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APPARATUS FOR CHARGING A LITHIUM ION BATTERY IN A NICKEL SYSTEM CHARGER

IP.com Disclosure Number: IPCOM000007780D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-23
Document File: 3 page(s) / 212K

Publishing Venue

Motorola

Related People

Scott M. Garrett: AUTHOR

Abstract

The problem of how to allow charging of a lith- ium ion based battery pack in a nickel cadmium/ metal hydride charger is currently broached by existing MicroTAC lithium ion batteries. These bat- teries contain circuitry comprising voltage sensing circuits and switches coupled in series with the lith- ium ion cell(s). In addition, the circuitry comprises a means for simulating a high temperature upon the cell voltage reaching some predetermined over- voltage threshold (typically 4.2V/cell). This fools the nickel system charger into terminating rapid charge, and keeps the cell(s) from experiencing an over- voltage condition. If the charger does not shut off within some time frame, such as, for example, two seconds, then the circuit opens a switch to electri- cally disconnect the battery from the charger.

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Technical Developments

APPARATUS FOR CHARGING A LITHIUM ION BATTERY IN A NICKEL SYSTEM CHARGER

by Scott M. Garrett

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  The problem of how to allow charging of a lith- ium ion based battery pack in a nickel cadmium/ metal hydride charger is currently broached by existing MicroTAC lithium ion batteries. These bat- teries contain circuitry comprising voltage sensing circuits and switches coupled in series with the lith- ium ion cell(s). In addition, the circuitry comprises a means for simulating a high temperature upon the cell voltage reaching some predetermined over- voltage threshold (typically 4.2V/cell). This fools the nickel system charger into terminating rapid charge, and keeps the cell(s) from experiencing an over- voltage condition. If the charger does not shut off within some time frame, such as, for example, two seconds, then the circuit opens a switch to electri- cally disconnect the battery from the charger.

Currently this approach is working with excel-

lent results. Patent applications have been filed with the USPTO on the specitic Motorola developed meth- ods and circuitry. However, this is not the only way, or necessarily the best way of solving the problem. This article describes an alternative means and method that can be applied to solving the same problem.

  In general, this problem as referred to in the art as "charging a battery with an incompatible charger? In this particular case a charger designed only to charge nickel system batteries is used to charge a lithium ion battery. Nickel cadmium batteries are typically rapid charged by applying a constant cur- rent of sufficient level to charge the battery in the desired time. During charging several battery param- eters are monitored and charging is terminated upon at least one of those parameters reaching a prede-

0 Motorola. Inc. ,996 95 August 1996

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termined magnitude. Typically chargers terminate rapid charging when the rate of battery temperature rise exceeds 1.6"C in 3 minutes. After this event is detected, the charger reduces the charging current to a small fraction of the original current to "top off' and maintain the battery fully charged. This is referred to as trickle charge.

  Similarly, lithium ion batteries also begin charg- ing with a constant current. However, unlike nickel systems, once the lithium ion cell(s) reach a voltage limit of, for example, 4.2 volts, the charge regime changes to a constant voltage mode. Because ofthis change from current to voltage regulation, the lith- ium ion charging regime is referred to as constant current/constant voltage. Upon entering the constant voltage portion of the charge cycle, the lithium ion cell(s) are 60%-80% charged. It is critical to main- tain the cells at the voltage limit to ensure contin- ued energy storage. The current applied to the cell(s) quickly drops durin...