Protecting the Li-ion Battery

For decentralization of the source of energy, it is hard to beat rechargeable lithium-ion batteries. A wide range of applications uses this electrochemical option of energy storage as a strategic imperative. That includes powering up units in the military sector,  storing and providing energy for personal use, keeping uninterruptible power supply systems operational for data centers and hospitals, storing energy from photovoltaic systems, and enabling the operation of battery electric vehicles and power tools.

The rechargeable battery pack is the most common design in the accumulator segment and accounts for the major share of battery-powered applications. Such a pack usually consists of multiple Li-ion cells. With continuous technological development, the economics of the Li-ion rechargeable battery pack is also becoming attractive enough to warrant a substantial increase in its use. This is also leading to the miniaturization of individual cells, resulting in an increase in their energy density.

However, even with the increased availability and use, the Li-ion rechargeable battery pack continues to carry a residual risk of hazards, especially due to the increase in energy density brought on by miniaturization. The disadvantage is in terms of safety.

The electrolyte in the Li-ion cells is typically a mixture of organic solvents and a conductive salt that improves its electrical conductivity. Unfortunately, this also makes the mixture highly flammable. During operation, the presence of an inordinate thermal load can lead to the point where the mixture becomes explosive. Furthermore, this safety hazard to the end-user is increasing with the constant efforts to further increase the energy density of Li-ion cells.

Most electric battery cells have a narrow operational temperature range, varying from +15 °C to +45 °C. That makes temperature the key parameter. When the cell exceeds this temperature range, its rising heat becomes a threat to its functional safety, and to the safety of the overall system.

Overcharging the battery substantially increases the statistical probability of the defect in the cell. This may lead to a breakdown of the cell structure, typically associated with the generation of fire and in some cases, an explosion.

Manufacturers of rechargeable battery packs try to mitigate this risk by including a battery management system, and primary and secondary protection circuits that they embed in the electronic safety architecture of the battery. This allows the battery to remain within its specified operating range during the charging and discharging cycles. But nothing is immune to failure, including components in the protection circuit, and the battery system can ignite and explode on an excessively high load.

As the battery powers up a load, excessive current flow can heat up the battery, and the primary protection circuit may not detect it even when it exceeds the permissible level. For the protection of batteries, RUAG Ammotec is offering a heat lock element, a pyrotechnical switch-off device that is entirely independent of the battery system. This comprises a physicochemical sensor to continuously monitor the environmental heat. As the temperature rises, the sensor blocks the flow of current permanently. The heat lock element causes an insulating piston to shear off a current conductor, thereby electrically isolating the battery.