VRLA Research Articles

04/01319 Enhanced performance of VRLA batteries with a novel spirally-wound electrode design: Wang, J. et al. Journal of Power Sources, 2003, 113, (2), 241–244

04/01319 Enhanced performance of VRLA batteries with a novel spirally-wound electrode design: Wang, J. et al. Journal of Power Sources, 2003, 113, (2), 241–244

04/01314 A unified discharge voltage characteristic for VRLA battery capacity and reserve time estimation: Pascoe, P. F. and Anbuky, A. H. Energy Conversion and Management, 2004, 45, (2), 277–302

04/01314 A unified discharge voltage characteristic for VRLA battery capacity and reserve time estimation: Pascoe, P. F. and Anbuky, A. H. Energy Conversion and Management, 2004, 45, (2), 277–302

04/01329 Progress in overcoming the failure modes peculiar to VRLA batteries: Cooper, A. and Moseley, P. T. Journal of Power Sources, 2003, 113, (2), 200–208

Fuel Cell Vehicles (FCVs) are now approaching wider spread consumer adoption, with some of the largest OEMs having released or about to release commercial products. However, research into consumer perceptions of FCVs has been extremely limited to date. This paper investigates automotive early adopter attitudes toward FCVs by performing interviews with high-end BEV owners. This sample was chosen as based on a preceding study it was expected that these adopters would have a greater level of awareness and knowledge of FCVs compared to the general population. It was expected that this would lead to more insightful results. In this study 5 consumer barriers to FCV adoption emerge. These are; the lack of hydrogen infrastructure, the source of hydrogen, the inability of FCVs to be recharged from home, cost issues and also concerns about hydrogen safety. This paper goes on to make recommendations on how to solve these consumer barriers stating that; hydrogen infrastructure needs to be pre-developed, hydrogen should be sustainably generated, FCVs should have the ability to be partially charged at home, hydrogen fuel will need to be subsidised in the early market, vehicles need to be positioned in the correct markets, and safety concerns can be mitigated partly through education and outreach. Finally this paper finds that consumers do desire the range of a FCV and there is also consumer demand for FCVs ability to provide emergency back-up power.

A Method for Battery Impedance Analysis

A model-based method is proposed for the impedance-spectra analysis of valve-regulated lead-acid batteries. The electrochemical and double-layer processes are analyzed in a wide range of frequencies based on current, voltage, and temperature measurements. Both slow and fast dynamics of the positive and negative electrodes are evaluated. The method is shown to be accurate, enabling the detection of differences in capacity or other parameters even in slightly differing batteries. An exact relation between battery impedance and the electrochemical cell model is established giving useful information on the contribution of different elements and processes on the impedance. The proposed method is computationally effective due to the special analytical formulation developed for impedance analysis. In addition to the basic processes, the method also allows analysis of the fast-rate discharge and overcharge processes. © 2004 The Electrochemical Society. All rights reserved.

Use of synthetic fibre reinforcement for improving the performance of AGM separators for VRLA batteries

Abstract Absorbent material made of 100% microglass fibre is well-known as a separator for valve-regulated lead-acid (VRLA) batteries and has been in use for about 20 years. As the VRLA battery market and demands on battery performance continue to grow, the electrical characteristics, productivity and reliability of VRLA batteries are being enhanced. Both the reliability and the electrical performance of the battery are linked to the AGM glass material. The properties of the AGM material impact the assembly route, defect rates, productivity, product cycleability, life, reliability, and recharge performance. An AGM product range which contains synthetic fibres up to 30% in mass (the remaining 70% is still glass) has been developed. The reinforcement of the material with a specific organic fibre is showing very positive effects, both on intrinsic separator characteristics and, as a consequence, on the manufacturing, electrical properties and cycle-life of batteries. The AGM product has been used in the battery market now for more than 20 years. Based on this experience, new testing methods such as the fatigue test and internal pressure measurement have been developed. It is clear from such measurements that reinforcement with synthetic fibre has a positive impact on battery characteristics.

