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Secondary protection of Li-ion batteries: function and development

SCPs provide secondary protection for lithium-ion batteries

Self Control Protector (SCP) is a secondary protection element that reliably shuts off overcharge and overcurrent of lithium-ion battery.

The world’s first lithium-ion battery was commercialized in 1991. The battery had about three times the voltage of nickel-metal hydride batteries, which had previously been the standard for rechargeable batteries.Lithium-ion batteries are suitable for laptop computers and other mobile electronic devices because of their low self-discharge rate when left unused. This article outlines the development and advancement of Self Control Protectors (SCPs), which contributes to improving the safety of lithium-ion batteries.

A product with a long history

It can lead to fire and/or explosion hazards if overcharged, since the electrolytes in Li-ion batteries consist of flammable organic solvent. For this reason, the cells and charge/discharge circuits of lithium-ion batteries currently on the market are always equipped with a control function called “primary protection” to prevent problems that could lead to accidents, such as overcurrent or overcharge.

However, even the very best electronic circuits can fail in rare cases. In the early 1990s, Sony Chemicals (now Dexerials Corporation) was asked to develop a component that would provide secondary protection to ensure safety when the primary protection function of the lithium-ion battery failed.

Condensation sensor technology

SCP’s development began from condensation sensors in video cameras. If a lithium-ion battery has a malfunction or signs of a malfunction, the primary protection circuit in the BMS (Battery Management System) responds first and ensures that the battery does not deviate from its safe range. In most cases, primary protection response is sufficient. However, secondary protection is necessary for lithium-ion batteries, since the consequences of a failure are serious.

The temperature of a rechargeable battery usually rises as the battery charge progresses. Since there is a relationship between the state of charge and battery temperature, a method of monitoring the battery temperature was considered first. At the time, Sony Chemicals was shipping condensation sensors for camcorders that detect changes in humidity and change the resistance value. We explored a possibility of creating a sensor that would work in the same way as a PTC thermistor (a device whose resistance value increases sharply after a certain temperature) by applying this technology. However, we could not achieve this approach due to various problems.

A new method leads to a breakthrough in circuit interruption

If a circuit is interrupted by conventional sensor technology, it can be used again once the temperature returns to its original state. However, it is dangerous to continue using lithium-ion battery that is already defective. Then, we shifted the concept to development of the product that completely shuts off the circuit once an overcharge or overcurrent occurs when the primary protection is not functioning.

After many trials and errors, the final idea was to incorporate a heater in the circuit to blow the fuse. This method can handle both overcharge and overcurrent and can also physically interrupt the circuit.

The electrical structure of SCPs is shown in the circuit diagrams below. On the left side is a generic circuit diagram, on the right side is a three-dimensional circuit diagram reflecting the internal structure of the SCP. The actual SCP has a structure where fuses are arranged in a three-dimensional intersection on top of a heater (resistor).

The diagrams below explain how SCPs operate.

The current flow during normal discharge and charging is shown below.

When an overcurrent occurs, the fuse element is melted by Joule heat, interrupting the circuit. When an overcharge occurs, the secondary protection IC detects the abnormality and turns on the FET, which operates the heater circuit. In this case, current flows to the heater from both T1 and T3 and generates heat. This heat is transferred to the fuse element, blows the fuse and interrupts the circuit. At the same time, the heater circuit gets disconnected, and the heat generation stops.

Initially, a flexible printed circuit board (FPC) was used as the base material for the circuit, but to accommodate surface mounting, the base material was changed to a ceramic substrate. Thus, we completed the prototype of SCP.

Growing demand for lithium-ion batteries and SCPs

In 1994, Sony Chemicals obtained a basic patent for the laminated structure of the SCP’s heater and fuse. Furthermore, in the same year, a lithium-ion battery with SCP was introduced to the market. Lithium-ion batteries with SCPs were soon adopted by many laptop computer manufacturers to use in their products. Although the basic patent protection period has expired, Dexerials’ SCPs are still used as a fuse in the secondary protection circuit of lithium-ion batteries in many products.

Demand for SCPs continued to grow as the lithium-ion battery market expanded. Today, lithium-ion batteries are used not only in laptops, but also in many large electrical products such as cordless power tools, industrial storage batteries, electric bicycles, and electric motorcycles. For smaller devices, it is used in laptops, tablets, quick-charging smartphones, and automated external defibrillators for medical devices. As the world is beginning to shift from gasoline engines to motors, SCPs will also be required to handle higher voltages in large devices.

Technology continues to change rapidly. Dexerials will continue developing products to meet a wide range of demands.

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