Brief description of lithium battery primary and secondary protection

Since the birth of lithium-ion batteries, safety issues have been accompanied. With the popularity of portable electronic devices such as mobile phones and tablet computers, the requirements for battery capacity are getting higher and higher, and at the same time, the size of batteries is becoming smaller and smaller. The energy density of the battery core is required to be high, and the danger is also greater. Therefore, the protection of lithium-ion batteries is indispensable. Different manufacturers have proposed different protection schemes for different types of lithium-ion batteries.

Battery type and its protection

Lithium-ion batteries are available in a variety of categories. They can be divided into cylindrical and square shapes according to their shape. They can be divided into lithium-ion and lithium-polymer batteries according to the state of the electrolyte. Currently, most of the cells used in mobile power supply are cylindrical 18650 cells (18mm in diameter and 65mm in length) and square lithium polymer cells. The safety is analyzed below from the battery structure.

1. cylindrical lithium ion battery (18650)

The battery core has a PPTC (Recoverable Safety Device) for over temperature and over current protection. When the battery core temperature is too high or the current is too large, the PPTC will become a high-resistance state, thereby blocking the battery core charging and discharging current, and avoiding the battery fire and explosion.

2. Square lithium battery

The design of MHP-TA and PPTC close to the battery core enables the MHP-TA and PPTC to better sense the battery temperature. When the battery temperature rises abnormally, it can exhibit high resistance, hinder the battery charge and discharge current, and ensure the safe use of the battery. .

The protection circuit can be divided into two parts: active component protection (protection IC and Mosfet), also known as primary protection, passive component protection (MHP, PTC, Fuse), also known as secondary protection. The primary protection circuit is mainly for the overcharge, over discharge, overload and short circuit of the battery. The IC detects the battery voltage and the charge and discharge current to control the Mosfet to turn on or off to ensure that the lithium battery works in a safe state.

Several different battery protection options

1. (SafetyIC+Mosfet)+Fuse

There are three types of Fuse in this type of release: thermal fuses, common current fuses, and slow-break current fuses.

The thermal fuse can better protect the battery core from fire and explosion caused by heat, and the cost is low. However, due to the current magnitude, ambient temperature, board temperature and cell temperature, the thermal fuse is easily caused to malfunction, and its unrecoverable characteristics make the application of such a release have certain limitations.

Ordinary current fuses have low cost and are not effective for overcharging of batteries because they cannot sense the temperature of the battery cells. The short circuit of the battery is easy to blow the fuse, it is not recoverable, and the battery is scrapped. Therefore, this protection scheme is mainly applied to the low-end lithium battery.

The slow-break current fuse operates longer than the SafetyIC's over current protection action time, which guarantees the first-level protection of the SafetyIC as an active component and does not trigger the fuse as a secondary protection. The battery is in a safe state. This kind of scheme has poor effect on the overcharge protection of the battery core, but under the premise of the safety of the battery core, such a scheme can meet the requirements of the LPS.

2. (SafetyIC+Mosfet)+PTC/MHP

Possible reasons for a lithium battery to explode:

A. The protection circuit is effective due to improper design of circuit parameters or component failure.

B. The lithium battery core itself is unqualified, and even if it is charged normally, it may explode.

Based on the above reasons, the international safety standards for lithium batteries clearly require that lithium batteries can be safely charged and discharged in the event of failure of the primary protection. Therefore, in order to make the application of lithium battery more safe, on the basis of the primary protection circuit (IC/Mosfet), a first-level passive component protection is added, and the temperature of the battery core is detected by a recoverable safety device (PTC or MHP). When the temperature rises abnormally, the PTC or MHP immediately exhibits a high-resistance state, hindering the charge and discharge of the battery, thereby preventing the lithium battery from igniting and exploding. The protection principle is as shown below. As can be seen from the figure, when the battery temperature rises, the PTC operates, the charging circuit is high impedance, the current is close to zero, and the battery temperature drops rapidly.

3. Double (SafetyIC+Mosfet)

The dual active component protection can provide the reliability of the protection circuit, reduce the failure probability of the protection component, and at the same time meet the safety requirements. However, the protection of the battery type is not perfect.

For whatever reason, lithium batteries appear to have a sharp rise in battery temperature before the explosion. If there is no passive component PTC/MHP to sense the battery temperature, even the double protection will not prevent the battery from exploding.

The dual protection circuit greatly reduces the probability of overcharging, short circuiting and reverse charging of the lithium battery cell. However, there is nothing to do with the battery core which has its own problems. According to statistics, about 85% of the batteries are fired and exploded because of the battery. The problem of the core itself, therefore, the protection of the battery from the protection circuit is limited.

Summary

As the energy density of lithium battery cells continues to increase, safety will be more valued. Based on the analysis and comparison of the above lithium battery protection schemes, (SafetyIC+Mosfet)+PTC/MHP protection scheme can effectively prevent lithium batteries from being used. In the process of fire and explosion, this program is currently the most widely used, and the highest cost performance.

The page contains the contents of the machine translation.