Lead-acid batteries in micro-hybrid applications. Part I. Selected key parameters

Micro-hybrid electric vehicles were launched by BMW in March 2007. These are equipped with brake energy regeneration (BER) and the automatic start and stop function (ASSF) of the internal combustion engine. These functions are based on common 14 V series components and lead-acid (LA) batteries. The novelty is given by the intelligent onboard energy management, which upgrades the conventional electric system to the micro-hybrid power system (MHPS). In part I of this publication the key factors for the operation of LA batteries in the MHPS are discussed. Especially for BER one is high dynamic charge acceptance (DCA) for effective boost charging. Vehicle rest time is identified as a particular negative parameter for DCA. It can be refreshed by regular fully charging at elevated charge voltage. Thus, the batteries have to be outstandingly robust against overcharge and water loss. This can be accomplished for valve-regulated lead-acid (VRLA) batteries at least if they are mounted in the trunk. ASSF goes along with frequent high-rate loads for warm cranking. The internal resistance determines the drop of the power net voltage during cranking and is preferably low for reasons of power net stability even after years of operation. Investigations have to be done with aged 90 Ah VRLA-absorbent glass mat (AGM) batteries. Battery operation at partial state-of-charge gives a higher risk of deep discharging (overdischarging). Subsequent re-charging then is likely to lead to the formation of micro-short circuits in the absorbent glass mat separator.     

Lead-acid batteries in micro-hybrid vehicles

More and more vehicles hit the European automotive market, which comprise some type of micro-hybrid functionality to improve fuel efficiency and reduce emissions. Most carmakers already offer at least one of their vehicles with an optional engine start/stop system, while some other models are sold with micro-hybrid functions implemented by default.But these car concepts show a wide variety in detail—the term “micro-hybrid” may mean a completely different functionality in one vehicle model compared to another. Accordingly, also the battery technologies are not the same. There is a wide variety of batteries from standard flooded and enhanced flooded to AGM which all are claimed to be “best choice” for micro-hybrid applications.A technical comparison of micro-hybrid cars available on the European market has been performed. Different classes of cars with different characteristics have been identified. Depending on the scope and characteristics of micro-hybrid functions, as well as on operational strategies implemented by the vehicle makers, the battery operating duties differ significantly between these classes of vehicles.Additional laboratory investigations have been carried out to develop an understanding of effects observed in batteries operated in micro-hybrid vehicles pursuing different strategies, to identify limitations for applications of different battery technologies.     

Micro-hybrid electric vehicle application of valve-regulated lead–acid batteries in absorbent glass mat technology: Testing a partial-state-of-charge operation strategy

The BMW Group has launched two micro-hybrid functions in high volume models in order to contribute to reduction of fuel consumption in modern passenger cars. Both the brake energy regeneration (BER) and the auto-start-stop function (ASSF) are based on the conventional 14 V vehicle electrical system and current series components with only little modifications. An intelligent control algorithm of the alternator enables recuperative charging in braking and coasting phases, known as BER. By switching off the internal combustion engine at a vehicle standstill the idling fuel consumption is effectively reduced by ASSF. By reason of economy and package a lead–acid battery is used as electrochemical energy storage device.     

Lead-acid and lithium-ion batteries for the Chinese electric bike market and implications on future technology advancement

China has been experiencing a rapid increase in battery-powered personal transportation since the late 1990s due to the strong growth of the electric bike and scooter (i.e. e-bike) market. Annual sales in China reached 17 million bikes year⁻¹ in 2006. E-bike growth has been in part due to improvements in rechargeable valve-regulated lead-acid (VRLA) battery technology, the primary battery type for e-bikes. Further improvements in technology and a transition from VRLA to lithium-ion (Li-ion) batteries will impact the future market growth of this transportation mode in China and abroad.     

