Glass Mat Separators Research Articles

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.

Three-layered absorptive glass mat separator with membrane for application in valve-regulated lead-acid batteries

Abstract During charge and discharge of the lead-acid cell equal amounts of H 2 SO 4 participate in the reactions at the two types of plates (electrodes). However, the charge and discharge reactions at the positive plates involve also 2 mol of water per every mole of reacted PbO 2 . Consequently, a concentration difference appears in the electrolyte between the two electrodes (horizontal stratification), which affects the reversibility of the processes at the two electrodes and thus the cycle life of the battery. The present paper proposes the use of a three-layered absorptive glass mat (AGM) separator, the middle layer playing the role of a membrane that divides (separates) the anodic and cathodic electrolyte spaces, and controls the exchange rates of H 2 SO 4 , H + ions, O 2 and H 2 O flows between the two electrode spaces. To be able to perform this membrane function, the thinner middle AGM layer (0.2 mm) is processed with an appropriate polymeric emulsion to acquire balanced hydrophobic/hydrophilic properties, which sustain constant H 2 SO 4 concentration in the two electrode spaces during cycling. Three types of polymeric emulsions have been used for treatment of the membrane: (a) polyvinylpyrollidonestyrene (MPVS), (b) polyvinylpyrrolidone “Luviskol” (MPVP), or (c) polytetrafluorethylene modified with Luviskol (MMAGM). It is established experimentally that the MMAGM membrane maintains equal acid concentration in the anodic and cathodic spaces (no horizontal stratification) during battery cycling and hence ensures longer cycle life performance.

Investigation of the porous structure of battery separators using various porometric methods

The aim of the investigation is to determine and compare the basic characteristics of the pores in battery separators using mercury porosimetry, which measures the volume of mercury penetrating into the pores, capillary flow porometry, which measures the flow rate passing through the pores, and scanning electron microscopy. Two groups of separators are investigated: PVC and glass mat.Two types of each group are analysed: PVC-R and PVC-E supplied by different manufacturers; and AGM and MAGM (modified AGM—new product developed by LABD at IEES). It has been established that: the PVC-R and PVC-E separators have similar porous structures; the AGM separator and MAGM separator have different pore size distribution, as clearly evidenced by the flow porometry data; though the glass mat separators have greater total pore volume (respective porosity), the PVC separators are characterized by greater permeability, because the pores in their narrowest part have greater diameters than those for the glass mat separators. The two methods used, mercury porosimetry and capillary flow porometry, give information about different characteristics of the porous structure. A combination of both methods will provide a more detailed information about the porous structure of the separators and a clearer idea about the dynamics of the processes that take place in the lead-acid batteries, than the data supplied by each of the techniques used alone.

Influence of float and charge voltage adjustment on the service life of AGM VRLA batteries depending on the conditions of use

Valve-regulated lead–acid (VRLA) batteries with absorptive glass mat (AGM) separators have been in use for over 20 years in different standby applications. These applications are increasingly varied, especially regarding environmental conditions. Standby batteries are not only for use in applications where conditions are strictly defined and controlled (air conditioning) and it is therefore necessary to review and clarify the key parameters for the use of VRLA batteries with respect to the optimum conditions. Several series of chemical and electrochemical reactions occur in VRLA batteries particularly when in a charge or float charge condition. These reactions give specific properties such as minimal water loss (low maintenance) but also create specific precautions for use. VRLA battery functioning is limited by four main phenomena that are positive grid corrosion, irreversible active mass sulfation, active mass degradation by cycling and dry-out by loss of water. Positive grid corrosion is the usual failure mode in float operation or due to persistent overcharge. Irreversible active mass sulfation occurs due to lack of charge. In cycling, dependent upon the frequency and depth of discharge, the active mass undergoes numerous structural changes that cause degradation. These four limiting phenomena define a framework inside which several parameters determine the service life of VRLA batteries. These parameters are commissioning, temperature, and frequency and depth of discharge. Commissioning is necessary to equalise and fully charge the cells before use. Temperature, and temperature dispersion, is the main factor determining the rate of corrosion. The frequency and depth of discharge determine how the active mass is utilised. This paper, by considering these parameters both qualitatively and quantitatively, attempts to indicate how and why to adjust the charge and float voltages to optimise the use of AGM VRLA batteries according to the environmental conditions.

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.

Advanced separator construction for long life valve-regulated lead-acid batteries

The performance of absorptive glass mat separators in valve-regulated lead-acid (VRLA) batteries is strongly influenced by the diameter of the fibres from which they are made. Coarser diameter fibres are beneficial for the compressive properties of separators while finer fibres maintain the uniform distribution of the electrolyte. Studies of cell compression and electrolyte stratification are reported using separators manufactured with segregated layers of fine and coarse fibres incorporated into a single sheet. This construction locates the different classes of fibre at their location of maximum effectiveness. Improvements in battery life in both cyclic and float charge applications are recorded, and compared with single layer separators.

