It is critical for Li-Ion battery pack manufacturers to build safe and reliable products for battery powered systems. Battery management electronics in battery packs monitor Li-Ion battery operating conditions, including battery impedance, temperature, cell voltages, charge and discharge current, and state of charge to provide detailed remaining runtime and battery health information to the system to ensure the right system decisions can be made. Additionally, to enhance battery safety, whenever at least one of the fault conditions, such as over current, short circuit, cell and pack over-voltage, over temperature occurs, the battery cells are disconnected from the system by turning off two back-to-back protection MOSFETs that are in series with the Li-Ion cells in the battery pack. Impedance Track technology-based battery management unit (BMU) monitors battery cell impedance over the entire battery life cycle as well as cell voltage imbalance, potentially capable of detecting the cell micro-short and preventing the cell from fire hazard or even explosion.
Li-Ion battery safety
Excessive high level operating temperatures accelerate cell degradation and causes thermal run-away and explosion in Li-Ion batteries. This is a specific concern with this type of battery because of its highly aggressive active material. Rapid temperature increase can occur if a battery is overcharged at high current or shorted. During overcharge of a Li-Ion battery, active metallic lithium is deposited on anode. This material dramatically increases the danger of explosion, because it can explosively react with a variety of materials including electrolyte and cathode material. For example, Li/carbon intercalated compound reacts with water and the released hydrogen can be ignited by the heat of the reaction. Cathode material, such as LiCoO2, starts reacting with electrolyte when the temperature exceeds its thermal run-away threshold of 175°C with 4.3V cell voltage.
Li-Ion cells use thin, micro-porous films such as polyolefin to electrically isolate the positive and negative electrodes as they provide excellent mechanical properties, chemical stability, and are of acceptable cost. The low melting point of polyolefin, ranging from 135°C to 165°C, makes it suitable to be used as a thermal fuse. As the temperature approaches the melting point of the polymer, porosity is lost. This is intentional so it will shutdown the cell because lithium ions can no longer flow between electrodes. Also, there is a PTC device and a safety vent to provide additional protection in the Li-Ion cells. The case, commonly used as the negative terminal, is typically Ni-plated steel. When the case is sealed, it is possible for the metal particles to contaminate the interior of the cells. Over time, the particles can migrate into the separator, degrading the insulating barrier placed between the anode and cathode sides of the cell. That creates a micro- short between anode and cathode, allowing electrons to flow freely, ultimately failing the battery. Most of the time, this type of failure leads to little more than the battery powering down and ceasing to function properly. In rare instances, however, the battery can overheat, melt, catch fire, or even explode. This was reported as the main root cause of some recent battery failures that resulted in mass recall by different manufacturers.