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Circuit for Full Charging of Series Lithium-Ion Cells

Differences among cells would no longer prevent full charging.

An advanced charger has been proposed for a battery that comprises several lithium-ion cells in series. The proposal is directed toward charging the cells in as nearly an optimum manner as possible despite unit-to-unit differences among the nominally identical cells.

The particular aspect of the charging problem that motivated the proposal can be summarized as follows: During bulk charging (charging all the cells in series at the same current), the voltages of individual cells increase at different rates. Once one of the cells reaches full charge, bulk charging must be stopped, leaving other cells less than fully charged.

To make it possible to bring all cells up to full charge once bulk charging has been completed, the proposed charger would include a number of “top-off” chargers — one for each cell. The top-off chargers would all be powered from the same DC source, but their outputs would be DC-isolated from each other and ACcoupled to their respective cells by means of transformers, as described below.

Each top-off charger would include a flyback transformer, an electronic switch, and an output diode. For suppression of undesired electromagnetic emissions, each top-off charger would also include (1) a resistor and capacitor configured to act as a snubber and (2) an inductor and capacitor configured as a filter. The magnetic characteristics of the flyback transformer and the duration of its output pulses determine the energy delivered to the lithium-ion cell.

It would be necessary to equip the cell with a precise voltage monitor to determine when the cell reaches full charge. In response to a full-charge reading by this voltage monitor, the electronic switch would be held in the “off” state. Other cells would continue to be charged similarly by their top-off chargers until their voltage monitors read full charge.

This work was done by William E. Ott and David L. Saunders of Honeywell, Inc. for Johnson Space Center.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)}, to Honeywell, Inc. Inquiries concerning licenses for its commercial development should be addressed to:

Honeywell, Inc.
P.O. Box 52199
Phoenix, AZ 85072-2199

Refer to MSC-23503, volume and number of this NASA Tech Briefs issue, and the page number.