High integrity VRLA batteries for telecommunications service

Telecommunications power requirements continue to evolve and to meet new patterns of use, new battery configurations have been developed to maximise energy density, optimise space utilisation and provide for ease of installation and maintenance. Front-access terminal batteries are well-matched to these requirements and are now widely applied in rack-mounted power supplies for telecommunications.A new range of sealed lead–acid batteries are described which embody front-access terminals but adopt a somewhat different approach where applications issues have been brought into sharper prominence rather than established design and manufacturing criteria. This range uses absorptive glass mat separators and state-of-the-art electrochemical designs to produce a range of batteries that has strong user benefits and enhanced performance.The technical characteristics of these batteries are described and compared to other constructions to demonstrate their behaviour under a wide range of operational conditions.

New modified AGM separator and its influence on the performance of VRLA batteries

A new modified AGM (MAGM) separator for VRLA batteries and the respective technology for its production have been developed. The new product is obtained by processing absorbent glass mat (AGM) separator with polymer emulsion, which is adsorbed onto the surface of the glass fibres and concentrates at the sites of contact between the fibres. Thus, a continuous porous network of interconnected glass fibres is formed. The two surfaces of the AGM sheet are treated with polymer emulsion of different concentration in view of the different processes that take place at the positive and the negative plates, respectively. As a result of the above treatment the surface properties of the AGM are modified to balance hydrophobic/hydrophilic properties. The MAGM separator has improved tensile strength as compared to untreated AGM, a slightly reduced H 2SO 4 absorption and wicking rate, and increased chemical and thermal stability. It does not disperse in H 2SO 4 solution as does AGM. The influence of the MAGM separator on the processes in the valve-regulated lead-acid cell has been investigated. An experimental method has been elaborated and used to establish the existence of horizontal electrolyte stratification between the plates. This stratification is a result of the different processes that occur at the two types of plates as well as of the resistance that the ion flows of H 2SO 4 and H 2O have to overcome when passing through the separator. In VRLA batteries, their movement is impeded by the AGM separator, which reduces the available capacity of the battery. The effect of the MAGM separator on the concentration of H 2SO 4 at the two separator interfaces during charge, discharge, open circuit (OC) and on cycling is investigated. MAGM influences the processes in the cells in such a way that the differences in H 2SO 4 concentration between the two types of plates are eliminated. These effects of the MAGM separator result in an increase of the available capacity of the battery on cycling and float service, which leads to a substantial increase in VRLA battery life. Moreover, MAGM improves the efficiency of the oxygen cycle in the cells and thus the water loss is reduced. Besides, MAGM separators reduce the differences in cell voltage when numerous cells are connected in long strings.

Investigation of the height and rate of capillary lift of electrolyte in the glass-mat separator of a sealed lead-acid battery

The height and rate of capillary lift of the electrolyte in the glass-mat separator of a sealed lead-acid battery was investigated. Dependence of the height and rate of rise of sulfuric acid on its density and separator compression degree was established.

Micro-structural design and function of an improved absorptive glass mat (AGM) separator for valve-regulated lead–acid batteries

Two important properties of absorptive glass mat (AGM) separators are examined in order to design optimum separators for advanced valve-regulated lead–acid (VRLA) batteries. Acid stratification in the separator depends on its micro-glass-fibre diameter, and it is found that the extent of stratification can be estimated based on hydrodynamics theory. Decreasing the plate-group pressure of the separator in the wetted state is also investigated, and it is considered that the phenomenon is caused by the balance between the fibre strength and the surface tension of acid solution. Given these results, the way to design AGM separators according to purpose has been identified. Accordingly, a new AGM separator has been developed and this functions both to suppress stratification and to maintain plate-group pressure.

Investigation of height of electrolyte retention by glass-mat separators in gravitation field

One of the limitations to use glass felt separators (AGM type) in sealed rechargeable cells of high capacity and big height can be their inability to keep the uniform distribution of electrolyte by the height in the gravitation field. The influence of various factors (separator height, degree of its compression, electrolyte density) on separator ability to keep the initial uniform distribution of electrolyte has been investigated.