Gelled electrolytes for use in absorptive glass mat valve-regulated lead-acid (AGM VRLA) batteries working under 100% depth of discharge conditions

Gelled electrolytes prepared from fumed silica for use in absorptive glass mat valve-regulated lead-acid (AGM VRLA) batteries and the effect of veratraldehyde addition on the electrochemical behavior and performance of AGM VRLA batteries are investigated. Cyclic voltammetry is used to investigate differences in the electrochemical behaviors of nongelled and gelled electrolytes and between gelled electrolytes with and without veratraldehyde. Battery performance is tested under 100% depth of discharge (100% DoD) conditions at both low- (0.1 C) and high- (1 C) rate discharges. The addition of silica or veratraldehyde does not affect the main reaction of the lead-acid batteries but tends to suppress the hydrogen evolution reaction. AGM VRLA batteries with gelled electrolytes have a higher discharge capacity and longer cycle life than the conventional nongel AGM VRLA batteries. The addition of 0.005% (w/v) veratraldehyde further improves battery performance, but higher (0.01%, w/v) veratraldehyde concentrations reduce it and correlate with the enhanced growth of lead sulfate crystals. The AGM VRLA battery prepared from a gelled electrolyte containing 0.005% (w/v) veratraldehyde provides the best battery performance in every operating temperature studied (0-60 °C).     

Carbon reactions and effects on valve-regulated lead-acid (VRLA) battery cycle life in high-rate, partial state-of-charge cycling

VRLA batteries in hybrid electric vehicles are operated at a partial state of charge with high current draws for acceleration and regenerative braking. Adding larger amounts of carbon particles to the negative plate material extends battery life. Water loss and increasing internal resistance are a cause of a subsequent failure mode that is related to the carbon and other organic additives in the negative plate. Previous studies of the composition and volume of gases vented from valve-regulated lead-acid (VRLA) batteries and acid-limited batteries at various temperatures and current levels are reviewed and used to develop an understanding of carbon reactions and their effects on battery state of health.     

Simulation of the current distribution in lead-acid batteries to investigate the dynamic charge acceptance in flooded SLI batteries

Measurements show that the dynamic charge acceptance (DCA) of flooded SLI lead-acid batteries during micro-cycling in conventional and micro-hybrid vehicles is strongly dependent on the short-term history, such as previous charge or discharge, current rate, lowest state of charge in the last 24 h and more. Factors of 10 have been reported. Inhomogeneous current distribution, especially as a result of acid stratification, has been suggested to explain the DCA variability.This hypothesis was investigated by simulation of a two-dimensional macrohomogeneous model. It provides a spatial resolution of three elements in horizontal direction in each electrode and three elements in vertical direction. For an existing set of parameters, different current profiles were analyzed with regard to the current distribution during charging and discharging.In these simulations, a strong impact of the short-term history on current, charge and acid density distribution was found as well as a strong influence of micro-cycles on both charge distribution and acid stratification.     

Studies of the pulse charge of lead-acid batteries for PV applications: Part II. Impedance of the positive plate revisited

In the second part of this publication series, dedicated to the pulse charge of the lead-acid battery, a special attention is paid to the impedance spectrum of the positive plate as a source for estimation of the electrostatic capacitance of the double layer (C_dl) on the surface of the positive active mass. The impedance spectra were measured at open circuit for different states of charge (SoC) in H₂SO₄ with specific gravity 1.24 and 1.28 g ml⁻¹. A substantial difference was observed in the impedance spectra of partially charged and partially discharged positive plates keeping the same value of the SOC. The impedance data were subjected to inductance error correction, followed by differential impedance analysis (DIA). Considering the results from DIA, the recently published equivalent circuits of the positive plate in charged and in discharged state and the gel-crystal model of the lead dioxide, we proposed a model of the positive plate in partial state of charge (PSoC). The analysis of the obtained experimental results using this model and DIA show that the double layer capacitance is not frequency distributed. The influence of the state of charge and state of health on the model parameters is discussed. One of the most interesting results is the dependence of C_dl on SOC—it features a hysteresis at which the values of C_dl during the charge are 5–6 times higher than the corresponding ones during the discharge. This result was discussed in terms of changes in the double layer structure considering the gel-crystal model of the lead dioxide. During the discharge in H₂SO₄ with specific gravity 1.28 g ml⁻¹ a passivation process was detected as a high frequency pseudo-inductive loop in the Nyquist plots in PSoC. The passivation time constant is higher at 50–60% SOC and decreases to zero in the end of the discharge. During the charge in both electrolytes, pseudo-inductive time constant was observed too. It was attributed to the phenomena of the dehydration of Pb(OH)₄, an intermediate in the reaction scheme of the PbSO₄ oxidation. The state of health influences mostly the ohmic resistance R_Ω, the charge transfer resistance R_ct and the parameters of the constant phase element accounting the diffusion in the pores (CPE_diff), when the plate is well charged.     

