Design specification for lithium batteries

　　Since there are hundreds of millions of mobile phones in the world, to achieve security, the failure rate of security protection must be less than one in 100 million. Because the failure rate of the circuit board is generally much higher than one in 100 million. Therefore, when the battery system is designed, there must be more than two safety lines of defense. A common design mistake is to use the adaptor to go directly to the charging pool. In this way, the overcharge protection task is completely handed over to the protection plate on the battery pack. Although the failure rate of the protection board is not high, even if the failure rate is as low as one in a million, there will be an explosion accident every day in the world. If the battery system can provide two safety protections for overcharge, overdischarge, and overcurrent, the failure rate of each protection can be reduced to one in 100 million if the failure rate of each protection is 1 in 10,000. The common battery charging system box diagram is as follows, including two parts: charger and battery pack.

　　The charger also includes an adapter (Adaptor) and a charging controller two parts. The adapter converts AC to DC, and the charge controller limits the maximum current and voltage of DC.

　　The battery pack consists of two parts: a protection plate and a battery core, and a PTC to limit the maximum current. In the figure below, the adapter AC to DC text box function: electric controller limit current and voltage. The role of the charger text box: protect the board from overcharge, over-discharge, over-current and other protection. Battery pack text box function: current limiting plate. Taking the mobile phone battery system as an example, the overcharge protection system uses the charger output voltage to be set at about 4.2V to achieve the first layer of protection, so that even if the protection board on the battery pack fails, the battery will not be overcharged and dangerous. The second protection is the overcharge protection function on the protection board, which is generally set at 4.3V. In this way, the protection board usually does not have to be responsible for cutting off the charging current, only when the charger voltage is abnormally high, it needs to act. Overcurrent protection is responsible for the protection plate and the current limiting plate, which are also two types of protection to prevent overcurrent and external short circuit. Since overdischarge will only occur during the process of electronic products being used. Therefore, the general design is to provide the first protection by the circuit board of the electronic product, and the protection board on the battery pack provides the second protection. When the electronic product detects that the power supply is lower than 3.0V, it should automatically shut down. If the product is not designed for this function, the protection board will close the discharge loop when the voltage is low to 2.4V.

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