With hundreds of millions of mobile phones worldwide, achieving safety requires a failure rate of less than one in a hundred million. However, the failure rate of circuit boards is generally much higher than one in a hundred million. Therefore, battery system design must include at least two lines of defense. A common design error is directly charging the battery pack with the adapter. This places the entire responsibility for overcharge protection on the protection board on the battery pack. Although the failure rate of the protection board is low, even at one in a million, explosions still occur daily globally. If the battery system provides two lines of defense against overcharge, over-discharge, and overcurrent, with each defense having a failure rate of one in ten thousand, the two lines of defense can reduce the failure rate to one in a hundred million.
Battery Pack Function: Current Limiter. Taking a mobile phone battery system as an example, the overcharge protection system uses the charger's output voltage set at around 4.2V to achieve the first layer of protection. This way, even if the protection board on the battery pack fails, the battery will not be overcharged and become dangerous. The second layer of protection is the overcharge protection function on the protection board, typically set to 4.3V. This means the protection board doesn't normally need to cut off the charging current; it only activates when the charger voltage is abnormally high. Overcurrent protection is handled by the protection board and current limiter, providing two layers of protection against overcurrent and external short circuits. Since over-discharge only occurs during the use of the electronic product, the circuit board of the electronic product typically provides the first layer of protection, while the protection board on the battery pack provides the second. When the electronic product detects a supply voltage below 3.0V, it should automatically shut down. If this function is not included in the product design, the protection board will close the discharge circuit when the voltage drops to 2.4V.
