The composition of the lithium battery protection board, the main function of the battery protection board
The composition and main function of the lithium battery protection board
This article mainly introduces the composition of the lithium battery protection board, the main function and working principle of the battery protection board. And the application range of the single-cell lithium battery protection circuit produced, electrical performance parameters, main materials, size specifications, and other related items. All project standards described in this specification can be used as quality inspection standards and basis.
Battery protection circuit board BMS application:
(1) Liquid lithium ion rechargeable battery
(2) Polymer lithium ion rechargeable battery
The composition of the battery protection board
The reason why lithium battery (rechargeable type) needs protection is determined by its own characteristics. Since the material of the lithium battery itself determines that it cannot be overcharged, overdischarged, overcurrent, short circuited, and ultra-high temperature charging and discharging, the lithium battery components of the lithium battery will always appear with a delicate protection board and a current fuse. The protection function of the lithium battery is usually completed by the protection circuit board and PT. The protection board is composed of electronic circuits. It can accurately monitor the voltage of the battery cell and the current of the charging and discharging circuit under the environment of -40℃ to +85℃. Control the on and off of the current loop; PTC prevents severe damage to the battery in a high temperature environment.
The protection board usually includes control IC, MOS switch, resistor, capacitor and auxiliary device NTC, ID memory and so on. Among them, the control IC controls the MOS switch to turn on under all normal conditions to make the cell communicate with the external circuit. When the cell voltage or loop current exceeds the specified value, it immediately controls the MOS switch to turn off (tens of milliseconds). Protect the safety of batteries. NTC is the abbreviation of Negative temperature coefficient, which means negative temperature coefficient. When the ambient temperature rises, its resistance decreases. The electrical equipment or charging equipment is used to respond in time and control internal interruptions to stop charging and discharging. ID memory is often a single-wire interface memory. ID is the abbreviation of Identification, which means identification, and stores information such as battery type and production date. Can play a role in product traceability and application restrictions.
The main function of the protection board
Generally, it is required that Control (IC) detect the voltage of the control cell and the working current and voltage of the charging and discharging circuit at -25℃～85℃. Under all normal conditions, the C-MOS switch tube is turned on, so that the cell and the protection circuit board are in contact with each other. Normal working state, and when the cell voltage or the working current in the loop exceeds the preset value of the comparison circuit in the control IC, the CMOS is turned off within 15-30ms (different control ICs and C-MOS have different response times) , That is, close the battery discharge or charging circuit to ensure the safety of the user and the battery.
The working principle diagram of the protection board:
1. Overcharge protection and overcharge protection recovery
When the battery is charged and the voltage exceeds the set value VC (4.25-4.35V, the specific overcharge protection voltage depends on the IC), VD1 flips to make Cout low, T1 ends, and charging stops. When the battery voltage drops to VCR (3.8-4.1V, the specific overcharge protection recovery voltage depends on the IC), when Cout becomes high, T1 is turned on and charging continues, VCR must be a fixed value less than VC to prevent frequent jumps.
2. Over-discharge protection and over-discharge protection recovery
When the battery voltage drops to the set value VD (2.3-2.5V, the specific overcharge protection voltage depends on the IC) due to discharge, VD2 reverses, after a short time delay, Dout becomes low level and T2 ends. Discharge stops. When the battery is placed in charge, the internal OR gate is flipped to make T2 turn on again to prepare for the next discharge.
3. Over current and short circuit protection
When the charging and discharging loop current of the circuit exceeds the set value or is short-circuited, the short-circuit detection circuit acts to turn off the MOS tube and the current is cut off.
The function introduction of the main parts of the protection board
R1: Reference power supply resistance; forms a voltage divider circuit with the internal resistance of the IC to control the level inversion of the internal overcharge and overdischarge voltage comparator; generally the resistance value is 330Ω and 470Ω; when the package form (that is, the standard component The length and width indicate the size of the component. For example, the length and width of the component in the 0402 package mark are 1.0mm and 0.5mm respectively). When it is larger, the resistance value will be marked by numbers. For example, the number mark 473 on the chip resistor indicates its resistance. The resistance value is 47000Ω or 47KΩ (the third digit represents the digit with 0 after the first two digits).
