, regardless of errors or
tolerances associated with real components.
3. The process consumes minimal energy, and can be used
continuously throughout charge and discharge sequences.
4. The hardware is modular, and can be configured on a
cell-by-cell or monoblock-by-monoblock basis. The
technology lends itself very well to miniaturization.
5. The inherent cost is low because no high-tolerance
components, controls, or specialty parts are needed.
6. Equalization proceeds independent of the charge process,
so external current and voltage limits can be set and enforced
without complications.
Assuming that equalization of voltage supports SOC
matching, what can be gained? First, it would be expected that
a series string of equalized cells would show life performance
like that of an individual cell. This is potentially the most
significant result – the 28-cycle performance of the
unequalized charge test in [5] should extend right up to the
manufacturer’s rating of 400 cycles with proper charge limits.
Second, failure modes associated with imbalance (repeated
undercharge of weak cells, ultimately leading to failure) are
avoided. Third, there is no need for forced overcharge as part
of a cycle (given cell-by-cell equalization). This last is
especially interesting, since it implies that perhaps the charge
voltage limits can be decreased when active equalization is in
place. Lower voltages make thermal runaway less likely, gas
the cells less, and should avoid the stress on strong cells
inherent in the passive equalization process.
Another important benefit is interchangeability. For
example, many users recommend that series strings be built
with tightly-matched cells. The basis for this is to start with as
close a match as possible, perhaps allowing more cycles
before cell imbalance becomes severe. With active
equalization, there is a process to drive the cells together, so
tight initial matching is not needed. What if a failure does
occur, perhaps because of a battery defect or other problem?
Active equalization would support changeout of the defective
battery or cell without introducing extra cell mismatch. The
combination of long cycle life with the ability to change
individual units rather than a whole string has potential for
significant cost reduction.
In summary, effective equalization should
-- Extend cycle life of a series string up to that of an
individual battery or cell.
-- Avoid failure modes based on cell imbalance.
-- Permit changeout of individual cells or monoblocks
when a failure does occur.
-- Prevent weakening of string performance caused by
individual undercharged cells.
All of these provide significant cost savings in battery
installations. Longer-term, it should be possible to alter the
charge process to take advantage of active equalization and
further extend battery cycle life.
IV. W
ILL EQUALIZATION REALLY EXTEND BATTERY LIFE?
The above results suggest that cycle life can be extended at
least up to the level promised by manufacturers for single cells
or monoblocks. But can this be proven with real data? In
fact, at least four published tests independently confirm the
performance benefits of active equalization. Each is discussed
individually here.
In [7], a distributed charging system was used. There were
multiple chargers, and the effect is independent charging of
small groups of cells. This is equivalent to an active
equalization process on monoblocks. The chemistry was
lithium-ion. Performance improvement was evident on the
very next cycle after the distributed charging system was
added: a 2% increase in total capacity was seen after just one
cycle. Subsequent cycles showed additional improvement.
In [5], active equalizers were tested with conventional
flooded lead-acid batteries. Active equalization maintained
cell-to-cell matching of better than 10 mV throughout an
intensive one-week accelerated test – even though a low float
limit of 2.30 V/cell was used. For a second test pack, a higher
voltage of 2.45 V/cell was used to drive a passive equalization
process, but the cell deviation began to rise above 10 mV after
only about six cycles. When water loss was measured over the
test interval, the active equalization approach showed 40%
less water loss than the passive method. The comparative
water loss indicates cycle life extension on the order of 66%.
In [10], several different equalization methods were
compared for effects on cycle life. Some of the methods
actually degraded pack performance. A low-cost system built
with the technology of [9] gave a 15% cycle life improvement
– even though the design was probably undersized for the
batteries being tested. A summary result from [10] is given in
Fig. 8. The “Control C” curve represents a control battery
pack that uses a manufacturer-recommended charge profile.
