Tech Briefs

Battery Cell Voltage Sensing and Balancing Using Addressable Transformers

A document discusses the use of saturating transformers in a matrix arrangement to address individual cells in a high-voltage battery. This arrangement is able to monitor and charge individual cells while limiting the complexity of circuitry in the battery. The arrangement has inherent galvanic isolation, low cell leakage currents, and allows a single bad cell in a battery of several hundred cells to be easily spotted.

The system is divided up into the battery cell array, the monitoring array, and the battery voltage sensing and balancing system. The battery cell array is the parallel/series connection of cells that store electrical energy. The monitoring array is the set of diodes, transformer cores, and interconnecting wires that allow the voltages of individual cells to be measured, and for the small amounts of charge to be put in individual cells as desired. In the battery voltage sensing and balancing system, the circuitry connects to the monitoring array and provides the pulses to do sensing and charging. It is separate from any main charge system. Electrical isolation is intrinsic to the design and there is no fusing necessary on sense wires.

Maximum array size is set by wire resistance and parasitic inductances in the sense winding loop that would cause waveform degradation. In practice, an array of several hundred cells could be monitored. Charge current is limited to a few hundred milliamperes, suitable for balancing cells after a main charge system is complete. Measurement accuracy is limited by leakage inductance in the cell/diode/core loop as well as in the sense winding loop. The length of the cable between the battery and the control electronics is limited by series resistance in the cable as well as inductance that degrades the shape of the measurement pulses.

This work was done by Francis Davies of Johnson Space Center.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-1003. Refer to MSC-24466-1.

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