Developments in absorptive glass mat separators for cycling applications and 36 V lead−acid batteries

The major markets for valve-regulated lead−acid (VRLA) batteries are undergoing a radical upheaval. In particular, the telecommunications industry requires more reliable power supplies, and the familiar 12 V electrical system in cars will probably be soon replaced by a 36/42 V system, or by other electrical systems if part of the automotive market is taken over by hybrid electrical vehicles (HEVs). In order to meet these new challenges and enable VRLA batteries to provide a satisfactory life in float and cycling applications in the telecommunication field, or in the high-rate-partial-state-of-charge service required by both 36/42 V automobiles and HEVs, the lead−acid battery industry has to improve substantially the quality of present VRLA batteries based on absorptive glass mat (AGM) technology. Therefore, manufacturing steps and cell components have to be optimized, especially AGM separators as these are key components for better production yields and battery performance. This paper shows how the optimal segregation of the coarse and fine fibres in an AGM separator structure can improve greatly the properties of the material. The superior capillarity, springiness and mechanical properties of the 100% glass Amerglass multilayer separator compared with commercial monolayer counterparts with the same specific surface-area is highlighted.

Changes in the demands on automotive batteries require changes in battery design

The electrochemical storage of energy in automobiles faces the possibility of a major change, in that the electrical systems planned for vehicles are expected to be far more extensive than hitherto. These advanced electrical systems will require the electrical energy-storage element (one or more batteries) to operate at a partial-state-of-charge for most of its life and at unprecedented high rates (so-called ‘HRPSoC duty’). The remarkable advances achieved with valve-regulated lead-acid (VRLA) battery technology for electric vehicles (EVs) during the past 8–10 years will be of only passing value in overcoming the challenges posed by HRPSoC duty. This is because the battery failure modes seen in HRPSoC operation are quite different from those faced in EV (deep-cycle) use. The replacement of the 12 V automotive battery will not take place rapidly. If the applications that take its place are to be satisfied by a lead-acid product (probably VRLA), rather than by a battery of a different chemistry, a programme of development as successful as that mounted for deep-cycle duty will be required.

Failure mechanism of valve-regulated lead–acid batteries under high-power cycling

Abstract A group of valve-regulated lead–acid (VRLA) batteries (12 V, 33 Ah) cycled under high power has exhibited premature failure. The only difference between failed and healthy batteries is the shedding of active material from the positive plates. The dislodged material has been examined by means of X-ray diffraction, scanning electron microscopy, and cyclic voltammetry. It is found that the material has a β-PbO2 structure. The particles, which are oval in shape with a diameter of about 100 μm, are uniform and well separated from each other. The activity of the material can be restored under pressure. It is concluded that the failure mode of VRLA batteries under high-power cycling is softening of the positive active-material, which eventually results in deterioration of the electrical conductivity and de-activation of the material.

Thermal runaway in valve-regulated lead-acid cells and the effect of separator structure

Abstract Thermal runaway is normally defined as the increase in charge or float current that occurs as a result of the increase in cell temperature from the initial applied constant potential. If left unchecked, the currents can reach high values and, ultimately, lead to the destruction of the cell. This definition does not explain why all cells floated at constant potential do not suffer from thermal runaway. The aim of this paper was to investigate and explain the cause of this transition from normal stable behaviour to unstable thermal runaway. A series of 6 V, 100 A h, valve-regulated lead-acid (VRLA) batteries were overcharged at potentials of up to 2.65 V per cell and the currents, temperatures and gas-evolution rates measured during thermal runaway. From these results, it was concluded that separator dry-out was the critical parameter that controls thermal runaway behaviour. This conclusion was reinforced by other data for the effect of saturation on the resistance, the normal float behaviour and the gas transport in VRLA separators. A model of the structure of partially saturated separators was developed to explain the observed behaviour, and was used to predict possible improvements in separator structure to increase resistance to runaway.

