Investigation of the response of NiMH cells to burp charging

Performance cycling and calorimetric analysis of nickel metal hydride cells have revealed that reverse pulse (“burp”) charging improves performance over other charging techniques. Burp charging periodically applies a short-duration, high-rate discharge pulse followed by a short rest during an otherwise galvanostatic charge. Results show improved charge input and output, lower heat generation during charge, lower charge overvoltage, and no loss of cycle life. Several hypotheses for the responsible phenomena were examined. In-situ video-microscopic analysis of the cell and its nickel electrode during charge revealed the significance of the gas evolution reactions. In-situ gas generation rate measurements made manometrically during charge determined that the burp pulse sequence results in less gas generation than galvanostatic charging. Evidence is presented that the “burp” pulse sequence electrochemically consumes gas bubbles generated during charge at the positive and negative electrodes. Removing bubbles increases the effective electrode surface area, results in a lower charge overvoltage, and reduces cell pressure.