INTELEC 2002 Paper 32.1
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The “BMSC” curve is for the technology of [9], with exactly
the same charge profile. (The profile target voltage was not
reduced to take advantage of active equalization.)
Fig. 8. Capacity vs. cycle for comparative equalizer test (from [10]).
In [11], cycle tests were performed on 48 V battery packs,
with and without active equalizers. Fig. 9 (from [11]) shows
the results – cycle life extension by about a factor of three. In
this case, the active equalizers are sized appropriately, and can
deliver sufficient charge to maintain cell balance. The
technology used in [11] shares many performance
characteristics of [9], although it requires precise control and
is inherently more expensive. It is important to recognize that
the result in Fig. 9 confirms that a series string should be able
to reach cycle life levels similar to those of single monoblocks
with an active equalization process in place. Pack 2, with
active equalization, achieves at least 400 cycles – consistent
with the manufacturer’s rating for a single monoblock. Pack 1,
which uses only a conventional charge cycle with passive
equalization, reaches only about 140 cycles. The overall result
is a tripling of cycle life for this 48 V pack.
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Fig. 9. Capacity vs. cycle, comparing passive and active equalization (from
[11]).
Fig. 10 (from [12]), is a telling result. In this case, four
different 24 V VRLA packs have been tested with two
different charge profiles and various equalization strategies.
Fig. 10a shows the capacity as it changes over repeated cycle
tests, while Fig. 10b shows the voltage standard deviation as
the test proceeds. Battery pack 1 – the only one for which the
standard deviation is held is about 10 mV – shows far better
cycle life than the others. The figure confirms that
performance degrades unless voltage imbalance is held to a
low level, and provides a linkage between voltage-based
equalization and cycle life performance. The results also
confirm that voltage balance must be better than 10 mV/cell to
provide benefits.
(a) Capacity vs. cycle number for four 24 V test packs
(b) Cell gradient (mV/cell) for four 24 V test packs
Fig. 10. Tests of four 24 V battery packs (from [12]).
Can active equalization technology provide precise enough
balance to realize the benefits of equalization? Fig. 11
compares two packs, now being cycle-tested in the laboratory.
The first uses passive equalization, while the second is
supplemented with switched-capacitor active equalizers [13].
The standard deviation is only a few millivolts. This match
has been maintained so far over several cycles, and results are
building quickly. The technology of [9,13] matches voltages
exactly. Provided sufficient charge exchange is supported,
voltage differences well below 10 mV can be obtained.
In applications [14,15], the benefits of equalization have
been immediately apparent. Overall pack performance
improves noticeably as soon as equalizers are in place. In [15],
a set of batteries revived from 0% SOC without developing
any imbalance, thanks to equalization.
INTELEC 2002 Paper 32.1
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V. C
ONTINUING RESULTS
At present, long-term cycle testing continues in our
laboratory. Fig. 12 shows an example of voltages over several
months of cycle testing. Fig. 12a are the cell voltages of the
passively equalized pack and Fig 12b are the cell voltages for
the actively equalized pack. Missing points reflect
datalogging problems. In general, the cycles have been
maintained without interruption for months. The gap at
approximately 700 hr shows a change in charge-discharge
sequence to match the recommendation of the manufacturer.
In Fig. 13, a few cycles are shown in detail. The active
equalization process, (Pack 2) shown in Fig 13b, brings
voltages together quickly during the charge sequence and
helps keep them close even during discharge. In Pack 2, one
of the cells was weak from the beginning. While the
equalizers have prevented it from getting worse, they do not
provide a “repair” function, but the cell can be changed out
without causing difficulty.
Fig. 14 shows an important summary result. Here, the cell
voltage standard deviation is shown at the end of each charge
process. The charge sequence recommended by the
manufacturer (which begins at cycle 49) appears to be holding
the voltage on the passive equalized pack reasonably well,
although a slow increase appears to be underway. The
actively equalized pack, on the other hand, shows very tight
voltage matching that is not degrading over time. Additional
data will be available at the conference.
