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e-School: EMERGENCY VRLAB- Methods and Techniques for Testing and Research B.Monahov Institute of Electrochemistry and Energy Systems (IEES) Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
ABOUT THE E-SCHOOL This e-School can be regarded as a specialized course from the group of POEMES “cd” training courses, organized for young scientists and Ph. D. students working in the field of Lead-acid batteries, but it can be also useful for accumulation of general knowledge about methods for testing and micro-structural investigations of batteries. The trends in emergency batteries testing, monitoring and diagnostics are marked. The capacity (rated, actual, expected), cycle life (expected for the particular application profile), actual state of charge and state of health (S.O.H.) are regarded as main battery parameters which should be controlled. Two groups of testing methods can be distinguished: invasive (electrolyte density determination and electrode potential measurements) and non-invasive (open circuit and float voltage measurement, battery/cell temperature monitoring, charge/discharge voltage measurement, DC current discharge, full or partial charge-discharge and AC pulses or impedance).
ABOUT THE E-SCHOOL The presented material is outlining the most suitable techniques, which can be used for the development of advanced methods for emergency batteries testing and investigating. The possibilities for bridging between different approaches are shown. Combined or single methods like in-situ XRD and SEM/TEM, PRO, RAM, system integrated EIS devices and reference electrode techniques are shown as promising tools for research and development. For convenience, the references concerning the presented information are given directly on the corresponding page.
Basic Abbreviations (I) AFM – Atomic Force Microscopy AGM – Absorption Glass Mat CC – Current Control CL – Corrosion Layer DMA – Dynamic Mechanical Analysis DSC – Differential Scanning Calorimetry DTA – Differential Thermal Analysis ED – Electron Diffraction EMPA – Electron Microprobe Analysis GC – Gas Control HC – Gas Control NAM – Negative Active Mass ND – Neutron Diffraction PAM – Positive Active Mass PCL – Premature Capacity Loss
Basic Abbreviations (II) PCL 1 effect – a series of phenomena related to increased electric resistance PCL 2 effect – a series of phenomena related to degradation of the positive active material PCL 3 effect – – a series of phenomena related to insufficient negative plate charge in VRLAB PM – Porometry RT – Radioactive Tracing SEM – Scanning Electron Microscopy S.O.C. – State Of Charge (of a battery) S.O.H. –State Of Health (of a battery) SS – Specific Surface STM – Scanning Tunneling Microscopy TEM – Transmission Electron Microscopy TGA – Thermogravimetric Analysis and DTGA UPS – Uninterrupted Power Supply VC – Voltage Control XPS – X-ray Photoelectron Spectroscopy XRD – X-ray Diffraction
INTRODUCTION 1. The development of testing methods for the electric parameters of lead-acid batteries during the first half of the last century, along with electrochemical research techniques resulted in creation of a rich battery knowledge bank. The structure of the active materials, however, was not known. 2. Structure research techniques (XRD, SEM, POR, RATM) used along with sophisticated polarization methods like constant or modulated potential, CLSV, RRDE etc. revealed the microstructure of the positive and negative active masses and of the corrosion layer, their phase composition and crystal structure, as well as the properties of the electrode systems formed on Pb in H2SO4and the mechanisms of action of the alloying additives. 3. Techniques for temperature and gas atmosphere control as well as for special porometry were developed during the last three decades along with pulse charge and impedance spectroscopy testing methods in order to study the processes in VRLAB, to improve their technology and to detect instantly any problems in them, and to make charging and maintenance of VRLAB easier than of the other battery types.
In this presentation various testing methods and techniques used to estimate the capacity, cycle life, S.O.C. and S.O.H. of VRLA batteries in EES are marked. • In addition to them, structure research methods that come to identify the degradation phenomena and to address their causes and influence are discussed . • The aim of the information introduced is: • To make a brief introduction into the principles of these techniques; • To show typical results; • To outline the main contributions to the understanding of lead-acid batteries created using the above techniques.
Aims of testing and research: To answer the consumer’s general QUESTION: When to replace the battery? • 1. Testing: to estimate consumer related battery characteristics: • Capacity (rated, actual, expected); • Cycle life (expected for the particular application profile); • Actual state of charge (S.O.C.); • Actual state of health (S.O.H.). • 2. Research: to solve producer related problems and save money: • to reveal the mechanisms of the technological and charge-discharge processes, and of the degradation phenomena for the particular battery; • to explain the influence of battery the exploitation conditions .
Battery testing methods • Invasive methods for on- and off-line testing. • (electrolyte samples are taken from the cell interior and analysed) • 1.1.Electrolyte density determination - digital hydrometers, • concentration, s.g. and T of the electrolyte (not applicable for • VRLAB) • Fact estimation: S.O.C.. • 1.2. Electrode potential measurements. Reference electrodes- PAM/NAM degradation. Test methods detect battery facts but don’t explain their reasons.