Development of a lead-acid battery for a hybrid electric vehicle

Abstract In September 2000, a project reliable, highly optimized lead-acid battery (RHOLAB) started under the UK Foresight Vehicle Programme with the objective of developing an optimized lead-acid battery solution for hybrid electric vehicles. The work is based on a novel, individual, spirally-wound valve-regulated lead-acid 2 V cell optimized for HEV use and low variability. This cell is being used as a building block for the development of a complete battery pack that is managed at the cell level. Following bench testing, this battery pack is to be thoroughly evaluated by substituting it for the Ni−MH pack in a Honda Insight. The RHOLAB cell is based on the 8 Ah Hawker Cyclon cell which has been modified to have current take-off at both ends—the dual-tab design. In addition, a variant has been produced with modified cell chemistry to help deal with problems that can occur when these valve-regulated lead-acid battery (VRLA) cells operate in a partial-state-of-charge condition. The cells have been cycled to a specially formulated test cycle based on real vehicle data derived from testing the Honda Insight on the various test tracks at the Millbrook Proving Grounds in the UK. These cycling tests have shown that the lead-acid pack can be successfully cycled when subjected to the high current demands from the vehicle, which have been measured at up to 15 C on discharge and 8 C during regenerative recharging, and cycle life is looking very promising under this arduous test regime. Concurrent with this work, battery development has been taking place. It was decided early on to develop the 144 V battery as four 36 V modules. Data collection and control has been built-in and special steps taken to minimize the problems of interconnect in this complex system. Development of the battery modules is now at an advanced stage. The project plan then allows for extensive testing of the vehicle with its lead-acid battery at Millbrook so it can be compared with the benchmark tests which have already been carried out on the vehicle with its Ni−MH batteries.

Valve-regulated lead-acid batteries for stop-and-go applications

Increasing levels of demand for electrical power for vehicles have prompted a considerable level of research into higher voltage systems. This has resulted in the definition of preliminary standards for 36/42 V systems. The implementation costs for these systems are high and this has led to improvements in 12/14 V power architectures. In particular, alternator power outputs at 14 V have increased and the need for lower emission levels and fuel economy is stimulating a demand for stop-and-go systems. In this type of application, the engine is stopped each time the vehicle comes to a halt, and is restarted when the accelerator is pressed again. The duty cycle that this applies to the battery is quite onerous with many shallow discharge cycles. Flooded lead-acid batteries are unable to meet the requirements and valve-regulated lead-acid (VRLA) batteries are essential to meet the demands applied. The background to stop-and-go battery applications is considered and test results on practical batteries are presented to show that under a simulated duty cycle, good performance can be achieved. There is also a need for a higher level of battery management for stop-and-go systems. A practical approach to battery condition monitoring to assess the state-of-charge and state-of-health of the battery is described.

Advanced management strategies for remote-area power-supply systems

Abstract An operating strategy based on partial-state-of-charge (PSoC) operation has been developed for a remote-area power-supply (RAPS) system in Peru. The facility will power an entire village and comprises a photovoltaic array, a bank of gel valve-regulated lead-acid (VRLA) batteries, a diesel generator, and a sophisticated control system. The PSoC schedule involves operation below a full state-of-charge (SoC) for 28 days, followed by an equalization charge. The schedule has been evaluated by operating a 24 V battery bank under simulated RAPS conditions in the laboratory. It is found that operation between 58 and 83% SoC causes the negative-plate potentials to move to significantly more negative values during charging as the PSoC duty progresses. This behaviour is undesirable, because it can lead to the activation of a preset limit and a subsequent reduction in system efficiency. Lowering the PSoC window to 47–72% SoC or 40–65% SoC during the 28-day cycle is found to stabilize the negative-plate potentials. The behaviour of the negative plates in gel batteries is very similar to that observed for absorptive glass mat (AGM) designs of VRLA batteries operated in hybrid electric vehicles.

Failure mode of valve-regulated lead-acid batteries under high-rate partial-state-of-charge operation

Abstract Within the next decade, there will be major changes in automotive technology with the introduction of several new features which will increase significantly the on-board power requirements. This high power demand is beyond the capability of present 14 V alternators and thus a 42 V power network is to be adopted. The new ‘PowerNet’ requires the lead-acid battery to be capable of providing a large number of shallow discharge–charge cycles at a high rate. High-rate discharge is necessary for engine cranking and power assist, while high-rate charge is associated with regenerative braking. The battery will operate at these high rates in a partial-state-of-charge condition, so-called HRPSoC duty. Under simulated HRPSoC duty, it is found that the valve-regulated lead-acid (VRLA) battery fails prematurely due to the progressive accumulation of lead sulfate mainly on the surfaces of the negative plates. This is because the lead sulfate cannot be converted efficiently back to sponge lead during charging either from the engine or from regenerative braking. Eventually, the layer of lead sulfate develops to such extent that the effective surface area of the plate is reduced markedly and the plate can no longer deliver the high cranking-current demanded by the automobile. A mechanistic analysis of battery operation during HRPSoC duty shows that high-rate discharge is the key factor responsible for the build-up of the lead sulfate layer. Such discharge causes a compact layer of tiny lead sulfate crystals to form on the surface of the negative plate and subsequent charging gives rise to an early evolution of hydrogen. Hydrogen evolution is further exacerbated when a high charging current is used.