VI. S
UMMARY
Effective use of series strings of battery cells requires cell-
by-cell SOC matching to maintain performance. In general,
SOC matching can be assured through precise voltage
matching, although the matching must be excellent (10 mV in
the case of lead-acid cells) for success. Conventional passive
equalization works for short series strings (six cells, and
perhaps up to twelve), but puts stress on strong cells and loses
effectiveness rapidly as the series string becomes longer. If
equalization can be assured, it provides substantial benefits
such as longer cycle life, fewer failure modes, and simpler
maintenance. These translate into major cost savings for large-
scale rechargeable battery applications. The technology of [9,
13] provides perfect voltage matching without any sensing or
control, and is the lowest-cost active equalization method
known. All results to date indicate that it can successfully
deliver on the promises of performance improvements under
equalization.
Fig. 11. Ongoing cycle test. Pack 1 uses passive equalization while Pack 2 uses active equalization.
INTELEC 2002 Paper 32.1
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0
500 1000 1500 2000 2500 3000
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
Hours
Cell voltage
Battery Pack 1 Cell Voltages
Fig. 12a: Passive equalization
0 500 1000 1500 2000 2500 3000
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
Hours
Cell voltage
Battery Pack 2 Cell Voltages
Fig. 12b: Active equalization
INTELEC 2002 Paper 32.1
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2095 2100 2105 2110 2115 2120 2125 2130
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
Hours
Cell voltage
Fig 13a: Passive equalization
2095 2100 2105 2110 2115 2120 2125 2130
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
Hours
Cell voltage
Fig 13b: Active equalization
20 40 60 80 100 120 140 160 180 200
0
2.5
5
7.5
10
12.5
15
Cycle number
σ (mV)
Pack1
Pack2
Fig 14: Standard deviation of cell voltages at end of charge
R
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... Moreover, the overcharge of the batteries in the string can cause explosion or fire in the case of sensitive batteries cells [7]. Therefore, a charge equalizer (i.e., balancing circuit) is essential to reduce the imbalances and consequently to improve the overall performance of the system [8], [9]. ...
... This process can be assumed instantaneous when compared to the rate that cells' voltages vary. As can be seen in (8), the power processing efficiency linearly depends on ∆V, which reflects on the voltage difference between the cells. This implies that when balance is achieved the converter exhibits ideal power processing efficiency, a consequence of the fact that no current circulates through the system when balanced. ...
... This implies that when balance is achieved the converter exhibits ideal power processing efficiency, a consequence of the fact that no current circulates through the system when balanced. It should be further noted that setting the resonant parameters reasonably high, (Q > 1.6), (8) can be approximated to: ...
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Full-text available
This work presents a battery management system for lead-acid batteries that integrates a battery-block (12 V) sensor that allows the online monitoring of a cell's temperature, voltage, and impedance spectra. The monitoring and diagnostic capabilities enable the implementation of improved battery management algorithms in order to increase the life expectancy of lead-acid batteries and report the battery health conditions. The novelty is based on the online monitoring of the evolution of electrochemical impedance spectroscopy (EIS) over a battery's life as a way to monitor the battery's performance. Active cell balancing is also proposed as an alternative to traditional charge equalization to minimize excessive electrolyte consumption. A battery-block sensor (VTZ) was validated by using the correlation between experimental data collected from electrochemical impedance spectroscopy lab-testing equipment and sensors that were implemented in a series of 12 V lead-acid battery blocks. The modular design and small size allow easy and direct integration into different commercial cell formats, and the proposed methodology can be used for applications ranging from automotive to stationary energy storage.
... The modularity of the system enables to extend the voltage of the energy 1 Fig. 1: Multilevel high voltage modular capacitor charger storage to much higher amplitudes than a single module, while still maintaining the advantage of the very low stress per component. The unique architecture utilizes cell equalization approach to keep the capacitors in the chain equally charged [8]- [23]. Additional advantage of the multilevel architecture is the ability to design multisource discharge pattern to accommodate multistage loads such as a multi-winding magnetic acceleration device targeted in this study. ...
