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Dynamic Rechargeable Battery End-Of-Life Prediction

IP.com Disclosure Number: IPCOM000061061D
Original Publication Date: 1986-Jun-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Balliet, L: AUTHOR [+3]

Abstract

This article describes a diagnostic technique and apparatus for predicting present charge status and end-of-life for nickel cadmium or similar rechargeable batteries. Rechargeable batteries can be used in portable computers as well as many other processor- and microprocessor-based equipment. In general, however, the life of batteries, including rechargeable batteries, has been considerably less than other electronic components. In computers especially, it is highly desirable to maximize this life and predict in advance when replacement is advisable. With dependable end-of-life prediction, important data can be retained in keep-alive memories, etc., that would not otherwise be practical. Known rechargeable battery diagnostic methods and apparatus have dealt with lead acid batteries, typically as applied to use in automobiles.

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Dynamic Rechargeable Battery End-Of-Life Prediction

This article describes a diagnostic technique and apparatus for predicting present charge status and end-of-life for nickel cadmium or similar rechargeable batteries. Rechargeable batteries can be used in portable computers as well as many other processor- and microprocessor-based equipment. In general, however, the life of batteries, including rechargeable batteries, has been considerably less than other electronic components. In computers especially, it is highly desirable to maximize this life and predict in advance when replacement is advisable. With dependable end-of-life prediction, important data can be retained in keep-alive memories, etc., that would not otherwise be practical. Known rechargeable battery diagnostic methods and apparatus have dealt with lead acid batteries, typically as applied to use in automobiles. Nickel cadmium batteries (and other battery types generally considered as nonchargeable) are more attractive for electronic equipments. These technologies have different characteristics and consequently require different diagnostic methods, equipment and algorithms. A key parameter for a battery cell is its capacity, generally stated in ampere hours (or milliampere hours). This parameter is a variable which changes depending on discharge rate, charge rate, temperature, age and number of charge/discharge cycles. Cell voltage also varies as a function of temperature, discharge rate and discharge status (percent of discharge). Monitoring voltage alone will not permit projecting the operational status or life expectancy of the battery. It is known that a battery can fail in catastrophic ways but that the common wear-out symptom is loss of capacity. The technique described herein is an adaptive method that calculates battery capacity to determine a figure of merit. It takes into account initial conditions, temperature, discharge rate, charge rate, number of charge cycles, and time in service. In a computer application, the output would be translated into life expectancy, charge status, and number of available deep charges or full charges. Battery capacity is determined in two phases, corresponding to both charge and discharge. A full charge/discharge cycle is employed to enhance battery life and eliminate variation in capacity due to charge/discharge memory. Voltage, current and temperature are monitored. The charge characteristics are shown in Fig. 1. Charge current is selected to be optimum for the battery type and temperature and occurs at a constant rate. Time to reach full charge results in the first indicator of capacity. The amount of current to charge will normally be greater than that available on discharge. Full charge is indicated by the inversion of the charge slope (point A) in Fig. 1. Fig. 2 illustra...