THE battery is a critical component in standby applications

Transcription

1 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 6, JUNE Self-Equalization of Cell Voltages to Prolong the Life of VRLA Batteries in Standby Applications William Gerard Hurley, Fellow, IEEE, Yuk Sum Wong, Member, IEEE, and Werner Hugo Wölfle Abstract The valve-regulated lead-acid battery has been the work horse of standby applications for several decades. Float charging is normally implemented in these systems. However, float charging tends to overcharge the battery, causing water loss and grid corrosion which shorten the service life of the battery. This limitation may be avoided by using cell voltage equalization and temperature-compensated interrupted charge control (TCICC). Cell voltage equalization reduces the voltage distribution range over many cells, which, in turn, means that there are fewer cells with either overvoltage or undervoltage, both of which shorten the life of the battery. TCICC can increase the service life of the battery by avoiding overvoltage. Experimental evidence is presented to validate the new approach by comparing float charging and TCICC in terms of battery voltage equalization and temperature response. Index Terms Batteries, charge equalization, emergency power supplies, float charging, temperature compensation. I. INTRODUCTION THE battery is a critical component in standby applications from telecom systems to emergency lighting to wind farms. Offshore and remote wind farms require standby batteries for blade pitch control in the event of power loss in extreme high wind conditions. Increased service life in the batteries improves reliability and reduces maintenance costs. Typically, a standby system has valve-regulated leadacid (VRLA) batteries with float charging. Overcharging is a salient feature of float charging that causes water loss and grid corrosion in the battery, and this shortens the service life of the battery. Temperature-compensated interrupted charge control (TCICC) [1] [4] prolongs the service life of the battery by avoiding overcharging [5] [7]. The Arrhenius equation predicts that reaction rate in the electrolyte doubles for every 10 Cincrease in temperature. The raised temperature increases the rates of positive grid corrosion and water loss. The standby volt- Manuscript received October 1, 2008; revised December 18, First published March 16, 2009; current version published June 3, This work was supported in part by Enterprise Ireland and in part by Convertec Ltd. through the Innovation Partnership Programme under Project IP W. G. Hurley is with the Power Electronics Research Centre, Department of Electronic Engineering, National University of Ireland, Galway, Ireland ( ger.hurley@nuigalway.ie). Y. S. Wong was with the Power Electronics Research Centre, National University of Ireland, Galway, Ireland. He is now with the Energy Studies Institute, National University of Singapore, Singapore ( yswong@nus.edu.sg). W. H. Wölfle is with Convertec Ltd., Wexford, Ireland ( wwolfle@ convertec.ie). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TIE age is typically V in emergency lighting and telecoms applications; in wind farms, the voltage is typically 288 V, which means that there are more cells in series, and this makes a compelling case for equalization of cell voltages. Float charging tends to overcharge the battery at the end of the charging process. The capacitances of the cells are not matched, and therefore, the cell voltages are not distributed equally, which means that some cells are overcharged while others are undercharged. Overvoltage in a cell causes hydrogen evolution and water loss at the negative electrode of the cell; grid corrosion occurs at the positive electrode [8], [9]. Undercharging causes sulphation [10], which reduces the active area of the plates and can even cause plate buckling. All of these effects shorten the service life of the battery [11] [14]. Equalization circuits exist to deal with this situation [15], [16], but of course, more circuitry means reduced reliability. The higher voltages encountered in wind farms give a proportionally greater exposure to the detrimental effects of a lack of equalization and lower reliability when equalization circuits are employed. The TCICC regime avoids overvoltages and gives an improved distribution of cell voltages. Experimental results are provided for both float charging and TCICC for comparison purposes. Field tests on the batteries used in a North Sea wind farm are provided to validate the improved performance of the TCICC regime. II. SELF-EQUALIZING ICC The interrupted charge control (ICC) regime is described in detail in [1], and the temperature compensation is described in [3]. The ICC regime and temperature compensation are summarized here for completeness. Fig. 1 shows the four operating modes of the ICC regime. In Mode 1, the battery is charged with a constant charging current of 0.1C rated, where C rated is the rated battery capacity in ampere hour. At the end of Mode 1, the state of charge (SoC) of the battery is typically over 85% [17], [18]. Mode 2 is triggered when the battery voltage reaches the upper threshold voltage (V ut ). The battery is in open circuit, and the voltage falls, leading to a reduction in internal resistance. Mode 3 is triggered when the open-circuit voltage drops below the lower threshold voltage (V lt ). There are two states in Mode 3, namely, pulse current charging state and the rest state. The battery is pulse charged with a peak current of 0.05C rated, in the pulse current changing state and left in open circuit in the rest state. The period of the states is 30 s, and the duty cycle of the pulse current charging state is 33.3% at 25 C. Mode 4 is triggered when the battery voltage again reaches /$ IEEE

