One of the most recent smartphone lithium-ion battery advancements is fast charging, which can dramatically cut charging time. It increases the amount of current sent to the battery in order for its capacity to refill quicker. Unfortunately, this performance comes with some safety and longevity issues that require overtemperature protection.

An integral element of advanced lithium-ion battery construction is the separator that plays a critical role in cell safety. Separators are used so there is no physical contact between the cathode and anode portions of the cell. Even though separators have evolved from simple single-layer sheets to multilayer sheets with shutdown features, they alone cannot ensure complete cell safety.  That’s because the shuttling back and forth of lithium-ions between the intercalating cathode and anode has a degrading impact on electrodes. Recent research has shown that lithium concentration on the surface of the electrode closest to the separator experiences a higher State of Charge (SOC).

Plus, cells are constructed with materials that are flammable and degradable where mechanical and electrical shocks can lead to thermal runaway, which can have catastrophic results. While lithium-ion cell materials are stable at lower temperatures, they can start to break down when temperatures exceed 130℃.

Fast charging of smartphones also requires that the charging cables be enhanced. The combination of small area, higher pin count and considerably higher power levels adds potential risks to these new cables. Fast charging over such cables can result in overheating if pins are damaged, the outer shell body is damaged or foreign material and liquids enter the plugs.

It is easy to see why designers need to be concerned about smartphone safety and reliability with fast charging batteries. Cell designers are wise to consider a multilayered approach to protect against various potential hazards. Individual cells require mechanical, electrical and thermal protection. Determining the right level of protection has become more complicated as cells are now networked into various battery pack arrangements.

Proven over the course of many years, mini-breaker thermal cut-off (TCO) devices are key components increasingly being used in the protection architecture of lithium-ion battery cell arrangements. In fact, Bourns’ broad line of mini-breaker TCO products have provided precision overtemperature protection in more than 3 billion circuits. Bourns® TCO products are self-preserving devices designed to stay tripped during abnormal temperatures, and then recover when power is cut and the temperature returns to a safe level. This allows for protection at lower temperatures compared with polymer PTCs and temperature fuses.

A new application note from Bourns gives readers a better understanding of lithium-ion cell construction and possible smartphone issues that can arise during fast charging. In it, you will learn about the specific aspects of fast charging functionality that can threaten the safe, reliable and long-life operation of not just lithium-Ion battery cells, but USB Type-C™ power delivery cables as well. Better yet, the paper provides the standards to be aware of, how TCO devices work to protect cells and precision protection layout solutions for Battery Management Systems that will help designers minimize any detrimental effects from overtemperature conditions.