Vrla battery rapid charging under stress management

This paper discusses an approach for closed-loop charge control of valve-regulated lead-acid (VRLA) batteries used in telecommunication standby power applications. This is an alternative to conventional preprogrammed, configuration-driven charge control. The developed approach uses the battery's response to the supplied charge to control the recharge process. In this way, only the energy that can be absorbed by the battery in the desired recharge operation is dealt with. Hence, excessive energy that causes battery stress, or lack of energy that slows the charging process is avoided. Two main sources of recharge stress are identified: (a) thermal stress, and (b) charge saturation stress. A charge control algorithm based on thermal management and charge saturation avoidance is suggested. The algorithm utilizes a fuzzy state-of-charge estimation model that derives the state of charge from real-time parameters. This in turn calculates the maximum recharge rate that maintains the battery's thermal rise within permissible limits.

High rate partial-state-of-charge operation of VRLA batteries

Abstract The world market for 12 V SLI batteries currently stands at around US$ 12 billion. The lack of a serious challenge from other battery types has allowed lead–acid products to serve this market exclusively, with minimal demand for product improvement through research and development, and a sharp competition has, over time, cut sales prices to commodity levels. The electrochemical storage of energy in automobiles now faces the possibility of a major change, in the form of the proposed 36/42 V electrical systems for vehicles that remain primarily powered by internal combustion engines, and of the hybrid electric vehicle. The duty cycle for these two applications sees the battery held at a partial-state-of-charge (PSoC) for most of its life and required to supply, and to accept, charge at unprecedented rates. The remarkable advances achieved with VRLA battery technology for electric vehicles during the past 8–10 years will be of only passing value in overcoming the challenges posed by high rate PSoC service in 36/42 V and HEV duty. This is because the failure modes seen in PSoC are quite different from those faced in EV (deep cycle) use. The replacement of the 12 V SLI will not take place rapidly. However, if the applications which take its place are to be satisfied by a lead–acid product (probably VRLA), rather than by a battery of a different chemistry, a program of development as successful as that mounted for deep cycle duty will be required. The present phase of the Advanced Lead–Acid Battery Consortium (ALABC) R&D program has begun to shed light on those aspects of the function of a VRLA battery which currently limit its life in high rate PSoC duty. The program is also pursuing the several technologies which show promise of overcoming those limits, including multiple tab plate design, mass transport facilitation and minor component (both beneficial and detrimental impurity) management. This paper presents a brief review of the changes which are taking place in lead–acid battery technology in response to the challenges of the new duty cycle.

Development of a long life 35Ah capacity VRLA battery for load-leveling applications

A load-leveling (LL) system is needed for the effective use of electric power to preserve the environment in Japan. Although the valve regulated lead-acid (VRLA) battery is considered to be one of the suitable candidates, high requirements of long cycle life such as 2000 cycles or a calendar life of 7 years needed to be solved. We are currently developing a new VRLA battery for this application, and have succeeded in achieving over 2000 cycles with a 35 Ah class VRLA battery. It was confirmed by the detailed investigation after a cycling test of the battery that this cycle performance was achieved by improving the charge acceptance of the negative plate, avoiding the grid corrosion by applying an optimized charge condition.

