How to design the protection board for 26650 lithium battery.

How to design the protection board for 26650 lithium battery.

The application of 26650 lithium-ion batteries is relatively widespread in the industry. Currently, the most popular type is 26650 lithium-ion batteries, while other types of 26650 batteries have gradually begun to withdraw from the market. So how to design the 26650 lithium battery protection board? This 26650 lithium battery protection board is not fixed, it is basically a customized battery protection board.

The protection function of lithium batteries is generally carried out by the collaborative work of the protection circuit board and current devices such as PTC. The protection board is composed of electronic circuits that accurately monitor the voltage of the battery cells and the current of the charging and discharging control circuit at all times in an environment of -40 ℃ to+85 ℃, and timely control the on/off of the current control circuit; PTC prevents severe damage to batteries in high temperature environments.
A regular 26650 lithium battery protection board generally includes a control IC, MOS power switch, resistor, capacitor, and auxiliary device FUSE PTC、NTC、ID、 Memory, etc. Among them, the control IC controls the MOS power switch to turn on and off under normal conditions, making the battery cell and the external circuit turn on and off. When the battery cell voltage or control circuit current exceeds the standard value, it immediately controls the MOS power switch to turn off, protecting the safety of the battery cell.

Under normal conditions of the lithium battery protection board, VDD is at a high level, VSS and VM are at a low level, and DO and CO are at a high level. When any of the parameters of VDD, VSS or VM is changed, the level of the DO or CO terminal will change.

1. Under normal circumstances

Under normal circumstances, both the "CO" and "DO" pins of N1 in the circuit output high voltage, and both MOSFETs are in the on/off state. The battery can be charged and discharged freely. Considering that the on/off impedance of MOSFETs is relatively small, generally less than 30 milliohms, their on/off impedance has a relatively small impact on the performance indicators of the circuit. The current consumption of the protection circuit in this state is μ A, generally less than 7 μ A.

2. Overcharge protection

The required charging method for lithium-ion batteries is constant current power supply/constant voltage. In the initial stage of charging, the constant current power supply is used for charging. As the charging process progresses, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require a constant voltage value of 4.1V), and then switch to constant voltage charging until the current decreases. During the entire process of charging the battery, if the charger circuit loses control, it will cause the battery voltage to exceed 4.2V and continue to charge with a constant current power supply. At this time, the battery voltage will continue to rise. When the battery voltage is charged beyond 4.3V, the chemical side reactions of the battery will intensify, leading to battery damage or safety issues. In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V (this value is determined by the control IC, and different ICs have different values), its "CO" pin will switch from high voltage to zero voltage, causing V2 to switch from on/off to off, thereby cutting off the charging control circuit and preventing the charger from charging the battery again, providing overcharge protection. At this point, considering the presence of V2's built-in body diode VD2, the battery can discharge external loads through this diode. Between the detection of battery voltage exceeding 4.28V by the control IC and the issuance of the shutdown V2 signal, there is also a delay time period. The length of this delay time is determined by C3 and is generally set to about 1 second to avoid misjudgment caused by interference.

3. Over discharge protection

During the entire process of discharging the external load, the voltage of the battery will gradually decrease. When the battery voltage drops to 2.5V, its capacity has been fully discharged. If the battery continues to discharge the load at this time, it will cause permanent damage to the battery. During the entire process of battery discharge, when the control IC detects that the battery voltage is below 2.3V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will switch from high voltage to zero voltage, causing V1 to switch from on/off to off, thereby cutting off the discharge control circuit and preventing the battery from discharging the load, providing over discharge protection. At this point, considering the presence of the built-in body diode VD1 in V1, the charger can charge the battery through this diode. Considering that the battery voltage cannot decrease further in the over discharge protection state, it is required that the consumption current of the protection circuit be extremely small. At this time, the control IC will enter a low-power state, and the power consumption of the entire protection circuit will be less than 0.1 μ A. Between the detection of battery voltage below 2.3V by the control IC and the issuance of the shutdown V1 signal, there is also a delay time period. The length of this delay time is determined by C3 and is generally set to around 100 milliseconds to avoid misjudgment caused by interference

4. Short circuit protection

During the entire process of discharging the battery to the load, if the control circuit current is large enough to make U>0.9V (this value is determined by the control IC, and different ICs have different values), the control IC will determine that the load is short circuited. Its "DO" pin will quickly transition from high voltage to zero voltage, causing V1 to transition from on/off to off, thereby cutting off the discharge control circuit and providing short-circuit protection. The delay time of short-circuit protection is extremely short, generally less than 7 microseconds. Its working principle is similar to overcurrent protection, but the judgment method and protection delay time are different. In addition to the control IC, there is another important component in the circuit, which is MOSFET. It plays the role of a power switch in the circuit. Considering that it is directly connected in series between the battery and the external load, its on/off impedance has an impact on the performance indicators of the battery. When a good MOSFET is selected, its on/off impedance is relatively small, the internal resistance of the battery pack is small, the load capacity is strong, and it consumes less energy during discharge.

5. Overcurrent protection

Considering the chemical characteristics of lithium-ion batteries, battery manufacturers have stipulated that the maximum discharge current cannot exceed 2C (C=battery capacity/hour). When the battery is discharged beyond 2C current, it will cause permanent damage to the battery or safety issues. During the entire process of discharging the battery under normal load conditions, when the discharge current passes through two MOSFETs connected in series, considering the on/off impedance of the MOSFETs, a voltage is generated at both ends, which is U=I * RDS * 2. RDS is the on/off impedance of a single MOSFET. The "V -" pin on the control IC detects this voltage value. If the load is abnormal for some reason and the control circuit current increases, when the control circuit current is large enough to make U>0.1V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to change from on/off to off, thereby cutting off the discharge control circuit. Make the current in the control circuit zero, providing overcurrent protection. Between the detection of overcurrent by the control IC and the issuance of the turn off V1 signal, there is also a delay time period. The length of this delay time is determined by C3, generally around 13 milliseconds, to avoid misjudgment caused by interference. In the entire process of the above control, it can be seen that the magnitude of the overcurrent detection value depends not only on the control value of the control IC, but also on the on/off impedance of the MOSFET. When the on/off impedance of the MOSFET is larger, the overcurrent protection value for the same control IC is smaller.

Design considerations for 26650 lithium battery protection board:
1. Overcharge detection voltage: In general, the voltage between VDD-VSS gradually increases as the voltage at the CO terminal changes from high to low.
2. Overcharge release voltage: In the charging state, VDD slowly decreases to the voltage between VDD-VSS when the CO terminal changes from low level to high level.
3. Overdischarge detection voltage: Generally speaking, in the state, the voltage between VDD-VSS gradually decreases until the DO terminal changes from high level to low level.
4. Over discharge release voltage: In the over discharge state, the voltage between VDD-VSS gradually rises to the level where the DO terminal changes from low to high.
5. Overcurrent 1 detection voltage: In general, VM gradually rises to DO from high level to low level, and the voltage between VM-VSS is maintained.