The MOSFET, a key component in the charge/discharge protection circuit, also experiences a certain percentage of short-circuit failures. If lithium-ion battery production volume is low, this effect won't be noticeable. However, the demand for lithium-ion batteries is enormous; in 2014, global shipments of small lithium-ion batteries reached 5.6 billion units. With such a large volume of shipments, even with a probability risk of 1 ppm, there are an average of 5,600 hazardous incidents per year. Therefore, in addition to main circuit maintenance, secondary maintenance has been added to further reduce risk. Among the components in secondary maintenance, there is generally only one useful component: a primary circuit safety device, a PTC (Potentially Transmitted Thermostat), or multiple components such as temperature-controlled safety wires. If a PTC is used, a fuse should not be used; if a fuse is used, a PTC should not be used. Maintenance devices compete with each other, like different alleles competing for the same position on a chromosome. However, since maintenance components are not the overall winner, multiple components coexist to meet different application needs.
However, with the rapid popularization of smartphones and the continuous increase in mobile phone battery capacity, the demand for fast charging is growing. Currently, there are several specifications, including OPPO VoOC, Qualcomm QC2.0, and MediaTek PumpExpressPlus. In fast charging, the current is very high, reaching around 3A, during the first 30 minutes.
The high current surge during the first 30 minutes of rapid charging, accompanied by increased heat and temperature, will alter the competitive landscape of secondary repair components for lithium-ion batteries, rather than the cooperative model: PTC+fuse will form a primary repair combination.
First, PTC + fuse has temperature complementarity and overcurrent protection functions. PTC has temperature maintenance function, but because of its relatively high temperature drop, the selection standard is larger, and its overcurrent maintenance capability is relatively weak, and the PTC's action speed is slower. Fuse is not sensitive to temperature and cannot perform temperature maintenance, but its temperature drop rate is also very low, so a smaller current standard can be selected, and its action speed is faster compared to its overcurrent protection capability.
Secondly, PTC+fuse is a low-cost processing method to pass UL2054. Under high-current charging, UL2054 struggles to pass a complete test on a single component because each component has its own advantages and disadvantages. First, there's the commonly used PTC. Due to the very high charging current, to ensure it doesn't shift during rapid charging and temperature rise, the standard must be 12066A/7A. Choosing such a high standard makes it difficult for lithium-ion batteries to pass the UL2054 LPS test, as it's difficult to limit the current below 8A within 60 seconds. Second, there's the commonly used fuse. Its biggest advantage is temperature insensitivity; choosing a 5A standard, a 5A safety lead is beneficial for lithium-ion batteries to pass the UL2054 LPS test; however, due to its temperature insensitivity and lack of maintenance functions, it's difficult to test after overfilling in UL2054 6V/1C and V2C. Third, there's the three-terminal staggered wire. Although it can maintain the processed temperature, the current standard is quite large, reaching up to 10A/12A, making LPS testing impossible; moreover, the current cost is very high. Fourth, some manufacturers choose the dual-IC method, which has good results but is relatively expensive. If a PTC is combined with a fuse, firstly, the temperature-insensitive 5A fuse can easily pass LPS, short circuit and other tests, and secondly, the 12066a/7A PTC can easily pass 6V/1C, 6V2C overcharge and other tests.
Finally, the PTC+fuse maintenance plan is safer than that of a single component. Combining the two components is equivalent to adding an extra layer of maintenance on top of the existing secondary maintenance, thus providing an additional layer of protection for the safety of lithium-ion batteries and significantly reducing risk factors.
