Valve Regulated Lead Acid Battery

scanning: time:2021-09-17 classify:Technology info

VRLABs operate under conditions of slight deficiency of H2SO4 electrolyte as a result of which, on deep discharge, the concentration of the H2SO4 in the plate pores drops down to very low values and thus it may become the capacity limiting active material


Thermal Models

Valve-regulated lead–acid batteries employ the oxygen recombination technology and they generate more heat than flooded ones during overcharging. In a tightly packed arrangement, the battery temperature can be considerably higher than the ambient. A high-temperature operation accelerates water loss and reduces battery life. This is why thermal models are developed to regulate (lower) the float charge voltage to mitigate the battery temperature fluctuations.

A two-dimensional time-dependent thermal model has been developed  to predict steady- and nonsteady-state behavior of VRLA batteries under overcharge. The effects of various design and operating parameters on the battery's thermal behavior demonstrate that the thermal runaway is a consequence of adverse combinations of the following parameters: number of cells packed together, case conductivity, heat-transfer coefficient, water loss fraction, float charge current, and the ambient temperature. Such a model could help design VRLA batteries that are more resistive to thermal runaway. An example showing the difference between a normal float charging and a thermal runaway is given in Figure 13.


Valve-Regulated Lead–Acid (VRLA) Batteries


Valve-regulated lead–acid batteries (VRLAB) with operating closed oxygen cycle. McClelland and Devitt invented the valve-regulated lead–acid cell with electrolyte immobilized in microporous absorptive glass mat (AGM) and a pressure relief valve.

The AGM separator absorbs electrolyte, provides gas transport channels between the two electrodes and minimizes the pulsation of plate volumes during cycling. AGM separators have to be under compression in the cells. The pore system of the AGN separator depends on the proportion between fine and coarse glass fibers in its structure. The electrical characteristics of VRLA cells depend strongly on the AGM saturation with electrolyte.

Depending on the state of charge of the cell, oxygen reduction and hydrogen evolution proceed at the negative plates during charge of VRLA batteries. The reactions of the closed oxygen cycle generate heat. The cell temperature increases also due to the Joule effect. A linear dependence is established between cell current and temperature. Self-accelerating interrelations are established between the processes on the two electrodes, which result in uncontrolled increase of cell temperature and current. This phenomenon is known as “thermal runaway.”

Reduction of oxygen at the negative plates may proceed via several mechanisms: electrochemical mechanism or reduction of oxygen with formation of PbO, H2O2, PbSO4 as intermediate products. Some of these mechanisms are discussed in this chapter.

Automotive absorptive glass-mat lead–acid batteries


For many years, valve-regulated lead–acid batteries with absorptive glass-mat (AGM) separator technology have been widely used in vehicles. Whereas the batteries were first fitted to luxury cars and special vehicles such as ambulances and taxis, the introduction of start–stop cars in Europe has opened the door for the global use of AGM batteries. The main advantage of the batteries is their excellent cycling performance at high depths-of-discharge due to a design that prevents acid stratification. On the road, a higher lifetime compared with conventional flooded batteries has been found. This feature, as predicted by laboratory tests, was successfully verified in early field trials with taxi fleets, even under elevated temperature conditions. AGM batteries are able to cope with the more demanding requirements of future vehicle applications, such as autonomous driving, due to their high reliability.

Lead–Carbon Electrodes

15.4.2 Additives to the Negative Active Material Reducing Hydrogen Evolution on the Lead–Carbon Electrode

Water loss in a valve-regulated lead–acid (VRLA) battery depends on the rates of hydrogen evolution, corrosion of the positive plate grids, and oxidation of the carbon particles.

Oxygen generated at the positive plates diffuses to the negative plates and is reduced there, thus lowering the potential of the negative plates. Consequently, the rate of hydrogen evolution is reduced.

Extensive research has been carried out recently aimed at finding appropriate additives to the negative active material that would suppress the hydrogen reaction and the sulfation of the negative plates. Fig. 15.23 illustrates the effect of gallium oxide (Ga2O3) or bismuth oxide (Bi2O3) additives to NAM on battery performance [29].


