The temperature of a charging battery is a critical factor in the proper control and the ultimate safety of equipment and personnel in the charging process. If the temperature of the battery increases significantly, the internal resistance of the battery decreases and consequently the charging current increases, causing further decrease of the internal resistance. This cumulative effect soon leads to "thermal runaway", eventually resulting in destruction of the battery and perhaps in catastrophic circumstances to surrounding equipment and personnel.
Prior battery charging systems have controlled charging voltage in response to the temperature of the battery. These systems generally reduce the battery charging voltage as the battery temperature rises in a simple linear ramp or single voltage step to a voltage or temperature limit. Most battery manufacturers recommend that the charging voltage be decreased or "compensated" for increasing temperature at a fixed value of X mV/.degree. C./cell. This is discussed by Y. Nagai and K. Ozaki in Proceedings, Twelfth International Telecommunications Energy Conference, Paper #CH2928-0/90/0000/0155, IEEE (New York 1990) pp. 155-160 (see FIG. 3) in which a change in the charging voltage of -3 mV/.degree. C./cell between 0.degree. and 50.degree. C. is shown. An alternative approach is charging at 2.275 V/cell from 0.degree. to 35.degree. C. and 2.215 V/cell from 35.degree. to 50.degree. C. A. I. Harrison, in Proceedings, Fourteenth International Telecommunications Energy Conference, Paper #0-7803-0779-8/92, IEEE (New York 1992), pp. 28-34 (see FIG. 5) shows a steady decrease in charging voltage from 2.36 to 2.20 V over the temperature range from 0 to 40.degree. C. Bellcore Technical Advisory TA-NWT-001515 Issue 1, December 1993, recommends a voltage decrease of 3 mV/.degree. C./cell to a limit of 2.17 V/cell when the cell or battery temperature is between 10.degree..+-.1.degree. and 15.degree..+-.1.degree. C. above the ambient temperature. The float voltage may alternatively be reduced by 1.5 mV/.degree. C./cell. Major alarms are sent when the battery temperature is at 10.degree..+-.1.degree. C. above the ambient temperature and also when the battery temperature is 15.degree..+-.1.degree. C. above ambient temperature. The voltage may also be reduced in a step function to 2.21 V/cell at a battery temperature 10.degree..+-.1.degree. C. above ambient and to 2.17 V/cell at a battery temperature 15.degree..+-.1.degree. C. above ambient. Since the measured ambient temperature can vary over a wide range, controlling voltage as a function of the difference between the ambient and battery temperatures is not a precise method of controlling the battery charging characteristic. Furthermore, measurement of ambient temperature requires one or more additional sensors and is highly dependent on sensor location, reducing reliability and increasing cost.
These step and linear ramp decreases in voltage with temperature rise protect the battery and environment under normal operating conditions in which the battery temperature does not get much higher than ambient temperature. However, these controlled charging voltage changes do not really address the situation of thermal runaway in which the battery current and temperature can continuously increase to a magnitude where the battery case materials can melt and/or explosive levels of hydrogen gas can be generated. Such a situation can exist if one or more cells in a battery string are at a low voltage level due to a short or a faulty seal while the remainder are being charged at an effectively higher float voltage.