Kote Polymer PTC solves the problem of overcharge/short circuit protection of lithium-ion batteries

Although primary protection is generally considered reliable, the protection IC or MOS-FET may be damaged when the electrostatic discharge voltage is too high or the temperature is too high, and the integrated circuit will oscillate when short-circuited. At the same time, most IC+MOS-FET circuits detect charging and discharging overcurrents indirectly, and cannot guarantee that overcurrent protection will be provided under all working conditions of the battery, and the reliability of protection is also reduced.

1. Lithium-ion battery protection challenges:

Compared with NiMH or NiCd batteries, lithium-ion batteries have a higher current density and are widely used in various portable devices.

Lithium-ion batteries are usually very sensitive to overcharging. When the voltage at both ends of the battery is too high, the risk of battery leakage, smoking, burning, and explosion (such risks are often quite severe) will increase. Overcharging may be caused by uncontrolled charging, electrode errors, or the use of an incorrect charger. When the charging and discharging current of a lithium-ion battery is too large or there is an external short circuit, the internal heat may damage the battery or burn other components, seriously shortening the battery's cycle life.

The task of the protection circuit is to monitor the parameters of the battery's charge and discharge status in response to various possible battery faults, so as to ensure the battery life and performance, and protect the battery and external devices (such as mobile phones, PDAs, laptops, etc.) from damage.

Mobile phone batteries usually use a primary protection circuit consisting of a battery protection IC and a metal oxide field effect transistor. The protection IC controls the on or off of the FET by monitoring the voltage across the battery and the discharge current to prevent overcharging, short circuit, over-discharging and other faults.

Although primary protection is generally considered reliable, it may damage the protection IC or MOS-FET when the electrostatic discharge voltage is too high or the temperature is too high, and the integrated circuit will oscillate when it is short-circuited. At the same time, most IC+MOS-FET circuits detect overcurrents in charging and discharging indirectly, and cannot guarantee that overcurrent protection will be provided in all working conditions of the battery, and the reliability of protection is also reduced. After adding the KOTE company's battery protection dedicated PTC to the protection circuit, even if the primary protection circuit fails or the temperature rise is high, the PTC can still provide protection against overcharge, overcurrent, short circuit, overtemperature and other faults, ensuring that the battery will not have safety problems when it is misused or abused.

2. Solution:

Early batteries used fuses or bimetallic strips as secondary protection devices in case of overcurrent, short circuit, and overtemperature. However, since battery overcurrent and other faults are often temporary and do not mean permanent damage to the battery, the non-recoverable disadvantage of the fuse increases the quality assurance cost of battery manufacturers; and although the bimetallic strip can self-recover, it will constantly operate/recover, which will cause early fatigue wear of the contacts and then adhesion, thus losing its protective function. In addition, the bimetallic strip is large in size and weight for the stringent requirements of portable devices on battery volume and weight. However, polymer PTC has advantages in providing secondary protection for batteries with its self-recovery, low resistance, and no additional volume requirements.

1. Overcharge protection :

In accordance with the requirements of GB/T18287-2000 for overcharge protection of lithium-ion batteries, we conducted overcharge tests on a battery with a nominal capacity of 720mAh without a protection circuit and with only the PTC protection of the battery protection provided by Cote, during which the battery voltage and surface temperature were monitored. (The charging power supply was set to constant current 2C A charging, and the constant voltage value was 2 times the nominal voltage).

Figure 2: Battery surface temperature rise curve with and without polymer PTC protection

From the above test results, it can be observed that the surface temperature of batteries without PTC protection can reach nearly 120℃ when overcharged, eventually causing leakage, fire or even explosion; while the surface temperature of batteries equipped with KT16-1750DL polymer PTC from Cote will not exceed 80℃.

2. Short circuit protection :

When the battery is short-circuited externally, the current can reach more than 15A. Under normal circumstances, the limit operating parameters of the power field effect tube will be easily exceeded; in addition, the working state of the protection IC cannot remain stable during a short circuit. The above problems lead to the incompleteness of the battery short-circuit protection function.

In this case, polymer PTC is a good solution. Polymer PTC can provide protection before the MOS tube is broken down, ensure the cycle life of the battery, and reduce the manufacturer's quality assurance costs.

The polymer PTC thermistor for battery protection from Shanghai Kete Polymer Materials Co., Ltd. is in strip form with nickel-plated copper leads and is suitable for use in high current density lithium-ion batteries in portable devices.