Melt Protection for Lithium Battery

During the use of lithium ion rechargeable batteries, overcharging, overdischarge, and overcurrent will all affect the battery's service life, performance, and safety. The fuse in the rechargeable battery is effectively monitored as a secondary overcurrent protection in conjunction with the IC control loop and prevents damage to the battery.

In recent years, with the miniaturization and popularization of mobile appliances, lithium-ion rechargeable batteries have the advantages of small size, light weight, high output voltage, stable discharge voltage, and long storage time, which has caused the use of lithium-ion rechargeable batteries for electrical appliances to soar.

Lithium-ion rechargeable batteries are one of the lithium batteries (rechargeable secondary batteries) that place protective lines in the battery and can effectively protect the battery from being damaged by overcharge, overdischarge, Overflow, or improper use. As an overcurrent protection device, the fuse can protect the battery damage caused by the large current and short circuit during charging or discharging of lithium ion rechargeable battery.

This protection loop consists of two MOSFETs and a control IC plus some resistance elements. The control IC is responsible for monitoring the battery voltage and controlling the two MOSFET Gates. The MOSFET acts as a switch in the circuit, controlling the switch and off of the charge loop and the discharge loop, respectively. The fuse acts as a secondary protection for the fuse. C2 is a delay capacitor. The circuit has the functions of overcharge protection, overdischarge protection, overcurrent protection and short circuit protection.

Normal working conditions

In normal charge and discharge, the high levels of C0 and D0 feet of the control IC are controlled in the circuit, and V1 and V2 are all turned on. The charging current flows in from Input +, charges the battery through the fuse, and flows out from Input after V1 and V2. During normal discharge, the current flows through Input-and V2 and V1 to the negative electrode of the battery, and its current direction is opposite to the charging current direction. Since V1, V2's conduction resistance Rds(ON) is minimal, the consumption current in this state is μA and therefore the loss is small.

Overcharge protection

The battery is charged for constant current at the initial stage of charging, and the battery voltage will increase with the extension of the charging time. When the IC detects that the battery voltage reaches the overcharge detection voltage(this value is determined by the control IC), The C0 foot is changed from Gaodianya to a low voltage, so that the MOSFET gate that controls charging is extremely switched off, that is, V2 is turned off from the pilot to the switch to cut off the charging loop, so that the charger can not protect the battery from charging. Overcharge detection corresponds to pulse charging and error actions due to noise. Delay time needs to be set. Delay time is determined by C2 and is generally set to about 1s.

Overdischarge protection

Overdischarge protection stops the discharge on the load when the battery voltage becomes lower. When the battery discharges the load, its voltage gradually decreases with the discharge process. When the battery voltage drops below the overdischarge test voltage, its capacity has been completely discharged. If the battery continues to discharge, it will cause permanent damage to the battery. Therefore, when the control IC detects that the battery voltage is lower than the over discharge point detection voltage, the D0 foot of the control IC changes from Gaodianya to a low voltage, so that the V1 is turned off from the switch to the off circuit to cut off the discharge loop, and the battery can not continue to discharge. Play a protective role. Since the battery voltage can no longer be reduced below the overdischarge test voltage, the control IC must be required to consume minimal current. Overdischarge detection corresponds to pulse charging and error actions due to noise, and the delay time that needs to be set is generally about 100ms.

Overcurrent Protection and Short Circuit Protection

Overcurrent protection stops discharging the load when a large current is consumed. The purpose of this function is to protect the battery and MOSFET to ensure the safety of the battery under working conditions. During the normal discharge process, the current passes through the two MOSFETs because the on-resistance causes a voltage drop across it. This voltage value U=I*[R1ds+R2ds], where the turned-on V1 and V2 are regarded as resistors, ie R1ds and R2ds, if the load causes an abnormality to increase the loop current for some reason, when the current surges so that the voltage also increases to the voltage value determined by the control IC, the D0 pin of the control IC rapidly changes from high voltage to low voltage. Turn V1 off to cut off the discharge loop and the loop current becomes zero. The delay time that needs to be set is generally about 13ms. When the voltage is increased to the voltage value determined by the control IC (when the IC determines that the load is short-circuited), V1 will be turned from on to off, and its working principle is similar to overcurrent protection. The short-circuit protection delay time is generally less than 7μs. If the current continues to increase in the event of an accident, the fuse will provide secondary protection during overcurrent, avoiding permanent damage to the control IC and MOSFET in the circuit.

The above describes in detail the working principle of the protective circuit of lithium-ion rechargeable batteries. In addition to controlling ICs and MOSFETs, there is an important component in the circuit, namely the fuse, which plays a secondary overflow protection role in the circuit. Since its resistance is very small and the energy consumed in the circuit is negligible, this has become the main reason why many battery manufacturers consider replacing the PTC polymer (internal resistance is too large) with a new laminated multilayer ceramic chip fuse.

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