Principle analysis of lithium ion battery protection circuit in Jump starter

Principle analysis of lithium ion battery protection circuit in Jump starter

---From AGA jump starter.

Now, seat heating, air conditioning, navigation, information entertainment, driving safety and other systems are designed to improve comfort and driving experience. From these systems, it is easy to understand the benefits of electronic systems powering various functions in the car. Now it's hard to imagine what happened just over 100 years ago when there were no electronic components in a gasoline-powered car. At the turn of the century, cars began to have hand cranks, headlights began to be illuminated with acetylene gas, and pedestrians could be prompted by bells. Nowadays, automobiles are becoming the junction of electronic systems, minimizing the use of mechanical systems, and becoming the largest and most expensive "digital tools" in people's lives.

As more and more mechanical systems are replaced by electronic systems, power consumption becomes more and more important. Whether the owner makes use of the car, there is a possibility that the car's battery power is not enough to cause the problem of not starting. So businessmen have developed an electronic product based on the demand for automotive power - automotive emergency start up power supply(Jump starter).

Let's talk about the safety of Jump starter today. Everyone knows that the core of jump starter is its built-in lithium-ion battery, and the characteristics of lithium-ion battery determine that it can not be overcharged, over discharge, over current, short circuit and ultra-high temperature charge and discharge, so the lithium-ion battery components will always follow a delicate protective plate and a current fuse. The protection function of lithium-ion batteries is usually accomplished by the protection circuit board and the current devices such as PTC. The protection board is composed of electronic circuits. The voltage of the battery and the current of the charging and discharging circuit are monitored accurately from - 40 85 C, and the current circuit is controlled in time. PTC stop the battery damage when the battery in the high temperature . Common lithium-ion battery protectors usually include control IC, MOS switches, resistors, capacitors and auxiliary devices FUSE, PTC, NTC, ID, memory, etc. The control IC controls the MOS switch on under all normal conditions, so that the battery cell and the external circuit on, and when the battery cells voltage or loop current exceeds the prescribed value, it immediately controls the MOS switch off to protect the safety of the battery cells.

Principle of protective plate for lithium ion battery:

As shown in the figure, the protection circuit consists of two MOSFETs (V1, V2) and a control IC (N1) plus some resistance and capacitance elements. The control IC monitors the battery voltage and circuit current, and controls the gate of two MOSFETs. MOSFETs act as switches in the circuit, respectively controlling the conduction and shutdown of charging and discharging circuits. C3 is a delay capacitor. The circuit has the functions of overcharge protection, over discharge protection, over current protection and short circuit protection.

1. Normal state

In the normal state, the "CO" and "DO" feet of N1 output high voltage, both MOSFETs are on state, the battery can be charged and discharged freely, because the MOSFET on impedance is very small, usually less than 30 m Ω the resistance of the circuit performance is very small. Under this condition, the consumption current of the protection circuit is a Class μA, usually less than 7 μA。

2. Overcharge protection

The lithium-ion batteries charge mode is constant -current/ constant -voltage charging. In the initial stage of charging, the voltage will rise to 4.2V (sometimes 4.1V, depending on the cathode material) for constant-current charging. Some batteries require constant-voltage charging until the current becomes smaller and smaller. If the charger circuit is out of control during the charging process, the battery voltage will continue to rise after the battery voltage exceeds 4.2V. When the battery voltage is charged to more than 4.3V, the chemical side effects of the battery will intensify, which will lead to battery damage or safety problems. In a battery with a protective circuit, when the control IC detects that the battery voltage reaches 4.28V (which is determined by the control IC, different IC has different values), its "CO" foot will change from high voltage to zero voltage, so that V2 will turn on and off, thus cutting off the charging circuit, so that the charger can not recharge the battery. It has over charge protection function. In this case, because of the existence of V2 with the body diode VD2, the battery can discharge the external load through the diode. There is a delay time between the detection of the battery voltage over 4.28V in the control IC and the signal of the switch-off V2. The delay time is determined by C3 and is usually set to about 1 second to avoid misjudgment due to interference.

3. Over discharge protection.

When the battery voltage drops to 2.5V, its capacity has been completely discharged. If the battery continues to discharge , it will cause permanent damage to the battery. When the control IC detects that the battery voltage is less than 2.3V (which is determined by the control IC, different IC has different values), the "DO" foot will change from high voltage to zero voltage, so that V1 will turn on and off, thus cutting off the discharge circuit, so that the battery can no longer discharge the load and play an over discharge. Protective effect. In this case, because of the existence of V1 with the body diode VD1, the charger can charge the battery through the diode. Because the voltage of the battery can not be reduced under over-discharge protection, the current consumption of the protection circuit is required to be very small. At this time, the control IC will enter a low-power state, and the power consumption of the whole protection circuit will be less than 0.1 mu A. There is also a time lag between the detection of battery voltage below 2.3 V in the control IC and the signal to turn off V1. The time lag is determined by C3 and is usually set to about 100 milliseconds to avoid misjudgment due to interference.

4. Over current protection

Because of the chemical characteristics of lithium-ion batteries, battery manufacturers have stipulated that the maximum discharge current should not exceed 2C (C = battery capacity / hour). When the discharge current exceeds 2C, the battery will cause permanent damage or safety problems. When the discharge current of the battery passes through two MOSFETs in series during normal discharge , a voltage will be generated at both ends of the MOSFET due to the conduction impedance of the MOSFET. The voltage value U = I * RDS * 2, RDS is the conduction impedance of a single MOSFET. The voltage value is detected by controlling the "V -" foot of the IC, if the load is different due to some reason. Often, when the loop current is large enough to make U > 0.1V (the value is determined by the control IC, different IC has different values), its "DO" foot will be changed from high voltage to zero voltage, so that V1 from turn on to turn off, thus cutting off the discharge circuit, so that the current in the loop is zero, playing an over current protection role. There is also a time delay between the over current detected by the control IC and the V1 signal being turned off. The time delay is determined by C3, usually about 13 milliseconds, to avoid misjudgment due to interference. In the above control process, the magnitude of the over current detection value depends not only on the control value of the control IC, but also on the conduction impedance of the MOSFET. The larger the conduction impedance of the MOSFET, the smaller the over current protection value of the same control IC.

5. Short circuit protection

In the process of discharge, if the circuit current is large enough to make U > 0.9V (the value is determined by the control IC, different IC has different values), the control IC will judge the load short circuit, and its "DO" foot will quickly change from high voltage to zero voltage, so that V1 from conduction to turn off, thus cutting off the discharge circuit and playing a short circuit protection. Use. The delay time of short circuit protection is very short, usually less than 7 microseconds. Its working principle is similar to over current protection, but the judgment method is different, and the protection delay time is also different.

The working principle of a single lithium-ion battery protection circuit is described in detail above. The protection principle of multiple series lithium-ion batteries is similar to that of single lithium-ion batteries. In addition to the control IC, there is an important element in the circuit, namely, MOSFET, which plays a switch role in the circuit. Because it is directly connected between the battery and the external load, its conduction impedance has an impact on the performance of the battery. The load carrying capacity is also strong, and it consumes less power when discharging.