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ADAPTIVE DELTA-T BATTERY CHARGE TERMINATION METHOD

IP.com Disclosure Number: IPCOM000006630D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2002-Jan-17
Document File: 3 page(s) / 140K

Publishing Venue

Motorola

Related People

Scott M. Garrett: AUTHOR

Abstract

Practical rapid charging of nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-Mh) batteries requires a reliable end-of-charge termination method. One such method is known as delta-T. This involves observing some signal indicating the temperature of the battery and ending rapid charge when the battery temperature changes some specified amount within a given time inter- val. Cell manufacturers recommend a delta-T of 1.6"C over 3 minutes to end rapid charge. By this scheme charg- ing ends when the battery temperature increases 1.6"C or more within a 3 minute interval.

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MOTOROLA INC. Technical Developments Volume 16 August 1992

ADAPTIVE DELTA-T BATTERY CHARGE TERMINATION METHOD

by Scott M. Garrett

  Practical rapid charging of nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-Mh) batteries requires a reliable end-of-charge termination method. One such method is known as delta-T. This involves observing some signal indicating the temperature of the battery and ending rapid charge when the battery temperature changes some specified amount within a given time inter- val. Cell manufacturers recommend a delta-T of 1.6"C over 3 minutes to end rapid charge. By this scheme charg- ing ends when the battery temperature increases 1.6"C or more within a 3 minute interval.

  The 1.6"C/3 min. scheme works very well while charging at the "C" rate with the battery and ambient temperature both at 25°C. The battery experiences little overcharge and receives full capacity. However, under less than ideal conditions, a single valued delta-T scheme is not reliable enough. For example, a battery stored in a cold place that has been brought into a room tempera- ture environment to be charged will increase in temper- ature. This warming can exceed the 1.613 criteria, depending on the heat transfer characteristics of the sys- tem, and cause the charger to stop rapid charge too early. Heat transfer of the system can also vary depending on the users; someone may set the charger on top of a television, for instance (the situation is worse for Ni-Mh batteries since, unlike Ni-Cds, they are exothermic dur- ing charge). A warm battery charged in a warm room (near 40°C) will overcharge somewhat with the 1.6/3 scheme, due to the inefficiency of the electrochemical reaction at higher temperatures. Although cells are designed to withstand some overcharge, when this occurs at higher temperatures it significantly decreases battery life.

  A solution to these problems is to change the delta-T criteria with the observed battery temperature while charging. When the battery temperature is low, a higher d&l&T value is used to help avoid early termination. At higher battery temperatures, a lower delta-T value is used to avoid overcharge. This solution takes advantage of the cells ability to withstand a short period of overcharge at cooler temperatures and corrects for charging

inefficiencies at warmer temperatures.

  The "adaptive delta-T" termination method may be implemented in hardware. or software. However, since most chargers use microcontrollers (and A/D convert- ers) to handle the wide variety of charging situations, it is easier to implement in software. Implementing this algorithm involves breaking the operational range ofbat- tery temperature into several subranges and determin- ing, through testing (which should be done anyway), an appropriate delta-T for each specifx subrange. The sot?- ware then uses the delta-T value that corresponds to the current battery temperature during charge as the cri- teria for terminating rapid charge. An...