Thin electrodes based on rolled Pb–Sn–Ca grids for VRLA batteries

Abstract Electrodes 0.5 mm thick (i.e. much thinner than conventional ones) and suitable for lead-acid batteries were prepared by using a special pasting procedure that allows plate thickness to be readily controlled. Novel rolled grids of Pb–Sn–low Ca alloys (0.35 mm thick) were used as substrates. Preliminary galvanostatic corrosion tests of the grids revealed an increased corrosion rate relative to conventional casted grids of Pb–Sn alloys (1 mm thick). Cells made with these thin electrodes were cycled under different discharge regimes and the active material at different charge/discharge cycling stages was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical analysis. At a depth of discharge (DOD) of 100%, the cell exhibited a premature capacity loss after the fifth cycle and delivered only a 20% of its nominal capacity after the 10th. By contrast, cycling performance of the electrode was significantly improved at a DOD of 60%. The capacity loss observed at a DOD of 100% can be ascribed to a rapid growth of PbSO4 crystals reaching several microns in size. Such large crystals tend to deposit onto the grid surface and form an insulating layer that hinders electron transfer at the active material/grid interface. For this reason, the cell fails after few cycles in spite of the high PbO2 content in the positive active material (PAM). On the other hand, at 60% DOD the submicronic particles produced after formation of the PAM retain their small size, thereby ensuring reversibility in the PbO2⇔PbSO4 transformation.

A VRLA battery simulation model

A valve regulated lead acid (VRLA) battery simulation model is an invaluable tool for the standby power system engineer. The obvious use for such a model is to allow the assessment of battery performance. This may involve determining the influence of cells suffering from state of health (SOH) degradation on the performance of the entire string, or the running of test scenarios to ascertain the most suitable battery size for the application. In addition, it enables the engineer to assess the performance of the overall power system. This includes, for example, running test scenarios to determine the benefits of various load shedding schemes. It also allows the assessment of other power system components, either for determining their requirements and/or vulnerabilities. Finally, a VRLA battery simulation model is vital as a stand alone tool for educational purposes. Despite the fundamentals of the VRLA battery having been established for over 100 years, its operating behaviour is often poorly understood. An accurate simulation model enables the engineer to gain a better understanding of VRLA battery behaviour. A system level multipurpose VRLA battery simulation model is presented. It allows an arbitrary battery (capacity, SOH, number of cells and number of strings) to be simulated under arbitrary operating conditions (discharge rate, ambient temperature, end voltage, charge rate and initial state of charge). The model accurately reflects the VRLA battery discharge and recharge behaviour. This includes the complex start of discharge region known as the coup de fouet.

Automated battery test system

Battery testing is required for a wide range of applications. This includes quality assurance, design verification and performance assessment purposes for battery manufacturers, validation purposes for battery users, and battery behavioural research purposes for engineers developing behavioural prediction algorithms. Regardless of the application, determining the behavioural characteristics of batteries is a non-trivial problem. The electrochemical processes of a battery are complex, involving many parameters that are non-linearly interrelated. Therefore it is important that a test system designed for investigating battery behaviour provides accurate and reliable control and data acquisition. Manual testing is a labour intensive and time-consuming process. Automated testing can alleviate the labour requirements, with the added advantages of limiting human error (through limited human involvement), maintaining consistency between tests and providing flexibility in test conduction time and test routine. This paper presents the architecture of an automated battery test system. The paper begins by providing details on battery testing procedures. A typical test cycle, where the battery passes through all operating phases of charge, float charge and discharge, is described. The battery testing technique of accelerated thermal ageing, a special case of cyclic testing, is also described. This allows the identification of the parameters that are to be acquired and controlled and hence provides the specification for the battery test system. A specific battery test system solution is presented, which was designed for conducting valve regulated lead acid battery behavioural research. The system employs commercial off the shelf (COTS) components. Versatility is a key attribute of the system. This versatility extends from the ability to accommodate various battery configurations (different number of blocks per string and number of strings) and battery types (single cells through to mono-blocks) to the ability to vary the way the test is conducted and the criteria for data acquisition.

Development of VRLA battery for hybrid bus

Abstract Valve regulated lead–acid (VRLA) batteries named SEH65 series for hybrid vehicles have long been used in hybrid inverter-controlled motor and retarder system (HIMR) buses developed by Hino Motors Ltd. In order to elucidate the points for improving the battery performance, the battery performance, which were obtained during field use and by bench test, was analyzed in details. As the result, a new VRLA battery, with which longer life performance can be expected, has been developed by improving the positive grid corrosion resistance and adopting a new additive for the negative plates. In addition, it was found that estimation of residual life of the battery is possible by measuring the battery voltage in high-rate discharging during bus use and by adjusting it with the field use conditions.