A field operational test on valve-regulated lead-acid absorbent-glass-mat batteries in micro-hybrid electric vehicles. Part I. Results based on kernel density estimation

In March 2007 the BMW Group has launched the micro-hybrid functions brake energy regeneration (BER) and automatic start and stop function (ASSF). Valve-regulated lead-acid (VRLA) batteries in absorbent glass mat (AGM) technology are applied in vehicles with micro-hybrid power system (MHPS). In both part I and part II of this publication vehicles with MHPS and AGM batteries are subject to a field operational test (FOT). Test vehicles with conventional power system (CPS) and flooded batteries were used as a reference. In the FOT sample batteries were mounted several times and electrically tested in the laboratory intermediately. Vehicle- and battery-related diagnosis data were read out for each test run and were matched with laboratory data in a data base. The FOT data were analyzed by the use of two-dimensional, nonparametric kernel estimation for clear data presentation.The data show that capacity loss in the MHPS is comparable to the CPS. However, the influence of mileage performance, which cannot be separated, suggests that battery stress is enhanced in the MHPS although a battery refresh function is applied. Anyway, the FOT demonstrates the unsuitability of flooded batteries for the MHPS because of high early capacity loss due to acid stratification and because of vanishing cranking performance due to increasing internal resistance. Furthermore, the lack of dynamic charge acceptance for high energy regeneration efficiency is illustrated. Under the presented FOT conditions charge acceptance of lead-acid (LA) batteries decreases to less than one third for about half of the sample batteries compared to new battery condition. In part II of this publication FOT data are presented by multiple regression analysis (Schaeck et al., submitted for publication [1]).     

A field operational test on valve-regulated lead-acid absorbent-glass-mat batteries in micro-hybrid electric vehicles. Part II. Results based on multiple regression analysis and tear-down analysis

In the first part of this work [1] a field operational test (FOT) on micro-HEVs (hybrid electric vehicles) and conventional vehicles was introduced. Valve-regulated lead-acid (VRLA) batteries in absorbent glass mat (AGM) technology and flooded batteries were applied. The FOT data were analyzed by kernel density estimation. In this publication multiple regression analysis is applied to the same data. Square regression models without interdependencies are used. Hereby, capacity loss serves as dependent parameter and several battery-related and vehicle-related parameters as independent variables. Battery temperature is found to be the most critical parameter. It is proven that flooded batteries operated in the conventional power system (CPS) degrade faster than VRLA-AGM batteries in the micro-hybrid power system (MHPS).A smaller number of FOT batteries were applied in a vehicle-assigned test design where the test battery is repeatedly mounted in a unique test vehicle. Thus, vehicle category and specific driving profiles can be taken into account in multiple regression. Both parameters have only secondary influence on battery degradation, instead, extended vehicle rest time linked to low mileage performance is more serious.A tear-down analysis was accomplished for selected VRLA-AGM batteries operated in the MHPS. Clear indications are found that pSoC-operation with periodically fully charging the battery (refresh charging) does not result in sulphation of the negative electrode. Instead, the batteries show corrosion of the positive grids and weak adhesion of the positive active mass.     