... To enable charge flow between the bottom capacitor, charged by the capacitor charger and the rest of the storage capacitors, a network of charge redistribution converters is used, the balancing circuits [8]- [23]. Each balancing circuit is built around a resonant switched-capacitor converter and designed to support two adjacent capacitors. ...
... Maintaining life-time of a battery string would not be an easy task to accomplish. According to [2]- [5], charge balancing or equalization has proven to be an effective way to extend life cycle of a battery string. Besides, charge balancing can effectively reduce battery failure risk due to voltage imbalance between batteries in the string and hence, reduce the maintenance cost especially for large-scale rechargeable battery application. ...
... After that the next battery on the stack would be connected to the auxiliary cell and so on. In this paper, a value of 11 seconds timeout and 10 mV termination voltage would be used with regard to reference [12] and following recommended value on reference [5]. A battery stack is deemed to be balanced when LTC3305 detects termination voltage in five consecutive cycle. ...
... O problema de desbalanceamento, assim como outras diversas causas de desgastes das células, aparece em diferentes tecnologias de baterias e pode ser contornado com soluções automáticas. A literatura técnica apresenta diversas topologias e estratégias para Sistemas Gerenciadores de Baterias (SGB), que integram eletrônica de potência com microcontroladores (Liu et al., 2019) e permitem assim estender a vidaútil dos elementos (Krein e Balog, 2002). ...
Conference Paper
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Este artigo apresenta uma revisão bibliográfica sobre os principais modelos para baterias de chumbo-ácido, dando foco aos modelos de circuito equivalente. Estes podem ser utilizados em simuladores comerciais e acelerar o desenvolvimento de eletrônica de potência voltada para carga e manutenção dessas baterias, especialmente em subestações de energia. Além de uma visão geral sobre ensaios com baterias, o artigo traz um levantamento dos dados fornecidos pelos principais fabricantes nacionais de baterias e apresenta métodos de extração paramétrica baseados em dados experimentais e fornecidos no manual. São ressaltadas a relação entre estado de carga, tensão, corrente e resistência interna da bateria. Por último, simulações de curva de descarga com diferentes correntes são realizadas, validando os métodos apresentados.
... Thus, to achieve optimal battery charging, a system for managing the charging state is necessary. To this end, the battery management system (BMS) was developed to ensure safety during charging via voltage and current monitoring [4][5][6][7]. The charging stage of the initial BMS comprised a constant current (CC) mode that charges based on a CC and a constant voltage (CV) mode that charges based on a constant CV; the charging scheme originated in the battery's internal composition. ...
Article
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With recent advancements in the electrical industry, the demand for high capacity and high energy density batteries has increased, subsequently increasing the demand for fast and reliable battery charging. A battery is an assembly of a plurality of cells, in which maintaining a balance between neighboring cells is crucial for stable charging. To this end, various methods have been applied to battery management systems. Representative methods for maintaining the balance in battery cells include a passive method of adjusting the balance using a resistor and an active method involving the exchange of energy between the cells. However, these methods are limited in terms of efficiency, lifespan, and charging time. Therefore, in this study, we propose a new charging method at the battery cell level and demonstrate its effectiveness through experiments.
... Battery charge equalisation enables batteries to be charged up to their maximum capacity while not being subjected to an overcharge state (Krein & Balog, 2002:516-523, Peiying Li, 2011. More so, this prevents the batteries from quick degradation and accelerated ageing. ...
