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Float Life Verification of a VRLA Battery Utilizing a High Purity Electrochemical System

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Float Life Verification of a VRLA Battery Utilizing a High Purity Electrochemical System Float Life Verification of a VRLA Battery Utilizing a High Purity Electrochemical System Frank A. Fleming*, HEPI1 Lei Gao and Philip R. Shumard, HEPI1 Rhodri Evans and Raju Kurian, HEPI2 1Hawker Energy Products Inc. Warrensburg, 64093, MO, USA 2Hawker Energy Products Ltd. Newport, Gwent, NP9 OXJ, UK *Please refer any e-mail correspondence to the principal author [email protected] The following is a brief explanation of the negative plate Abstract - Claims made at recent INTELEC battery run down mechanism whilst in a float application. Figure workshops suggest that VRLA batteries may have a 1 depicts the flow of current into and out of a VRLA reliable life limited to as little as 2 years in standby battery whilst under float charge conditions. applications. Furthermore, several published papers propose that one of the inherent failure modes is loss of Recharge capacity of the negative electrode. 2+ 4+ - 2+ - Pb = Pb + 2e Pb + 2e = Pb This paper examines float life verification of Dry Out Pb + H2SO4 -> PbSO4 + H2 commercially available high purity VRLA batteries in (Self-discharge) light of this recent concern regarding negative electrode 4+ - 2+ - Pb= Pb + 4e Pb + 2e = Pb capacity loss and includes data from real-time room (Grid Corrosion) (Recharge Self-discharge) temperature float testing which has been ongoing well in Pos Neg + - + - excess of 10 years. Org = Org + e 2H + 2e = H2 (Expander Oxidation) (Hydrogen Evolution) The condition of the negative electrode, from both new Heat Generator and real-time aged product, has been carefully examined <100% - O 2 lost to atmosphere + - + - by both electrochemical measurements of the product and 2H20 = 4H + 4e +O2 4H + 4e +O2 = 2H20 =100% also by analyzing the morphology and crystallography of (Oxygen Evolution) (Recombination) the plates. The condition of the positive grid has also been examined to determine the extent of corrosion. Figure 1 – VRLA Current Flow In conclusion, this paper demonstrates that by using a There are three areas to consider, namely: high purity VRLA technology, which has been commercially available for 25 years, it is possible to far 1. Recharge – immediately following a discharge the exceed 2 years of reliable service. Properly designed predominant mechanisms are the recharge of both batteries using this technology have been proven to electrodes. Due to the inefficiencies associated with deliver greater than 13 years in real-time float service. these recharge processes, the two electrodes tend to recharge at different rates dependent upon, amongst I. Introduction other things, the state-of-charge of the individual electrode. For the purpose of this paper we assume Recent INTELEC workshops have focused on the that recharge has been completed. perceived shortfalls of VRLA batteries in telecommunication applications. In particular, the ability 2. Dry Out – the consumption of water through the of the negative electrode to maintain a full state of charge corrosion of the positive grid material and the loss of throughout life has been questioned. Several papers have hydrogen gas either via self-discharge (local action) been published that elegantly postulate a mechanism for or via hydrogen ion reduction. Other minor reactions the decline in negative electrode capacity throughout the can contribute to the current flow within this region batteries float life (1). such as oxidation of organic expanders that have leached from the negative electrode or the loss of oxygen gas from the cell. 2. Impurities – the purity of the raw materials that make up the battery is absolutely critical due to the 3. Heat Generator – this describes the recombination deleterious local action of certain metallic impurities process whereby oxygen generated at the positive is at the negative electrode resulting in self-discharge reduced to water at the negative electrode, with the (2). This is made even more critical with VRLA subsequent generation of heat. If the recombination technology since, due to the depolarizing nature of process is 100% efficient then the two partial the recombination reaction, the negative electrode currents perfectly balance one another. has less ability to recharge any self-discharge resulting from local action, relying only upon the If we make the assumptions during float operation that, partial current from the positive grid corrosion a) the battery is fully recharged, b) that it is behaving in a reaction. (In a flooded design any partial current 100% efficient recombination mode and, c) that side associated with oxygen evolution at the positive will reactions such as organic expander oxidation at the recharge/polarize the negative