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TIME-BASED TEMPERATURE INFLECTION TERMINATION ALGORITHM FOR NICD/NIMH BATTERY CHARGER

IP.com Disclosure Number: IPCOM000008372D
Original Publication Date: 1997-Dec-01
Included in the Prior Art Database: 2002-Jun-11
Document File: 3 page(s) / 144K

Publishing Venue

Motorola

Related People

Brian Pozsgay: AUTHOR [+3]

Abstract

In battery and battery charger design, the thermistor within the battery pack connects to the microcontroller's analog-to-digital (A/D) port such that a temperature range spanning 90°C can be accurately read. An R-bit AID provides significant resolution (approximately 0.4"Cibit). However, the temperature swing of a battery under rapid charge is typically less than 20°C (50 bits) over 90 minutes.

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MOTOROLA Technical Developments

TIME-BASED TEMPERATURE INFLECTION TERMINATION ALGORITHM FOR NICD/NIMH BATTERY CHARGER

by Brian Pozsgay, Martin Ramsden and Dale Tittensor

PROBLEM

  In battery and battery charger design, the thermistor within the battery pack connects to the microcontroller's analog-to-digital (A/D) port such that a temperature range spanning 90°C can be accurately read. An R-bit AID provides significant resolution (approximately 0.4"Cibit). However, the temperature swing of a battery under rapid charge is typically less than 20°C (50 bits) over 90 minutes.

  Most rapid chargers evaluate the battery temperature periodically. A typical evaluation period is every 3 minutes. A longer evaluation period risks overcharging the battery. A shorter evaluation period risks being unable to identify an inflection when using two consecutive slope calculations. Over three minutes, the change in the battery temperature is usually less than 5 COUPES. When examining the temperature's second-derivative (the change in the change), values of 0 and I counts are tyical. As a result, any value greater than 0 counts is considered a positive inflection! Detecting an inflection requires 6 minutes since 3 temperature values are necessary to calculate a single second- derivative value. This h-minute time period can be too long for identifying a fully charged battery.

SOLUTION

  Since boundaries between the microcontroller's A/D counts are consistent, greater resolution in the temperature change can be achieved by examining the amount of time between temperature changes.

For the purpose of rapid charge termination, this algorithm does not evaluate the rate of temperature change until the temperature changes. The faster the temperature changes, the more often the software algorithm assesses the temperature fluctuations.

SECONDARY PROBLEM AND SOLUTION

  Data collected from a battery under charge shows that small oscillations occur in the tempera- ture curve. These may be the result of the chemical reactions taking place within the cells or the different heat transfer properties of the battery. Regardless of the cause, the oscillations present a problem since they occasionally produce data similar to that produced during an inflection in the rate of temperature change. Looking at 5 pieces of data at a time provides a solution to this problem.

ALGORITHM DESCRIPTION

  Each time a change occurs in the temperature's A/D count, the microcontroller records the time since the previous A/D count change. This docu- ment refers to this operation as "time-stamping" the count change. The microcontroller saves the 5 greatest time-stamps (the top S), which correspond to the slowest temperature changes. If the most recently recorded time-stamp is greater than the least of the top 5, then the algorithm adds the most recently recorded time-stamp to the 5 in place of the least. This filters the oscillations in the time-stamp data, and it ignores negative inflections.

  Mul...