Thesis
Due to the fickle nature of weather upon which renewable energy sources mostly depend, a shift towards a sustainable renewable energy system should be accompanied with a good intermediate energy storage system, such as a battery bank, set up to store the excess supply from renewable sources during their peak periods. The stored energy can later be utilised to supply a regulated and steady power supply for use during the off-peak periods of these renewable energy sources. Battery banks, however, are often faced with the challenge of charge imbalance due to the disparities that occur in the operating characteristics of the batteries that constitute a bank. When a battery bank with charge imbalance is repeatedly used in applications without an effective battery management system (BMS) through active charge equalisation, there could be an early degradation, loss of efficiency and reduction of service life of the entire batteries in the bank. In this research, a universal battery management system (BMS) in stand-alone photovoltaic application was proposed and designed. The BMS consists majorly of a switched capacitor (SC) active charge equaliser, designed with a unique configuration of high capacitance and relatively low switching frequency, which can be applicable to common battery types used in stand-alone photovoltaic application. The circuit was mathematically optimised to minimise losses attributed to impulsive charging and tested with lead acid, silver calcium, lead calcium and lithium ion batteries being commonly used in stand-alone photovoltaic application. The SC design was verified by comparing its simulation results to the digital oscilloscope results, and with both results showing similar values and graphs, the design configuration was validated. The design introduced a simple control strategy and less complicated circuit configuration process, which can allow an easy setup for local usage. The benefit of its multiple usage with different stand-alone photovoltaic battery types saves the cost of purchasing a different charger and balancer for different battery types. More so, the design is solar energy dependent. This could provide an additional benefit for usage in areas where energy dependence is off-grid.
... For cell balancing, the BMS should implement a charge equalization technique. A charge equalizer (i.e., balancing circuit) reduces the charge imbalances and consequently improves the overall performance of the system [6], [7]. Different BMS configurations have been developed to equalize the battery cells by adjusting the SOC values of the individual battery cells, and prevent unsafe conditions by applying cell protection techniques [8][9]. ...
Thesis
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Batteries used in photovoltaic systems are subjected to penalizing operating conditions due to the intermittency of the solar resource. Their effects may be reduced by the optimisation of energy management strategies. This study deals with the pulse charge of lead-acid batteries, the most used ones for this application. The effects of this charge mode are shown both on experimental cells and commercial batteries. The influence of the parameters, frequency, duty cycle and charge factor, on the voltage profiles is precisely studied. In a second approach, a simplified model of the lead-acid battery is developed and experimentally validated after the analysis of its sensitivity to the adjustable parameters. It shows that transport phenomena in the electrolyte may be well described by a global characteristic time depending only on the state of charge. It accounts for the voltage response of the battery after adding the terms describing the non ideal behavior of the interfacial voltages.
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This paper presents experimental verification of th e impact of individual battery equalization on batt ery life and performance. A study of two battery syste ms, one equipped with PowerCheq equalizers, a new battery equalization device, and one without was co nducted over nearly 450 charge and discharge cycles simulating an electric scooter drive routine. The data clearly shows that the batteries equipped with the individual battery equalizers outperform those with no equalization as well as those employing string equalization. In fact, the battery without the equ alizers reached its end of life (80% of initial cap acity) after 140 drive cycles while the one with the equal izers was still at 100% capacity. In fact, the bat tery equipped with the PowerCheq battery equalizers last ed for more than drive 425 drive cycles resulting i n approximately 300% improvement in battery life, range, and capacity. The use of modular, real-time individual battery equalizers allows batteries to s tay equalized during charge as well as discharge cy cles thus improving battery life and longevity.
Article
The charge characteristics and construction details of a maintenance-free sealed lead-acid battery are discussed. The battery contains spirally wound very thin pure lead grid plates separated by a thin glass matte separator material. The water-loss problem during overcharging is eliminated because the gases generated are recombined by using a highly retentive separator material, and problems associated with corrosion are minimized because the plates are compressed and not self-supporting. Graphs of charge voltage, charge rate, and percent capacity returned vs. time are presented which show that the battery is very easily fast-charged and that 90% to 100% of the rated capacity can be returned to it in less than one hour with a charge voltage somewhat higher than 2.5 V/cell and charge currents of the 2C rate minimum.