2 2116 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 6, JUNE 2009 Fig. 1. Operating modes of the ICC regime. Fig. 2. Voltage responses of a VRLA battery to a current pulse. V ut. The battery is expected to be fully charged at the start of Mode 4. The battery is in open circuit and in self-discharge in Mode 4. Mode 1 restarts the cycle when the voltage drops to V r, when the SoC is 97%. There are many secondary reactions taking place in the battery. For example, oxygen evolution at the positive terminal is reduced at the negative terminal, thus establishing an internal oxygen cycle. The acceleration factor (F a ) due to a rise in temperature is given by the Arrhenius equation F a =2 10(V 2.27) 2 (T 25)/10 (1) where V is the cell voltage (in volts) and T is the temperature (in degree Celsius). The temperature aging factor (F ta ) is and the effective life of the battery (L e ) is F ta = F 1 a (2) L e = L p F ta (3) where L p is the projected service life of the battery. This shows that for every 10 C rise in battery ambient temperature, the expected life is reduced by 50%. Conversely, properly chosen temperature compensation can greatly improve service life. Temperature compensation in float charging of VRLA batteries normally involves adjusting the float voltage [19] [23]. The float voltage would decrease linearly from a maximum value at the minimum operating temperature to a minimum value at the maximum expected operating temperature. Self-discharge is mitigated at low temperature, and thermal runaway is prevented at high temperature. Typical values might be 2.37 V at 10 C and 2.15 V at 50 C. This simple approach will not suffice for the ICC regime. The ICC regime must maintain the battery in a high SoC state in Mode 4. Mode 4 is triggered when the battery voltage reaches V ut. Fig. 2 shows the voltage response of a 12-V 16-Ah VRLA battery to a 0.05C rated current pulse at different temperatures, when the battery is at 99.5% SoC. There is a fivefold increase in battery voltage when the temperature drops from 45 Cto15 C. Evidently, the trigger for Mode 4 is strongly influenced by temperature, i.e., the battery voltage will reach Fig. 3. Fig. 4. Temperature compensation algorithm for ICC regime. Laboratory setup of the battery test system. V ut too early when the temperature is low, resulting in an undercharged battery. We can compensate for this by reducing the duty cycle of the pulse current charging state in Mode 3

3 HURLEY et al.: SELF-EQUALIZATION OF CELL VOLTAGES TO PROLONG THE LIFE OF VRLA BATTERIES 2117 Fig. 5. Block diagram of a TCICC regime battery charger. at low temperature. To prevent overcharging, we need to reduce the threshold voltage at high temperature. The overall temperature compensation scheme in the TCICC regime is shown in Fig. 3 where T rated is the rated temperature of the battery. The duty cycle and threshold voltage are controlled at temperature T as follows: { Dmin +(0.333 D D(T )= min )(T 5)/20, T T rated (4) D rated, T>T rated { V rated V ut (T )= ut, T T rated Vut rated (5) N cell (V tcpc )(T T rated ), T >T rated where T is the ambient temperature of the battery, D rated is the rated duty cycle (0.33), and N cell is the number of cells. V tcpc is the temperature compensation factor in volts per cell (0.004 V). The minimum duty cycle D min is determined by the minimum average charging current in Mode 3 (typically 0.167). Above rated temperature, the duty cycle is constant to shorten the charge time and to prevent thermal runaway. A number of experiments were carried out in the laboratory and in the field to establish the self-equalizing effect of the TCICC regime, and these will be described in the next section. III. EXPERIMENTAL VALIDATION AND FIELD TESTING The TCICC regime was compared to the float-charging regime in the laboratory prior to field testing. The laboratory setup is shown in Fig. 4 [3], the main components being a data acquisition system, electronic power supplies and loads, and a temperature-controlled oven, all controlled in a Lab- VIEW environment to facilitate data acquisition. Fig. 5 shows a block diagram of the implementation of the TCICC charger [3]. It essentially consists of a flyback dc dc converter with current-mode control. The microcontroller is programmed to implement the TCICC regime; a PWM regulator controls the duty cycle of the MOSFET switch in the converter through an optocoupler. Fig. 6. Battery compartment in the wind turbine. Tests were carried out at different temperatures to establish the improved temperature compensation provided by the TCICC regime. Tests were carried out on equalization voltages to show that the TCICC regimes provided a more equal distribution of cell voltages. Fig. 6 shows the battery compartment in the wind turbine and the battery that was tested. The hub of the 5-MW wind turbine that contains the battery pack is shown in Fig. 7. The temperature tests were carried out on a 16-Ah 12-V battery, model no. G12V16CP, at 15 C, 20 C, 25 C, 30 C, 35 C, 40 C, and 45 C. The test parameters for the TCICC regime and the float-charge regime are shown in Table I. The battery was charged for 7 h and 45 min followed by discharge at 0.5C rated for 15 min. The charge performance at 15 Cis shown in Fig. 8, and the charge performance at 35 Cisshown in Fig. 9. For the float-charge regime, the float-charge voltage is 0.48 V lower at 35 C. On the other hand, the corresponding voltage difference between 15 C and 35 C is halved in the TCICC regime. In the case of the float-charge regime, the final voltage is much higher in the upper temperature range, which

