![]() If the cells are losing SoC due to self-discharge, this loss needs to be factored into range calculations. If the battery-powered device is an electric vehicle, the designer will want to build a battery management system (BMS) that very accurately knows the SoC of the battery in order to deliver that maximum range to the driver. If the battery-powered device will be stored for a long time, such as an emergency defibrillator, the designer must be able to predict when the cells will have self-discharged to the point where they will be no longer able to support a life-saving event. It’s important for engineers who design battery-powered devices powered by lithium-ion cells to know the expected self-discharge. Small cells of 100 mAH might exhibit less than 10 ♚ of self-discharge while large cells of 100 AH might exhibit 500 ♚ of self-discharge. Generally, self-discharge is on the order of microamps. The amount of self-discharge depends on many factors: cell materials/chemistry, SoC of the cell, temperature of the cell, and capacity of the cell. ![]() Note that self-discharge is a continuous process, constantly draining the cell and reducing the SoC and OCV. This self-discharge, which is sometimes called leakage, reduced the SoC and ultimately reduced OCV. The loss of stored energy wasn’t caused by an external load (such as a light bulb or motor), but rather by self-discharge, where the energy was lost as a tiny amount of heat to an internal load inside the cell. ![]() If the SoC is lower, that means the amount of stored energy (i.e., stored Amp*Hours) in the cell is lower. Come back to the cell some time later and you will find the cell has a lower OCV, indicating that the cell is at a lower state of charge (SoC). The concept of self-discharge is simple: Take a cell, charge it up, measure its open-circuit voltage (OCV), and let it stand with nothing connected to it. One characteristic of lithium-ion cells is a phenomenon called self-discharge.
0 Comments
Leave a Reply. |