Conference Paper
There is a growing trend among users of lead acid batteries towards use of sealed maintenance free designs. These offer the user many advantages in freedom of battery placement, increased safety, battery size and weight, no need to water, and in some instances superior performance. In standby service, two battery types are rivalling the traditional flooded lead acid stationary battery. Both are sealed, contain immobilized electrolyte and are "maintenance-free" by operating in the oxygen cycle. The difference in these sealed cell designs will be discussed together with a comparison of performance characteristics.
Article
Printout. Thesis (M.S.)--University of Illinois at Urbana-Champaign, 1997. Includes bibliographical reference (leaf 106).
Conference Paper
The importance of state-of-charge (SOC) balance, or equalization, is well known. Results of accelerated life testing are presented to evaluate equalization requirements and to compare passive and active equalization approaches for valve-regulated lead-acid (VRLA) batteries. In both heavy cycling duty and high-temperature duty, battery degradation appears very early during expected life in the absence of equalization. The degree of equalization is critical: results show that voltage differences should be held to less than 15 mV/cell to prevent SOC separation in repeated cycling. The tests confirm that conventional overcharge-based passive equalization is effective for VRLA batteries-if there is sufficient time to ensure SOC matching among cells. Most proposed active voltage equalization methods in effect transfer the problem of SOC matching to external voltage matching of sensors and magnetic elements. Matching at the 15 mV/cell level is costly. A switched-capacitor approach has been identified that avoids this limitation. Test results show that switched-capacitor equalization is useful, particularly when the series string is too long to support enough time for passive equalization
Conference Paper
The requirements for state-of-charge and voltage control for lithium ion batteries are reviewed. Strategies for controlling the state-of-charge of the individual Li-ion cells that comprise a battery are described. The design and test results for several of these charge control strategies are presented
Conference Paper
Hybrid electric vehicles (HEV) are approaching commercial realization, many using advanced high power, lead-acid batteries. The HEV application is quite challenging due to the high currents going to and from the batteries. Although cycle efficiencies of thin plate lead-acid batteries are as high or better than other available power sources, the heat generated by HEV mode operation is not a trivial issue. The maintenance of a desired state of charge and cell-to-cell balance offer additional complications. These issues present the vehicle integrator with additional complexity in an already complex system. Although the application is relatively new, much information exists or is being developed to help the performance of lead-acid batteries in this difficult task of HEV operation. Operating parameters are offered to help in the transition to these and other advanced transportation systems. They consist of recommendations for discharge, charge and battery management
Conference Paper
A project was undertaken to evaluate the benefits of battery charge management systems to control charge and provide equalization to high voltage series-strings of valve regulated lead acid (VRLA) batteries. The goal of the project was to compare the resulting cycle life of charge managed packs to nonmanaged (control) packs. Project results indicated some benefit with use of battery charge management systems and the enormous influence of the charge algorithm applied to the batteries. This paper summarizes the project results, including a description of the test methodology and the battery charge management systems
Conference Paper
Delphi Automotive began commercial production of a valve-regulated lead acid (VRLA) battery in 1996 that was specifically designed to deliver the power and energy demands required of electric vehicles such as the EV1. Field results support the need for thermal control of the battery pack in order to maintain capacity, state-of-charge balance, and overall performance. The influences of manufacturing variance, self-discharge rate, monoblock age, and charge acceptance are quantified and discussed
Conference Paper
A new charging method for valve-regulated lead-acid (VRLA) batteries is presented which limits the maximum of all block voltages in a battery string. A commercially available VRLA battery was equipped with cell-pressure and cell-voltage sensors. The new method was applied to the battery in comparison to conventional IU- and IUIa-charging methods. Voltage and pressure measurements during these experiments show clearly that the conventional charging methods lead to excessive overcharge of single cells correlated with irreversible water losses. These problems are efficiently avoided by the new charging method. Capacity data from cycling operation prove as well that the new charging method guarantees charge-equalizing between the inhomogeneous cells while avoiding additional hardware