electrode). positive electrode are minimized, then we need only Furthermore, in recent years, VRLA battery consider the simplified ‘Dry Out’ portion as shown in manufacturers have developed more and more Figure 2. corrosion resistant positive grid materials. This inadvertently has had the effect of reducing the amount of partial current available to offset any self- discharge of the negative electrode. Therefore, the Dry Out concentration of impurities that the negative Pb + H2SO4 -> PbSO4 + H 2 electrode can endure has decreased with the (Self-discharge) development of VRLA technology. Indeed, the worst case scenario would involve a corrosion Pb2+ + 2e- = Pb resistant positive grid and an impure system. (Recharge self-discharge) Pos Pb = Pb4+ + 4e- Neg Several researchers have since advocated the deployment of catalysts in the gas space of VRLA cells (3). This action (Grid Corrosion) will undoubtedly polarize and offset the rundown of the + - 2H + 2e = H2 negative electrode, indeed may well offer benefits in (Hydrogen Evolution) abusive situations, but is unnecessary if the VRLA cell/battery is manufactured with due consideration to the aforementioned run-down mechanisms. Figure 2 – VRLA Positive Grid Corrosion Partial Current The purpose of this paper is to demonstrate that even with Flow the highest corrosion resistant grid material, i.e. pure lead or pure lead alloyed with a small amount of tin, the As can be seen from Figure 2, the partial current negative is not the limiting electrode, even after up to 13 responsible for positive grid corrosion offsets the self- years real-time float duty, provided that the VRLA discharge of the negative electrode and, if sufficient, can cell/battery is manufactured to a high purity standard. polarize the negative electrode with subsequent Furthermore, that the use of catalysts is not required for electrochemical generation of hydrogen gas. normal float applications. If the partial current responsible for positive grid Commercially available CyclonÒ and SBS pure lead and corrosion, i.e. the rate of positive grid corrosion, is lead-tin VRLA batteries were used for this float life greater than the partial current required to recharge the verification study. The manufacturers of these products negative self-discharge then the negative electrode will take significant precautions to ensure that the entire maintain its capacity throughout its float life. If, however, system is free of contaminants. The grid lead of both the partial current responsible for positive grid corrosion electrodes, as well as the oxides used in both electrodes, is less than the partial current required to recharge the are manufactured from primary lead from sources known negative self-discharge then the negative electrode will to have low impurity levels. The electrolyte is progressively lose capacity. manufactured from highest reagent grade acid and de- ionized water. The separator material is also of the There are two principal reasons for the self-discharge of highest possible purity. All raw materials and in-process the negative electrode: materials are monitored to ensure that a high system purity is consistently maintained. 1. Oxygen ingress – if external oxygen is allowed into the VRLA system then the negative electrode will II. Experimental readily react with the oxygen resulting in a reduction of its SOC. It is imperative, therefore, that the VRLA All the cells and batteries used in this study were taken manufacturer ensures the batteries are leak free and from real-time float experiments, i.e. floated under that the safety valve operates in a correct manner ambient temperature conditions, and were not taken from throughout the product’s life. accelerated float life tests which can artificially alter the sulphuric acid and were then dried in a vacuum oven. The condition of the negative electrode. Capacity tests on the electrodes were then stored in an inert atmosphere in aged product were conducted immediately following the order to prevent further chemical reaction, in particular float duty without any boost charging, which also can oxidation of the negative active mass. Accurate analysis artificially alter the condition of the negative electrode. In of the sulphate concentration on both electrodes was all cases, a mercury/mercurous sulphate reference conducted employing wet chemistry techniques. X-ray electrode (Hg/Hg2SO4) was inserted into the cell in order diffraction (XRD) was employed to determine the to determine the capacity-limiting electrode during the crystallographic composition of both the positive and discharge. Finally, teardown analysis was carried out to negative active masses (PAM & NAM). Mercury determine the chemical and morphological condition of porosimetry was used to characterize the porosity of the both the positive and negative active masses and also to electrodes and BET surface
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