The Behavior of Oxygen Transport in Valve-Regulated Lead-Acid Batteries with Absorptive Glass Mat Separator

In the oxygen cycle of valve-regulated lead-acid (VRLA) batteries, there are two ways in which oxygen can move from the positive to the negative plates, namely, either horizontally to penetrate the absorptive glass mat (AGM) separator, and/or transport vertically via the gas space. It is found that the oxygen transport depends on the passageway with big void space in the AGM separator and its rate is proportional to the oxygen partial pressure. The rate constant of vertical transport is about three orders higher than that of horizontal transport because of the large void space between the AGM separator and plates. However, in the horizontal direction, the area is very large and the transport path is very short. So the way and the rate of oxygen transport actually depend on the level of saturation in VRLA batteries. The horizontal transport is dominant when the saturation is less than 93%, while the vertical transport becomes dominant when it is higher than 93%. The experiments also indicate that with decreasing saturation, the recombination of more oxygen at the negative plate may oxidize more active atoms and therefore, the overpotential of hydrogen evolution increases obviously. © 2001 The Electrochemical Society. All rights reserved.

00/01359 A study of the effects of compression on the performance of the positive active mass in lead-acid cells using absorptive glass mat separators

00/01359 A study of the effects of compression on the performance of the positive active mass in lead-acid cells using absorptive glass mat separators

00/00840 Synthetic fibre reinforcement of absorptive glassmat separators for valve-regulated lead-acid batteries

00/00840 Synthetic fibre reinforcement of absorptive glassmat separators for valve-regulated lead-acid batteries

00/00818 A multi-layered approach for absorptive glass-mat separators

00/00818 A multi-layered approach for absorptive glass-mat separators

A study of the effects of compression on the performance of the positive active mass in lead–acid cells using absorptive glass mat separators

Abstract A new electrochemical cell was designed and constructed to enable in situ measurements of the performance and internal resistances of lead–acid test electrodes in a vertical position during cycling at well-defined degrees of compression. The AGM separators used were combined with microporous paper separators to prevent short-circuiting by conducting bridges of lead dioxide. The test cells were subjected to pressures in the range 0–8 N/cm2; the compression decreased the positive active mass resistance and increased its cycle life.

A multi-layered approach for absorptive glass-mat separators

The traditional method of manufacturing absorptive glass mats (AGMs) for valve-regulated lead–acid (VRLA) batteries is to blend two or more types of fibres together in an aqueous acidic solution and deposit this blend on to either a moving endless wire or a roto-former (another version of an endless wire). The sheet acquires consistency as the water is withdrawn; it is then pressed and dried against heated drums. The methods of fibre dispersion and deposition can be changed so that the different constituent fibre types of an AGM separator are processed separately in distinct and separate layers. This fibre segregation results in the enhancement of some key characteristics of the separator and thus brings some very definite advantages to the performance of VRLA batteries, e.g., the ability to deliver higher currents at higher discharge rates. Also, important AGM characteristics, such as wicking, porosity/pore-size and stratification, are radically modified by adoption of the multi-layered AGM design. As a consequence, the high-rate and cycling performances of VRLA batteries are equally affected.

Synthetic fibre reinforcement of absorptive glass-mat separators for valve-regulated lead–acid batteries

The contribution of synthetic fibres to the puncture resistance of absorptive glass-mat (AGM) battery separators when they are combined with glass microfibres is presented. Other mechanical characteristics necessary for the automated manufacture of batteries are also improved by the addition of well-chosen synthetic fibres. The selection of these synthetic fibres is of paramount importance given that specific physical parameters such as wicking, porosity or wettability have to be taken into account to design a useful and long-life AGM battery separator.

Absorptive glass-mat separators for valve-regulated lead/acid batteries — thoughts on compression

In the past few years, valve-regulated lead/acid (VRLA) batteries have come under increased study. Their use has become more widespread, yet their expected life has not always been realized. This paper discusses some thoughts relating to the property of compression of the microglass separator and the impact of compression on VRLA battery life. Ideas are suggested for the design engineer to consider in selecting a battery separator. Additionally, several long-term battery separator tests are described. As more is learned about the complex interactions that are taking place in the VRLA recombination process, a greater appreciation is being given to the role of the separator. Today, battery designers can help improve expected battery performance by incorporating the latest information regarding battery separators, compression factors, and impact on life.

Development of valve-regulated lead/acid batteries for distributed power requirements

Valve-regulated lead/acid (VRLA) batteries provide the very high levels of reliability required for stand-by service. Various types are available and, in particular, cells with lead-calcium-tin alloy grids and absorptive glass-mat (AGM) separators, cells with pure-lead grids and AGM separators, and cells with tubular plates and gelled electrolyte. The cell types are subject to a number of factors that affect durability in floating service. The factors are reviewed and it is shown that grid corrosion is the usual failure mode. As a result, practical cell and battery designs need to ensure that all other potential causes of failure are either eliminated or occur at a slower rate. Test results based on thermal acceleration are presented and have been correlated with real-time tests. The attainment of satisfactory product life under practical conditions is fully demonstrated. Techniques for battery monitoring and surveillance also have a strong impact on reliability and can be used to define the best strategy for replacement. The overall result is better levels of protection which, together with precise specification and careful consideration of the service conditions, enable user requirements to be met in full.