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Can We Find Solutions to Deal with the Safety Issues?

The battery management system (BMS) is a crucial element between lithium batteries and end users; it plays a very important role in the daily management and protection of batteries.

At last, Samsung has announced the cause of exploding Note 7 smartphones at the end of the Chinese lunar calendar year; the conclusion was: faulty batteries, but of course, Samsung must be held accountable for its oversight. The industry has had different responses to this conclusion, but this major public incident which took place in 2016 is finally coming to an end. For Samsung, however, the journey to regain consumer trust is just starting.

After the Note 7 incident, safety issues related to lithium-ion batteries (hereinafter referred to as lithium batteries) have been placed in the forefront once again. Based on the current technical specifications of rechargeable batteries, the dominance of lithium batteries is still untouchable; they can be called "perfect" batteries based on the high energy density, high operating voltage, low self-discharging rate, large number of charges, and no memory effect. If we look at lithium batteries by considering the principles of thermodynamics, however, they are simply unstable to begin with and have many corresponding safety concerns. From battery production to end-product applications, each step must be performed carefully in order to prevent accidents. When a manufacturer as experienced as Samsung can make mistakes like these, others certainly cannot afford to be so careless.

 

Lithium-ion battery

Nickel-cadmium (NiCd) battery

Nickel-metal hydride (NiMH) battery

Battery voltage range (v)

3.0-4.2

1.0-1.3

1.0-1.4

Energy density (Wh/kg)

150

50

75

No. of charges

1000

500+

400-500

Battery life (years)

5+

4-6

4-6

Max. charging/discharging rate

1C/1C

20C/10C

5C/2C

Self-discharge rate/month

<10%

20%

25%

Charging method

Constant current/Constant pressure

Constant current

Constant current

Operating temperature (℃)

-20-50

-40-50

-20-50

Charging temperature (℃)

0-50

0-40

0-40

Table 1 - Performance comparison of three different types of rechargeable batteries

 

Lithium batteries contain lithium-ion compounds (such as lithium cobalt oxide) positive electrode, graphite carbon in the negative electrode, as well as electrolyte, which serves as a reaction medium between the anode and the cathode. The electrolyte can be further divided into liquid, gel, and all solid-state polymer electrolytes. Inside the liquid electrolyte structure, a diaphragm is also installed to separate the positive and negative electrodes. Currently, the most common battery type used for mobile phones and other portable devices is lithium polymer batteries; due to the chemical stability of electrolytes as well as their high machinability.

The biggest and most common safety risk with respect to lithium batteries is caused by internal short circuits. After a short circuit has occurred, the high current going through the battery's internal resistance will generate a lot of heat, which activates various unstable substances inside the battery. The temperature of the electrolyte, which is a flammable material, is increased to cause gas release or start a fire. This type of short circuit is mainly caused by errors during the manufacturing process or from users overcharging the battery. For example, exploding batteries used in Note 7 were caused by battery deformation and issues related to the welding process during production, which led to the higher probability of a short circuit. When batteries are overcharged, lithium-ion forms dendrites at the cathode which can internally short out the battery. Objectively speaking, measures for ensuring the safety of lithium batteries must be taken by the battery manufacturer as well as the end-product manufacturer. Success can only be achieved by having various groups working together. An error during any stage which causes a short circuit can have life-threatening consequences.

From a battery manufacturer's perspective, in addition to following a set of strict production and testing standards when making battery cells, incorporating additional protection circuits is also a necessary step. Typical protection circuits include a temperature switch which prevents high current surges, a circuit cut-off device to prevent overcharging, as well as a gas relief valve for releasing pressure. Currently, there are many organizations involved in the research of safer materials and electrochemical structures of lithium batteries; however, many studies are still in the experimental phase, or they choose to sacrifice energy density and/or increase production costs as compromise solutions. Based on the current situation, we can foresee that battery manufacturers will continue to perform their "safety dance while being handcuffed" in the future.

For end-product manufacturers such as Samsung, it is absolutely critical to establish a set of safe and controllable lithium battery charging management mechanisms. When charging lithium batteries, we must follow a strict "constant current/constant pressure" management process; charge up to 4.1V under a constant current source, then continue to charge up to 4.2V by switching the charger to a constant pressure charging method to prevent overcharging the battery. The good news is that many semiconductor manufacturers such as TI, ST, Maxim, and Microchip all have well-developed battery charging management solutions for end-product manufacturers to select from. However, companies like Samsung must continue to balance functionality and safety very carefully during product design. New technologies such as fast charging and creative product designs are used as tools to attract buyers, but security risks of lithium batteries must not be overlooked.

In this way, lithium batteries are like powerful lords with unpredictable tempers; we must tread lightly when dealing with them. It will cost Samsung at least US$2 billion to deal with the Note 7 incident; this proves the point that investing a bit of effort beforehand to have preventative measures in place is worth it.

ST lithium battery charger solution

Figure 1: Demo board of ST's lithium battery charger solution

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