3D electro‐thermal modelling and experimental validation of lithium polymer‐based batteries for automotive applications

This article presents an electro‐thermal model of a stack of three lithium ion batteries for automotive applications. This tool can help to predict thermal behaviour of battery cells inside a stack. The open source software OpenFOAM provides the possibility to add heat generation because of Joule losses in a CFD model. Heat sources are introduced at the connectors and are calculated as a function of battery discharge current and internal resistance. The internal resistance is described in function of temperature. Simulation results are validated against experimental results with regard to cooling air flow field characteristic and thermal behaviour of the cell surface. The validation shows that the simulation is capable to anticipate air flow field characteristics inside the battery box. It also predicts correctly the thermal behaviour of the battery cells for various discharge rates and different cooling system conditions. The simulation supports the observation that batteries have a higher temperature close to the connectors and that the temperature increase depends highly on discharge rate and cooling system conditions. Copyright © 2016 John Wiley & Sons, Ltd.     

Advances in Fe(VI) charge storage: Part II. Reversible alkaline super-iron batteries and nonaqueous super-iron batteries

Reversible thin film Fe(VI/III) cathodic charge/discharge storage in alkaline batteries is presented. Whereas ultra-thin (e.g., 3 nm) Fe(VI/III) films exhibit a high degree of reversibility, thicker films are increasingly passive toward the Fe(VI) charge transfer. An extended conductive matrix facilitates a 100-fold enhancement in charge storage for reversible Fe(VI/III) super-iron thin films. The thicker (100s of nanometers) films deposited on extended conductive matrixes composed of high-surface-area Pt, Ti, and Au can sustain high reversibility, which provides the possibility of using Fe(VI) salts as the cathode materials for rechargeable Fe(VI)/metal hydride batteries. Super-iron cathodes can also be discharged in conjunction with a Li anode in nonaqueous media. Optimization of the nonaqueous primary super-iron/Li batteries is summarized. Fe(VI) cathodes are also reversible in nonaqueous electrolyte systems. The charge/discharge process of super-iron cathodes in nonaqueous media involves both the lithiation/delithiation of the active mass and the reduction/oxidation of the Fe(VI/III), while only the thin film Fe(VI/III) electrodes can sustain high reversibility involving the full theoretical capacity in the nonaqueous batteries.     

Studies of the pulse charge of lead-acid batteries for PV applications: Part III. Electrolyte concentration effects on the electrochemical performance of the positive plate

In the third part of this work the effects of the sulphuric acid concentration on the positive plate discharge capacity, impedance and oxygen overvoltage are discussed. It has been found that the full discharge capacity of the positive plate is available down to electrolyte concentrations of 3 mol l⁻¹ (s.g. 1.18 g ml⁻¹). At further acid dilution, capacity of the positive plate declines, keeping the utilization of the sulphuric acid about 50%. Decreasing the acid concentration, the oxygen overvoltage decreases with a factor of 12–18 mV M⁻¹, excluding the effect of the equilibrium potential of the oxygen electrode as a function of pH. The capacitance of the electrical double layer decrease linearly with the dilution of the sulphuric acid suggesting strong adsorption effects. This suggestion has been confirmed from the measurements of potential of the zero charge of the positive plate, which increases from 1.11 to 1.34 V vs. Ag/Ag₂SO₄ in the region 1.11–4.60 M H₂SO₄. From the measurement of the time constant of the electronic transfer through the gel part of the lead dioxide (T_gel) as a function of the acid concentration and the applied potential, a change in the mechanism of the lead dioxide hydration has been estimated—below 1 M H₂SO₄T_gel increases sharply, showing sharp increases of the extent of the hydration. The dilution of the electrolyte increases substantially the value of average double layer current in the beginning of the charge. During the pulse overcharge at the employed frequency of 1 Hz, the average double layer current is equal to the pulse amplitude, suggesting that the maximal efficiency of the pulse charge is reached.     