The battery with 0.01% Ga2O3 in NAM, corresponding to 2% Ga2O3 in the activated carbon, has a cycle life three times longer than that of the battery with no Ga2O3 additive (Fig. 15.23A). This loading level of Ga2O3 increases the overvoltage of hydrogen evolution by 120 mV, which results in reduced water loss and hence longer cycle life under HRPSoC conditions [33].

The battery with 0.02% Bi2O3 in NAM, corresponding to 4% Bi2O3 in the activated carbon, reduces the rate of hydrogen evolution and thus prolongs the cycle life of the battery by a factor of 2.6 as compared to the battery with no Bi2O3 additive (Fig. 15.23B[33].

Addition of indium oxide (In2O3) to NAM also increases the overvoltage of hydrogen evolution, thus reducing the rate of the hydrogen reaction and eventually extends the HRPSoC cycle life of VRLA batteries [34]. Batteries with 0.02% In2O3 in NAM have at least four times longer cycle life than the reference batteries without this additive. Besides, the In2O3 additive in NAM suppresses the accumulation of lead sulfate in the negative plates and thus inhibits their progressive sulfation facilitating the reaction of lead sulfate reduction back to lead, i.e., improves the reversibility of the processes at the negative electrodes on battery cycling.

Different carbon additives to NAM reduce the overvoltage of hydrogen to different extent. Thus for example when 0.02% carbon black (CB) or 0.5% acetylene black (AB), or 2.0% flake graphite (FG), or 1.5% expanded graphite (EG) are added to the negative active material, the hydrogen overvoltage is increased only by 20–30 mV. However, addition of 0.5% activated carbon (AC) in NAM reduces the polarization of the electrode by 190 mV, because it expands significantly the active surface of NAM [35].

Addition of rare earth metal oxides to the electrolyte increases the overvoltage of hydrogen evolution on the negative plates, containing 0.2% carbon black, immersed in 1.28 s.g. H2SO4 solution containing 0.025% of the respective additive. Fig. 15.24A shows the polarization curves of hydrogen evolution on negative plates with 0.2% carbon black in NAM, in the presence of different additives in the electrolyte. The positive effect of the particular additive on retarding the hydrogen evolution reaction is in the order:


Figure 15.24. Polarization curves of hydrogen evolution at the negative plates containing: (A) 0.2% carbon black in negative active mass (NAM) and 0.025% rare earth oxides in 1.28 s.g. H2SO4 solution and (B) 0.5% active carbon in NAM and 1.28 s.g. H2SO4 solution containing different concentrations of polytetrafluoroethylene [35].

Gd2O3 > La2O3 > Dy2O3 > Nd2O3 > Sn2O3

These oxides are soluble in H2SO4 solutions and the rare earth metal ions adsorb on the surface of NAM thus impeding the diffusion of H+ ions to the surface and probably the electron transfer through the phase boundary as well.

Polytetrafluoroethylene (PTFE) added to the paste for the negative plates increases the overvoltage of hydrogen evolution. Fig. 15.24B presents the polarization curves of the hydrogen reaction in solutions with different concentrations of PTFE. At 0.025% PTFE content in the electrolyte, the additive has the strongest effect on the cathodic polarization of the electrode as a result of the hydrogen reaction, when the NAM contains 0.5% active carbon [35].

SECONDARY BATTERIES – LEAD– ACID SYSTEMS | Valve-Regulated Batteries: Absorptive Glass Mat

The absorptive glass mat (AGM) separator in the valve-regulated lead–acid battery plays an active rather than a passive role and has a critical influence on the performance and life of the battery. In this article, some historical background to the development of the AGM separator is given, and the manufacturing techniques and properties of the AGM separator are described. Important properties of the separator that are discussed in this article are pore volume, acid absorbency, compression characteristics, and gas-transfer properties. The separator properties and degree of saturation influence the oxygen cycle and recombination efficiency. The influence of the separator microstructure on the oxygen transfer rate is also discussed.

