An improved charge-termination technique has been developed to obtain a more accurate balance between charge and discharge of rechargeable lithium-ion-based electrochemical cells and batteries. The technique has been demonstrated experimentally and is now in use in a laboratory battery charger. The technique could readily be implemented in the electronic control circuits of battery chargers for consumer electronic equipment.
The conventional charging technique, recommended by manufacturers, involves (1) charging a cell in a current-limited, constant-supply-voltage mode until the cell potential reaches 4.100 V, then (2) allowing the charging current to taper off for about an hour before terminating the charging process. Measurements have shown that the conventional arbitrary time-based cutoff does not guarantee balance between the amounts of electric charge withdrawn and restored during discharge and recharge, respectively.
In the improved technique as in the conventional technique, one charges a cell in a constant-supply-voltage, current-limited mode. However, instead of terminating the charging process after an arbitrary amount of time, one terminates it when the charging current has decreased to C/100 amperes, where C denotes the nominal charge capacity of the cell in ampere•hours. [If the amount of time available for charging is not sufficient for tapering down to C/100, then an alternative minimum current (e.g., C/50) can be used.] In experiments at operating temperatures from 10 to +40 °C, the C/100 cutoff criterion has been demonstrated to return between 0.99 and 1.01 times the amount of charge removed during the previous discharge.
For safety, it would be advisable to augment a C/100 charge-termination trigger with a secondary trigger that terminates charging when the charge/discharge ratio reaches 1.01. This secondary trigger would help to prevent further damage in the event that a short circuit develops in the cell, preventing tapering of the charging current.
This work was done by David Perrone of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
NPO-20358
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Improved Charge-Termination Technique for Lithium-Ion Cells
(reference NPO-20358) is currently available for download from the TSP library.
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Overview
The document presents an improved charge-termination technique for lithium-ion cells and batteries, developed by David E. Perrone at NASA's Jet Propulsion Laboratory. This technique addresses the limitations of conventional charging methods, which typically rely on an arbitrary time-based cutoff after reaching a specific voltage (4.100 V) and then allowing the current to taper off for about an hour. Such methods do not guarantee a precise balance between the charge withdrawn during discharge and the charge restored during recharge.
The new approach involves terminating the charging process when the charging current decreases to C/100 amperes, where C represents the nominal charge capacity of the cell in ampere-hours. This method has been shown to reliably return between 0.99 and 1.01 times the amount of charge removed during the previous discharge, demonstrating a significant improvement in charge/discharge balance. The technique has been validated across a temperature range from -10 to 40 degrees Celsius, making it versatile for various operating conditions.
For safety, the document recommends augmenting the C/100 cutoff with a secondary trigger that terminates charging when the charge/discharge ampere-hour ratio reaches 1.01. This precaution helps prevent overcharging in the event of an internal cell short circuit, ensuring the safety and longevity of the battery.
The improved technique is not only more accurate but also practical, as it can be integrated into the electronic control circuits of battery chargers for consumer electronics. This advancement could lead to better performance and reliability in devices that utilize lithium-ion batteries, which are widely used in smartphones, laptops, and electric vehicles.
Overall, the document highlights a significant innovation in battery charging technology, emphasizing the importance of precise charge termination methods to enhance battery performance and safety. The work is part of ongoing efforts to improve energy storage solutions, which are critical for the advancement of modern technology and sustainable energy systems.
