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# DYNAMIC BATTERY GAUGE SAMPLING FREQUENCY FOR POWER CONSERVATION

IP.com Disclosure Number: IPCOM000009474D
Original Publication Date: 1999-Sep-01
Included in the Prior Art Database: 2002-Aug-27
Document File: 3 page(s) / 120K

Motorola

## Related People

Charles Batey: AUTHOR [+3]

## Abstract

The purpose of this publication is to describe an algorithm for dynamically adjusting the sampling frequency of a battery gauge, based upon the remaining energy in the power source.

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Technical MOTOROLA @ Developments

DYNAMIC BATTERY GAUGE SAMPLING FREQUENCY FOR POWER CONSERVATION

by Charles Batey, Francisco Gonzalez and Jack Van Zile

PURPOSE

The purpose of this publication is to describe an algorithm for dynamically adjusting the sampling frequency of a battery gauge, based upon the remaining energy in the power source.

BACKGROUND

The purpose of a battery gauge is to display the remaining energy of a power source, which is indicative of the length of time that the device will remain operational. The granularity of the battery gauge varies. Some gauges display remaining energy on a scale of 0 to 100% (such as the PageWriterTM 2000), while others simply notify the user when the remaining energy is at or near 0% (such as Bravo Plus). In all cases, the power source voltage must be periodically sampled in order to gauge the remaining energy.

One of the most critical parameters for deter- mining the sampling frequency of a battery gauge is the slope of the discharge curve at the end of life of the power source. It is usually at this point that the slope is greatest. It is also the most critical point because the system is about to run out of energy and data could be lost. Figure 1 shows an example of a

3.6V NiMH battery discharge curve. Notice at about 10% the slope begins to drop off quickly. Among other things, an effective gauge must do the following:

1. Sample fast enough to detect the fast drop off at end of life. If it does not, the system will run out of energy without ever alerting the user.

2. Sample slow enough to conserve energy. By sampling too fast the system will drain the power source by trying to gauge itself.

It is up to the designer to balance both require- ments effectively.

DESCRIPTION OF ALGORITHM

A very effective method for balancing the requirements list described above is to dynamically adjust the sampling frequency based upon the remaining energy. This maximizes energy savings while reliably detecting the end of life drop off point.

The algorithm works as follows an...