SECONDARY BATTERIES – LEAD– ACID SYSTEMS | Valve-Regulated Batteries: Oxygen Cycle

Valve-regulated lead–acid batteries operating under the oxygen cycle have had a major impact on the battery market over the last 25 years. They differ from conventional flooded batteries in that the electrolyte level is controlled to ensure that some gaseous porosity remains in the separator. This allows oxygen transport to occur, on overcharge, from the positive to the negative plate, where it is reduced to water, thus giving a battery that is largely maintenance-free. Critical to the successful operation of this oxygen cycle are the properties of the separator, and microfiber glass is universally used for this purpose. The large pore anisotropy shown by separators made from this material allows significant oxygen transport in the gas phase at high degrees of acid saturation. Lead–acid cells operating under the oxygen cycle have significant electrochemical differences from flooded cells that affect their capacity, life, float charge characteristics, and thermal properties.

Electro-chemical energy storage technologies for wind energy systems

10.10.3 Valve regulated lead–acid (VRLA) batteries

Valve-regulated lead–acid (VRLA) batteries are also referred to as ‘recombinant’ batteries. Unlike flooded batteries, which lose water as a result of oxygen and hydrogen evolution at the positive and negative electrodes respectively during charging, in VRLAs, oxygen will recombine with the hydrogen to reform water [10]. A valve is used a safety feature in case the rate of hydrogen evolution becomes too high. Since the battery system is designed to eliminate the emission of gases on overcharge, room ventilation requirements are reduced and no acid fumes are emitted during normal operation. As there is no need to top up water lost due to electrolysis, this reduces inspection and maintenance, so that VRLAs are also referred to as ‘maintenance free’.

The first VRLA batteries had the sulphuric acid electrolyte immobilized as a gel by the addition of 5–8 wt% of fumed silica. Unlike a traditional wet-cell lead–acid battery, these ‘gel-type’ batteries do not need to be kept upright and virtually eliminate the electrolyte evaporation and spillage common to the wet-cell battery. They also have greater resistance to extreme temperatures, shock and vibration. These batteries are sometimes referred to as sealed lead–acid batteries, but they are not completely sealed. The valve regulation system allows for excess gas to be vented.

The second system employs a glass-microfibre separator or absorbent glass mat (AGM) which has a high porosity and can absord a high volume of electrolyte. AGM batteries are more widely used than the gelled type of cells because of lower cost and higher power ratings [10].

AC/DC microgrids

10.1.2 Connected mode

When the VRLA batteries discharge, the voltage of the DC bus drops to the lowest threshold, and the system starts to charge the VRLA batteries. In that case, the operation mode is changed to the connected mode. In this mode, the thyristors (SCR) serving as ACSW are turned on alternately every half cycle. In the connected mode, the bidirectional converter operates not only as a rectifier for batteries charging but also as an active filter for rejecting harmonic current from loads. When the bidirectional converter charges the VRLA batteries, the bidirectional converter is initially operated by constant current (CC) control. When the DC bus voltage increases to the highest threshold, the system changes to the islanding mode. Furthermore, we can change each value of the current via CC control.


Valve-Regulated Lead–Acid Batteries with Absorptive Glass Mat Separators

The great majority of VRLA batteries produced, either as cells or as monoblocs, use pasted plates with Pb–Ca–Sn grids and AGM separators (Figure 5). Sizes range from 12 V, 1 Ah monoblocs to single cells with a capacity of 4000 Ah or more. Cell cases may be polypropylene (PP), ABS, polycarbonate (PC)/ABS, or PVC. The majority of types specified for telecommunications applications use flame retardant ABS cases but PP cases are often used for batteries for UPS applications for economic reasons. The positive grid alloys have evolved over recent years from moderately high levels of calcium (0.07–0.08%) to lower levels (0.04–0.05%) with a corresponding increase in the tin level from 0.7–0.8% to ∼1.2%. This improves the corrosion behavior of the alloy and also the cycle life. One manufacturer has used Pb–Sb–Cd alloys but this has not been followed elsewhere in the industry. Grid design and thickness are important in achieving the required life on float. For a life of 10–12 years at 20–25 °C, a grid thickness of 4–6 mm is required.