R2: Over-current and short-circuit detection resistance; the current of the protection board is controlled by detecting the voltage of the VM terminal. Poor welding or damage will cause the battery to over-current and short-circuit without protection. The general resistance values are 1KΩ and 2KΩ.
R3: ID identification resistor or NTC resistor (described earlier) or both. Summary: The resistance is a black patch in the protection board. The resistance value can be measured with a multimeter. When the package is large, the resistance value will be represented by numbers. The method of expression is as described above. Of course, the resistance value of All resistors have precision specifications. For example, if the 10KΩ resistance specification is +/-5% accuracy, the resistance value is within the range of 9.5KΩ to 10.5KΩ.
C1, C2: Since the voltage at both ends of the capacitor can't change suddenly, it plays the role of instant voltage stabilization and filtering.
Summary: The capacitor is a yellow patch in the protection board. There are many types of 0402 packages, and there are also a few 0603 packages (1.6mm long and 0.8mm wide); the resistance value of the multimeter is generally infinite or MΩ level; the leakage of the capacitor will be generated from High power consumption, no self-recovery in short circuit. FUSE: Ordinary FUSE or PTC (abbreviation of Positive Temperature Coefficient, meaning positive temperature coefficient); to prevent unsafe high current and high temperature discharge from occurring, and PTC has a self-recovery function.
Summary: FUSE is generally a white patch in the protection board. The FUSE provided by LITTE will mark the character DT on FUSE. The character means the rated current that FUSE can withstand. For example, it means that the rated current of D is 0.25A, S is 4A, T It is 5A, etc.; currently, most of our company's FUSEs with a rated current of 5A, that is, the character'T' is marked on the body.
U1: Control IC; all functions of the protection board are realized by the IC by monitoring the voltage difference between VDD-VSS and the voltage difference between VM-VSS and controlling the C-MOS to perform switching actions.
Cout: Overcharge control terminal; the switch of the MOS tube is controlled by the gate voltage of the MOS tube T2.
Dout: Over-discharge, over-current, short-circuit control terminal; control the switch of MOS tube through the gate voltage of MOS tube T1.
VM: Over-current and short-circuit protection voltage detection terminal; realize the circuit's over-current and short-circuit protection (U(VM)=I*R(MOSFET)) by detecting the voltage of VM terminal.
Summary: IC in the protection board is generally a 6-pin package. The method of distinguishing the pins is: the first pin is marked near the black dot on the package, and then the second pin is rotated counterclockwise. , 4, 5, 6 pins; if there is no black dot mark on the package body, look at the lower left of the characters on the package body as the first pin, and the remaining pins are analogously counterclockwise) C-MOS: field effect switch tube; protection function Realizers; continuous welding, virtual welding, false welding, and breakdown will cause unprotected batteries, no display, low output voltage and other undesirable phenomena. Summary: CMOS in the protection board
Generally, it is a package with 8 pins. It is composed of two MOS transistors, which are equivalent to two switches, which control overcharge protection and overdischarge, overcurrent, and short circuit protection respectively. The pin distinction method is the same as that of IC.
Under normal conditions of the protection board, Vdd is high, Vss, VM are low, Dout, Cout are high; when any parameter of Vdd, Vss, VM is changed, the level of Dout or Cout will occur Change, at this time the MOSFET performs corresponding actions (opening and closing the circuit), so as to realize the protection and recovery function of the circuit.
Main performance test methods of battery protection board
1. NTC resistance test: use a multimeter to directly measure the NTC resistance value, and then compare with the "temperature change and NTC resistance comparison guide".
2. Identification resistance test:
Use a multimeter to directly measure the identification resistance value, and then compare it with the "Protection Board Important Project Management Table".
