1. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES Temperature Control (in situ) The temperature of VRLAB changes during operation. Heat is evolved due to the presence of a small but greater than zero ohmic resistance of the cell. In addition, heat can be generated or consumed by the electrochemical and chemical reactions taking place during discharge and charge.

2. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES VRLA batteries evolve intensively HEAT during charge. In addition to the charge processes in flooded lead-acid batteries, in VR extra heat is evolved due to the oxygen recombination reaction proceeding on the negative plate. This exothermic process is an intensive heat source: 68.32 kcal/mol. The negative plate is a good thermal conductor and it dissipates actively heat. The electrolyte, which is a water solution, has a large heat capacity. When the temperature of the positive plate increases, the rate of oxygen evolution increases rapidly and a bigger portion of oxygen recombines at the negative plate, giving rise of a further temperature rise there. The temperature can easily exceed 100oC, the electrolyte starts boiling over 107oC, the polymer container becomes plastic and is destroyed – thermal runaway effect. The cell temperature can be monitored during cycling and the temperature distribution across the plate surface can be monitored by IR thermometers. When a VRLAB operates at elevated temperatures, the following processes are accelerated: gassing rate/water losses, capacity, corrosion rate, PAM degradation. The capacity increases. In the same time the thermal runaway danger increases, and cycle life and recombination efficiency decrease.

3. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES VRLA batteries evolve HEAT during operation (charge / discharge) : Discharge: Q d = Q d ECR + Q d J Charge: Q ch = Q ch ECR + Q ch H2 / O2 evol + Q ch COC + Q ch J Oxygen recombination: main contribution (exothermic – 68.32 kcal/mol). negative plates: good thermal conductors electrolyte: large heat capacity (water solution). cell temperature at the end of charge: > 80-90 o C. temperature distribution along plate surface: a set of thermosensors radiation thermal detectors When a VRLAB operates at elevated temperatures: increases: gassing rate / water losses, capacity, corrosion rate, PAM degradation, thermal runaway danger decreases: cycle life, oxygen recombination efficiency Temperature changes in VRLAB under operation Source: Energy balance of the closed oxygen cycle and processes causing thermal runaway in valve-regulated lead-acid batteries, D. Pavlov, Journal of Power sources 64 (1997) 131.

4. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES Calorimetric studies of VRLAB Heat rate / mW Source: A. Pesaran and M. Keyser, Thermal Characteristics of Selected EV and HEV Batteries, 16-th annual battery conference, Long Beach, California, USA, January 9-12, 2001. heat capacity: C p = Q / m (T o – T c ) heat rate for C/1 discharge heat rate: heat generated (HG) / cycle time Energy efficiency: 1 – HG / EEI (EEO) el. energy input el. energy heat ChD ToCToC C, Ah Eff, % HR, W 08.596.37.3 2510.496.17.7 4513.198.82.4 Q VRLA (16.5 Ah) – 660 J/kg. o C NiMH (20 Ah) – 677 J/kg. o C Li ion 6 Ah – 795 J/kg. o C NiMH (6.5 Ah) – 521 J/kg. o C Li polymer 4 Ah – 1012 J/kg. o C thermoelectric sensors battery T o, m, T i bath fluid heat sink heat

5. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES Source: P. Haering, H. Giess, J. Power Sources, 95 (2001) 153. Temperature monitoring of UPS VRLAB during operation Cell temperature evolution during cycling: A – rest B – discharge C – CC charge D – CV charge Maximum cell temperature during CC charge as a function of current I 10 3 I 10 + 7 I 10 U=const

6. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES 116 A discharge to 1.60 VPC. The central area of the plate is warmer than the edge parts. The current lug is warm. Source: P. Haering, H. Giess, J. Power Sources, 95 (2001) 153. Temperature mapping of UPS VRLAB Thermal imaging: ABS essentially transparent to the analysed infrared radiation.

7. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES Temperature changes during constant current and constant voltage charge. CC charge – small heat evolution Source: P. Haering, H. Giess, J. Power Sources, 95 (2001) 153. CV charge – high heat evolution Temperature mapping of UPS VRLAB

8. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES Cycle # 38 Cycle # 350 Influence of cycling on VRLAB temperature Source: http://www.ctts.nrel.gov, M. Keyser, A. Pesaran, M. Mihalic, B.Nelson, Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles, Presented at the 17th Electric Vehicle Symposium, Montreal. Canada, October 16-18, 2000. Cell T as a function of cycle number: end of discharge, end of charge

9. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES Local shorts cause local temperature rise Source: http://www.ctts.nrel.gov, Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles, M. Keyser, A. Pesaran, M. Mihalic, B.Nelson. Presented at the 17th Electric Vehicle Symposium, Montreal. Canada, October 16-18, 2000.

10. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES The enhanced heat evolution and the operation of a closed oxygen recombination cycle in VRLAB are the reason for developing a new complex monitoring method for these batteries. The current and voltage are monitored along with cell temperature and gassing rate. The next page presents 3 graphs: The first one shows the current and temperature changes with time. The temperature increases at the end of the charge, when oxygen evolution and recombination become intensive. In the second graph the gassing rate is plotted as a function of time along with the current. This is possible using a gassing rate monitoring system. It comprises two glass crucibles filled with water and a photoelectric counter. The gas leaving the cell forms bubbles in the crucibles. The bubbles are counted by the photo electric detector and the passing gas volume is calculated. The chemical composition of the gas can also be monitored. In the third graph the potentials of the positive and negative plates are shown as a function of time. The voltage is constant, but the electrode potentials vary due to the electrochemical reactions taking place there. Both potentials decrease with the temperature rise. The negative plate is additively depolarised by oxygen recombination. VRLAB Complex Monitoring Source: D. Pavlov, B. Monahov and A. Kirchev, to be published

11. Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN ACADEMY OF SCIENCES Polarization parameters (voltage and current) and temperature VRLAB complex monitoring gassing rate, volume and chemical composition of the evolved gas Potential of the positive and negative plates Source: D. Pavlov, B. Monahov and A. Kirchev, to be published Light source s Photo detector to PC Photo detector P cell P atm H2OH2O gas bubble Gas from the cell