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that the conventional arbitrary time-based cutoff does not guarantee balance between the amounts of electric charge withdrawn and restored during discharge and recharge, respectively.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.