4 2118 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 6, JUNE 2009 Fig. 7. Wind turbine of 5 MW. Fig. 9. Charge performance at 35 C. TABLE I CHARGE PARAMETERS OF TCICC AND FLOAT-CHARGE REGIMES Fig. 10. Voltage distribution for battery modules for full charging cycle. Fig. 8. Charge performance at 15 C. gives rise to the increased reaction rates of the overvoltage effects. The influence of overvoltage on service life has been described in Section I. The consistency of the voltages and its proximity to the ideal voltage in the TCICC regime mean that there will be very little effects arising from elevated temperature. The effects of equalization were tested on a field battery rated at 288 V. The battery was tested for five cycles with TCICC charging followed by five cycles with float charging and then five cycles with TCICC charging. The distribution of voltage was checked over V battery modules at the end of the charging. The batteries were initially discharged at 50 A, typical of load conditions in the field, followed by charging. The tests were carried out at 25 C. Fig. 10 shows the voltage distribution on the 12-V battery modules for float charging; the voltage range is V ± V at 25 C. The corresponding range for the TCICC regime is 12.8 V ± V. Clearly, the smaller range means that there is more equalization and a proportionate improvement in the service life of the battery. The improvement in the voltage distribution of the TCICC regime is attributed to the higher voltages at the end of Mode 3 (typically, V ut =14.7 V per battery module compared to the charging voltage of V for float charging). When the battery cells are in series, the same amount of electrical charge passes though the cells. When the charge passes though the cells, the fully charged cells undergo secondary reactions only. They do not undergo primary reactions to absorb the energy. On the other hand, the undercharged cells undergo both secondary reactions and primary reactions, which absorb the charging energy such that the SoC and battery voltages can increase. The pulse current charging state in Mode 3 uses a large current pulse to force the cells to undergo primary reactions to equalize the cell voltages. The rest states in Mode 3 and Mode 2 release the overpotential that built up during Mode 1 and Mode 3. Fig. 11 shows the initial discharge at 50 A. The average voltages after the initial charge are 330 V (13.75 V 24)

