Battery protection is an important and growing area of research and improvement. Protection strategies should address (1) cell, (2) module, and (3) pack.
(1) Maintain quality control in cell manufacturing. Good battery management can ensure balanced charge/discharge of individual cells to avoid stress.
(2) Focus on the prevention of thermal runaway propagation, such as high-quality connections and insulating barriers between cells.
(3) Take measures to reduce the risk to passengers caused by battery damage and thermal runaway.
The Automotive Industry Standard-156 (AIS-156) requires Rechargeable Electronic Energy Storage System (REESS) to have pressure release vent provided, to avoid building up of internal pressure and release of gases in case of internal single cell short circuit.
During an internal short circuit of a battery, the two electrode materials are internally and electronically interconnected, giving rise to high local current densities. Internal short circuits may occur in a lithium-ion battery due to, for instance, lithium dendrite formation or a compressive shock. A prolonged internal short circuit results in self-discharge in combination with a local temperature increase. The latter effect is important because the electrolyte may start to decompose by exothermic reactions if the temperature reaches above a certain threshold, causing thermal runaway with potential health and safety hazards.
The UN GTR 20, “Global Technical Regulation on the Electric Vehicle Safety (EVS)” and China’s “Electric Vehicles Traction Battery Safety Requirements” regulations also express using vents to give passengers a few minutes to safely leave a vehicle after the occurrence of thermal runaway in the first battery cell.
In thermal runaway events, large volumes of vent gas are created in a short time which leads to a steep increase of pressure inside the battery pack. So, devices must be integrated into the battery pack which enables fast and controlled release of gases to limit massive pressure build-ups. A proven technique to achieve this is to add a VENT functionality that enables pressure equalisation between the battery pack interior and its environment. PorVent® Screw-in Series PTFE membrane vents allow for such pressure equalisation under standard operating conditions.
These vents are chemically stable and super hydrophobic. They are suitable for lifetime use in automotive applications, achieving both protection class IP66 and IP67.
Also, a burst function is integrated into the vent: if the pressure inside the pack rises over the pre-defined level, the membrane is pushed beyond its holding capacity thus rupturing the membrane and opening a larger area to remove gases from the pack very quickly. Burst pins or specialized valves can also be integrated to give an added level of control over the minimum pressure at which the membrane bursts.
As larger and more energy-dense/high-voltage battery packs are being demanded, thermal runaway can never be completely eliminated. Adopting appropriate safety measures at all levels is the safest long-term strategy.