Figure 5. Valve-regulated lead–acid (VRLA) battery for standby applications with front access terminals (12 V, 105 Ah).

The AGM separator is a key element for VRLA batteries. The glass mat needs to fill the space between the electrodes, absorb a maximum volume of electrolyte, and retain a small volume of connected gas porosity to permit oxygen transport between the plates for effective recombination. High porosity is required (90–95%) with a small pore size (5–8 μm). The material also needs to be fully inert in the cell environment. These requirements are met with glass microfibers (<1 μm) that are made into a separator by a paper-making process. The separator is normally formulated with larger diameter fibers to reduce the cost and increase the tensile strength of the material. Organic polymer fibers may also be added, which not only increase the tensile strength but may also be used to weld the material into a pocket around the plate. Dimensional stability is important as the material needs to retain pressure on the plates during charge and discharge. Higher levels of resilience are achieved with higher fine fiber contents and the overall formulation is adjusted to give the correct balance of properties.

The details of the electrochemical design of the cell, separator specification and compression, and acid filling are all important in achieving reliable performance over life. For higher performance with shorter discharge times, thinner plates are used but at the expense of service life. Pillar seals use a variety of rubber sealing rings, mechanical compression or thermosetting resins. The lid to case seal may also use a resin to bond the case and lid together or they may be heat sealed together with a hot plate welder. Valves may be simple Bunsen valves or more complex arrangements and normally have a flame filter to prevent any external ignition of hydrogen from penetrating the cell. Venting pressures are low and the more important requirement is that air should not enter the cell from outside.

VRLA cells with AGM separators may be built with pure lead or pure Pb–Sn grids instead of cast Pb–Ca–Sn grids. These are fabricated from continuously cast or wrought strip by punching the grid pattern into the strip. The strip is then pasted continuously and the plates are then processed as for VRLA cells with Pb–Ca–Sn grids. The grids are typically 1.0–1.2 mm thick. Active material utilization is much better than cells with thicker plates and the use of pure lead reduces the corrosion such that the life is equivalent to cells with Pb–Ca–Sn grids with much thicker plates. Alloying with a small amount of tin (0.4–0.6%) improves performance on cycling but slightly reduces the corrosion resistance. Other details of construction are similar to normal types of VRLA cells.

The gravimetric energy density of thin plate pure lead cells is up to 50% better than conventional cells at moderate rates of discharge and the relative improvement increases at higher rates. Somewhat greater improvements are achieved in the volumetric energy density.

Thicker plate VRLA cells with pure lead plates are also manufactured. Here the corrosion life is extended but the rate performance is unaltered. Service lives in excess of 25 years at 25 °C and up to 10 years at 40 °C are claimed. For higher temperature service, the case and lid material need to be in a polymer such as PC/ABS with a higher softening point to avoid any tendency of the container to distort over time.


Oxygen Cycle

The so-called recombination in VRLA batteries is not a reaction of hydrogen and oxygen to water as it takes place in recombination plugs, but an oxygen cycle inside the cell.

The oxygen evolved at the positive electrode during overcharge diffuses to the negative electrode and discharges the negative active material by oxidation followed by the forming of sulfate.

Figure 8 shows schematically the oxygen cycle in a porous gel structure (silica and sulfuric acid) with oxygen evolution on the positive lead dioxide and oxygen reduction on the negative lead electrode.

Figure 8. Recombination.

This process suppresses the hydrogen evolution at the negative electrode, so that only very small hydrogen gassing rates are measured. The initial gassing rates are 0.2–0.7 mL day−1 cell−1 only and decrease by aging of the gel during life, so that the efficiency of the oxygen cycle becomes about 99%.

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