5 HURLEY et al.: SELF-EQUALIZATION OF CELL VOLTAGES TO PROLONG THE LIFE OF VRLA BATTERIES 2119 Fig. 11. Voltage distribution in battery modules for initial charging cycle. for float charging and 307 V (12.8 V 24) for TCICC. This means that the grid corrosion rate is higher for float charging and therefore the state of health of the battery will decrease faster. IV. CONCLUSION This paper has described a self-equalizing charging regime that extends the service life of VRLA batteries in standby applications. It has been shown that proper temperature control can improve the service life of the battery by reducing the reaction rates that are detrimental to the operating life of the battery. The TCICC regime has been described and compared to the float-charge regime. The TCICC regime equalizes battery cell voltages. Field tests revealed that TCICC resulted in a much improved voltage distribution in the battery cells when compared with float-charge regime. The improved voltage distribution means that there is less overvoltage and undervoltage in individual cells, and this, in turn, mitigates the aging effects of grid corrosion and sulphation. ACKNOWLEDGMENT The authors would like to thank the engineers at Convertec Ltd., particularly E. Hinterleitner and T. Whelan. REFERENCES [1] M. Bhatt, W. G. Hurley, and W. H. Wolfle, A new approach to intermittent charging of valve-regulated lead-acid batteries in standby applications, IEEE Trans. Ind. Electron., vol. 52, no. 5, pp , Oct [2] D. P. Reid and I. Glasa, A new concept: Intermittent charging of leadacid batteries in telecommunication systems, in Proc. IEEE INTELEC, 1984, pp [3] Y. S. Wong, W. G. Hurley, and H. Wölfle, Temperature compensation algorithm for interrupted charge control regime for a VRLA battery in standby applications, in Proc. 23rd IEEE APEC, Austin, TX, 2008, pp [4] L.-R. 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Electron., vol. 55, no. 6, pp , Jun [16] IEEE guide for the protection of stationary battery systems, IEEE Standard, [17] M. Coleman, C. K. Lee, C. Zhu, and W. G. Hurley, State-of-charge determination from EMF voltage estimation: Using impedance, terminal voltage, and current for lead-acid and lithium-ion batteries, IEEE Trans. Ind. Electron., vol. 54, no. 5, pp , Oct [18] A. H. Anbuky and P. E. Pascoe, VRLA battery state-of-charge estimation in telecommunication power systems, IEEE Trans. Ind. Electron., vol. 47, no. 3, pp , Jun [19] D. Berndt, R. Brautigam, and U. Teutsch, Temperature compensation of float voltage-the special situation of VRLA batteries, in Proc. IEEE INTELEC, 1995, pp [20] Y. S. Wong, W. G. Hurley, and W. H. Wölfle, Charge regimes for valveregulated lead-acid batteries: Performance overview inclusive of temperature compensation, J. Power Sources, vol. 183, no. 2, pp , Mar [21] Y.-S. Lee and M.-W. Cheng, Intelligent control battery equalization for series connected lithium-ion battery strings, IEEE Trans. Ind. Electron., vol. 52, no. 5, pp , Oct [22] J. Chatzakis, K. Kalaitzakis, N. C. Voulgaris, and S. N. Manias, Designing a new generalized battery management system, IEEE Trans. Ind. Electron., vol. 50, no. 5, pp , Oct [23] J. A. Mills, Results with advanced, in situ monitoring of electric-vehicle and stationary batteries, J. Power Sources, vol. 78, no. 1/2, pp , Mar William Gerard Hurley (M 77 SM 90 F 07) was born in Cork, Ireland. He received the B.E. degree (with first-class honors) in electrical engineering from the National University of Ireland, Cork, in 1974, the M.S. degree in electrical engineering from the Massachusetts Institute of Technology, Cambridge, in 1976, and the Ph.D. degree from the National University of Ireland, Galway, Ireland, in He was with Honeywell Controls in Canada as a Product Engineer from 1977 to He was a Development Engineer in transmission lines with Ontario Hydro from 1979 to He lectured in electronic engineering at the University of Limerick, Limerick, Ireland, from 1983 to He is currently a Professor of electrical engineering with the National University of Ireland, Galway, where he is the Director of the Power Electronics Research Centre. He was a Visiting Professor of electrical engineering with the Massachusetts Institute of Technology, Cambridge, from 1997 to His research interests include high-frequency magnetics, power quality, and renewable energy systems. Prof. Hurley is a Fellow of the Institution of Engineers of Ireland and a member of Sigma Xi. He has served as a member of the Administrative Committee of the IEEE Power Electronics Society and was General Chair of the IEEE Power Electronics Specialists Conference in He received a Best Paper Prize from the IEEE TRANSACTIONS ON POWER ELECTRONICS in 2000.

6 2120 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 6, JUNE 2009 Yuk Sum Wong (M 04) received the B.Eng., M.Phil., and Ph.D. degrees in electrical and electronic engineering from the University of Hong Kong, Hong Kong, in 1997, 2000, and 2008, respectively. He was a Postdoctoral Research Fellow with the Power Electronics Research Centre, National University of Ireland, Galway, Ireland, from 2007 to He is currently a Research Fellow with the Energy Studies Institute, National University of Singapore, Singapore. His research interests include system optimizations of hybrid and plug-in hybrid electric vehicles and optimizations of charge regimes for batteries in cyclic and standby applications. Werner Hugo Wölfle was born in Bad Schussenried, Germany. He received the Diplom-Ingenieur degree in electronics from the University of Stuttgart, Stuttgart, Germany, in 1981, and the Ph.D. degree in electrical engineering from the National University of Ireland, Galway, Ireland, in From 1982 to 1985, he was a Development Engineer for power converters in space craft applications with Dornier Systems GmbH, where he was a Research and Development Manager for industrial ac and dc power from 1986 to Since 1989, he has been a Managing Director of Convertec Ltd., Wexford, Ireland, a company that develops high-reliability power converters for industrial applications. He is an Adjunct Professor of electrical engineering at the National University of Ireland, Galway.