Advances in Fe(VI) charge storage: Part I. Primary alkaline super-iron batteries

Recent advances in super-iron batteries, based on an unusual Fe(VI) cathodic charge storage, are presented. Fe(VI) cathodes that have been demonstrated in super-iron batteries include the synthesized Fe(VI) compound with three-electron cathodic charge capacity Na₂FeO₄, K₂FeO₄, Rb₂FeO₄, Cs₂FeO₄ (alkali Fe(VI) salts), alkali earth Fe(VI) salts BaFeO₄, SrFeO₄, and also a transition Fe(VI) salt Ag₂FeO₄ which exhibits a five-electron cathodic charge storage. This paper focus on the primary alkaline Fe(VI) charge storage in aqueous electrolyte systems. Primary alkaline super-iron batteries exhibit a higher capacity than conventional alkaline batteries. Configuration optimization, enhancement and mediation of Fe(VI) cathode charge transfer of primary Fe(VI) alkaline batteries are summarized. Composite Fe(VI)/Mn(IV or VII), Fe(VI)/Ag(II) and zirconia coating stabilized Fe(VI)/Ag(II) cathode alkaline batteries are also illustrated.     

A critical review of electric vehicle charging using solar photovoltaic

The application of renewable sources such as solar photovoltaic (PV) to charge electric vehicle (EV) is an interesting option that offers numerous technical and economic opportunities. By combining the emission‐free EV with the low carbon PV power generation, the problems related to the greenhouse gases due to the internal combustion engines can be reduced. Over the years, numerous papers, including several review work, have been published on EV charging using the grid electricity. However, there seems to be an absence of a review paper on EV charging using the PV as one of the energy sources. With growing interest in this topic, this review summarizes and updates some of the important aspects of the PV‐EV charging. For the benefit of a wider audience, it provides the background on the EV fundamentals, batteries and a brief overview on the PV systems. Two types of PV‐EV charging, namely the PV‐grid and the PV‐standalone, are comprehensively covered. Moreover, a case study is carried out in comparison to the grid‐only charging to critically analyse the technical and the economical feasibilities of both types using Matlab simulation. At the end, recommendations and future directions are presented. It is envisaged that the material gathered in this paper will be a valuable source of information for the researchers working on this topic. Copyright © 2015 John Wiley & Sons, Ltd.     

The novel use of phase change materials in refrigeration plant. Part 2: Dynamic simulation model for the combined system

A dynamic mathematical model for coupling the refrigeration system and PCMs has been developed in this paper. Overall the model consists of the following basic components: a compressor, a condenser, an expansion valve, an evaporator cooler and a PCM heat exchanger. The model developed here, is based on a lumped-parameter method. The condenser and evaporator were treated as storage tanks at different states, which have a superheat region, a two-phase region and a sub-cooled region. In the single-phase region the parameters are considered homogeneous whereas in the two-phase region, the intensive properties are considered as in thermal equilibrium. The compressor model is considered as an adiabatic process; an isentropic efficiency is employed in this process. The expansion process in the thermostatic expansion valve is considered as an isenthalpic process. The PCM is treated as a one-dimensional heat transfer model. The mathematical simulation in this study predicts the refrigerant states and dynamic coefficient of performance in the system with respect to time. The dynamic validation shows good agreement with the test result.     

Regulatory framework and business models for charging plug-in electric vehicles: Infrastructure,agents, and commercial relationships

Electric vehicles (EVs) present efficiency and environmental advantages over conventional transportation. It is expected that in the next decade this technology will progressively penetrate the market. The integration of plug-in electric vehicles in electric power systems poses new challenges in terms of regulation and business models. This paper proposes a conceptual regulatory framework for charging EVs. Two new electricity market agents, the EV charging manager and the EV aggregator, in charge of developing charging infrastructure and providing charging services are introduced. According to that, several charging modes such as EV home charging, public charging on streets, and dedicated charging stations are formulated. Involved market agents and their commercial relationships are analysed in detail. The paper elaborates the opportunities to formulate more sophisticated business models for vehicle-to-grid applications under which the storage capability of EV batteries is used for providing peak power or frequency regulation to support the power system operation. Finally penetration phase dependent policy and regulatory recommendations are given concerning time-of-use pricing, smart meter deployment, stable and simple regulation for reselling energy on private property, roll-out of public charging infrastructure as well as reviewing of grid codes and operational system procedures for interactions between network operators and vehicle aggregators.