3. Self-consumption test:
Adjust the constant current source to 3.7V/500mA; set the multimeter to uA, insert the test leads into the uA jacks, and connect them in series with the constant current source to the protection boards B+ and B- as shown in the figure below: the reading of the multimeter is the protection board at this time If there is no reading, use tweezers or tin wire to short circuit B- and P- to activate the circuit.
4. Short circuit protection test:
Connect the battery core to the protection board B+, B-, use tweezers or tin wire to short-circuit B-, P-, and then short-circuit P+, P-; after short-circuit, use a multimeter to measure the open circuit voltage of the protection board (as shown in the figure below); repeat; Short-circuit 3-5 times. At this time, the multimeter reading should be consistent with the battery cell, and the protection board should be free from smoke or bursting.
Common bad analysis of lithium battery protection board
No display, low output voltage, no load:
For such defects, first eliminate the bad battery (the battery has no voltage or the voltage is low). If the battery is bad, the self-consumption of the protection board should be tested to see if the self-consumption of the protection board is too large and the battery voltage is low. If the cell voltage is normal, it is because the entire circuit of the protection board is blocked (components are soldered, false soldered, FUSE is poor, the internal circuit of the PCB board is blocked, the via is blocked, MOS, IC is damaged, etc.). detailed analysis
Proceed as follows:
(1) Connect the black test lead of a multimeter to the negative electrode of the battery cell, and connect the red test lead to the FUSE, R1 resistor terminals, the Vdd, Dout, and Cout terminals of the IC, and the P+ terminal (assuming the battery cell voltage is 3.8V), and analyze it section by section. These test points should all be 3.8V. If not, there is a problem with this section of the circuit.
1. There is a change in the voltage across the FUSE: test whether the FUSE is turned on, if it is turned on, the internal circuit of the PCB board is not turned on; if it is not turned on, there is a problem with the FUSE (poor incoming material, overcurrent damage (MOS or IC control failure), There is a problem with the material (the FUSE is burned out before the MOS or IC is activated), then use a wire to short the FUSE, and continue to analyze later.
2. There is a change in the voltage across the R1 resistor: test the resistance value of R1. If the resistance value is abnormal, it may be a virtual weld and the resistor itself is broken. If the resistance value is not abnormal, there may be a problem with the internal resistance of the IC.
3. There is a change in the voltage of the IC test terminal: the Vdd terminal is connected to the R1 resistor. Dout, Cout end abnormal, it is due to the IC weak welding or damage.
4. If there is no change in the previous voltage and the voltage between test B- and P+ is abnormal, it is because the positive via of the protection plate is blocked.
(2) Connect the red test lead of the multimeter to the positive electrode of the battery. After activating the MOS tube, the black test lead is connected to the MOS tube 2, 3, 6, 7 and P- terminal in turn.
1. There is a change in the voltage of MOS tube 2, 3, 6 and 7 pins, which indicates that the MOS tube is abnormal.
2. If there is no change in the voltage of the MOS tube and the voltage at the P- terminal is abnormal, it is because the negative through hole of the protection plate is blocked.
No short circuit protection:
1. There is a problem with the resistance of the VM terminal: use a multimeter to connect one test pen to the IC2 pin, and one test pen to the MOS pin connected to the VM terminal resistor to confirm the resistance value. Check whether the resistance and IC, MOS pins are soldered.
2. IC and MOS abnormality: Since the over-discharge protection and over-current and short-circuit protection share the same MOS tube, if the short-circuit abnormality is due to a MOS problem, the board should have no over-discharge protection function.
3. The above are defects under normal conditions, and short-circuit abnormalities caused by poor IC and MOS configurations may also occur. For example, in the BK-901 that appeared in the previous period, the delay time in the IC with the model of ‘312D’ was too long, which caused the MOS or other components to be damaged before the IC took the corresponding action control.
Note: The easiest and direct way to determine whether an IC or MOS is abnormal is to replace the suspected components.