Since there are hundreds of millions of mobile phones worldwide, to achieve security, the failure rate of security protection must be less than one in 100 million. Because the board's failure rate is generally much higher than one billion. Therefore, when designing a battery system, there must be more than two safety lines. A common mistake is to use a charger (adaptor) to directly charge the battery pack. In this way, the overcharged protection is completely transferred to the protection board 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 is an explosion accident every day in the world. If the battery system can provide two safety protections for overcharging, overdischarging and overcurrent respectively, if the failure rate of each protection is one in ten thousand, the two protections can reduce the failure rate to one hundred thousandth.