2. Non-invasive methods. 2.1. Open circuit and float voltage measurement – off- and on-line operation.S.O.C./S.O.H. 2.2. Battery / cell temperature monitoring (IR) - on-line. Shorts, S.O.H., TRA. 2.3. Charge / discharge voltage measurement - on-line (requires additional info about battery pre-history) . S.O.C./S.O.H. 2.4. DC current discharge(capacity estimation and conductivity problems detection) - off-line. A load is connected to the battery - I, U, t. Capacity. 2.5. Full or partial charge-discharge for capacity estimation - off-line, (Digatron/Firing circuits, Bitrode, Arbin Instruments,Solartron Analytical,ChenTech Electric Mfg., etc). Battery connected to a PC-controlled stand.Expensive! All characteristics! 2.6. AC pulses or impedance spectroscopy - on-line, (Midtronics Inc., BatteryCorp Inc.). A low amplitude periodic signal is passed through the battery, the response is analyzed.S.O.C./S.O.H, shorts. Test methods detect battery facts but don’t explain their reasons.
Midtronics Inc., U.S.A. - conductance technology Cell capacity vs. conductance • capacity of cells in good condition, • degradation processes, shorts and open circuits, • no load on the battery( no heat or sparks), • powered directly by the battery under test. recognized by IEEE as a standard for testing of LAB with proven correlation to battery capacity. Source: “Field and laboratory studies to assess the state of health of VRLA batteries: Part I – conductance / capacity correlation studies”, D.O.Feder, T. Croda, K. Champlin and M. Hlavac, A paper presented to INTELEC ’92, Washington D.C., Oct. 4-8, 1992, Reprinted October 1992, http://www.midtronics.com/techpapers.html.
3 Battery testing and monitoring AC 4 cell monitoring and control 1 1 2 DC DC 2 vent vent maintenance free LA battery full maintenance LA battery 1.1. DC current 1.2. DC voltage 2.1. Electrolyte s.g. 2.2. Electrolyte T. 2.3. Ref. Electrode. 3. AC u/i – impedance (EIS) 4. Cell monitoring and control 1.1. DC current 1.2. DC voltage 2.1. Electrolyte s.g. 2.2. Electrolyte T. 2.3. Ref. electrode.
2 AC Blue tooth 1 3 DC line valve valve monitoring and control intelligent self controlling cell cooling 1. Optimal current 2. Optimal voltage 3. Optimal temperature 4. Optimal recombination control 4. Precise SOC control 5. Complex SOH diagnosis 6. Remaining C and cycles. VRLA battery future battery 1.1. DC and pulse current 1.2. DC voltage 1.3. AC u/i – impedance (EIS) 2.1. No electrolyte s.g. 2.2. External cell ToC. 2.3. No ref. electrode. 2.4. Remote data /control
Structure Research Techniques (general) The objects for the structure research techniques are either extracted post mortem from real batteries, or artificially prepared using one of the electrochemical polarisation methods – potential, current or voltage control. The controlled parameter in the particular method can be kept constant, swept linearly once or cyclically or controlled in a pulse mode. There are two groups of structure research methods. In the first one the ambient conditions of the sample are controlled, while in the second one special physical or physico-chemical techniques are directly applied to the sample.
Structure Research Techniques (general) Temperature and gas atmosphere control belong to the sample treatment techniques. Most often thermostats are used to keep the temperature constant during a test. Calorimetric equipment allows to measure and control the heat flows absorbed or emitted by the sample, as well as to estimate the thermal parameters of the samples. Temperature monitoring and mapping of cells/batteries provides in-situ information about the electrochemical processes and about defects like shorts, dry-out etc. The differential thermal analyses phase transitions and structure changes occurring in very small amounts of samples on rising temperature can be identified very precisely. The control or detection of the chemical composition, pressure or flow rate of the gas inside the battery or around the sample during polarisation provides important information about the structure of the active materials, as well as about the operation of the so called oxygen recombination cycle in VRLAB.
Structure Research Techniques (general) Pure structure research techniques are the optical (metallographic), SEM and TEM, the recently developed STM and AFM. The diffraction from crystals exposed to X-rays, electrons or neutrons is used to investigate the properties of the crystalline battery compounds. Special techniques are used to study the pore structure, size distribution, volume and surface of the battery active materials and separators which are extremely porous. Radioactive isotopes are used to study the location and rate of the charge-discharge processes inside the battery.
Electrochemical polarization methods One or some of these methods are used along with sample treatment or structure research ones to investigate battery components
Combination of methods used to investigate the components and the gas phase processes in VRLAB • Alloy structure: Opt.Micr., XRD, SEM, EMPA, DTA. • Active mass phase composition and structure: CC/CV, TC, XRD, ND, ED, SEM, EMPA, TEM, DTA, GC, Rad.T, PM/SS, • Corrosion layer phase composition and structure: CC/CV , TC, XRD, ED, SEM, EMPA, TEM, DTA, GC, Rad.T. • Separator structure and porosity: SEM, DTA, PM/SS. • Hydrogen and oxygen evolution: CC/CV, GC, XRD, SEM, TEM. • Closed oxygen recombination cycle: CC/CV , TC, GC, SEM, DTA, PM/SS.