Analysis and solutions of common bad causes of lithium battery protection board

1. No flash, low output voltage, no load
For such defects, we should first rule out the bad battery (the battery originally has no voltage or the voltage is low). If the battery is bad, the self-consumption of the protection board should be checked to see if the self-consumption of the protection board is too large and the battery voltage is low. Assuming that 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, PCB internal circuit is blocked, vias are blocked, MOS, IC is damaged, etc.). detailed analysis

The process is 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). These check points should all be 3.8V. If not, there is a problem with this circuit.

1. The voltage at both ends of the FUSE has been changed: check whether the FUSE is turned on, if the lead is normal, the internal circuit of the PCB board is not open; if the lead is not normal, the FUSE has problems (bad incoming materials, 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. The voltage at both ends of the R1 resistor has changed: check the resistance value of R1. If the resistance value is abnormal, it may be a false welding and the resistor itself is cracked. If the resistance value is not abnormal, there may be a problem with the internal resistance of the IC.
3. The voltage at the IC test terminal is changed: the Vdd terminal is connected to the R1 resistor. The Dout and Cout terminals are abnormal, it is because the IC is soldered or damaged.
4. If there is no change in the previous voltage, check that the voltage between 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. If the voltage of MOS tube 2, 3, 6 and 7 has changed, it indicates that the MOS tube is abnormal.
2. If the voltage of the MOS tube is unchanged and the P- terminal voltage is abnormal, it is because the negative via of the protection board is blocked.


2. No short circuit protection
1. There is a problem with the resistance of the VM terminal: a multimeter can be used to connect one meter pen to the IC2 pin, and one meter pen to the MOS pin connected to the VM terminal resistance to confirm its resistance value. Check the resistance and IC, MOS pin for false soldering.
2. IC and MOS malfunction: Because the over-discharge protection and over-current and short-circuit protection share a MOS tube, if the short-circuit malfunction is due to MOS problems, the board should have no over-discharge protection function.
3. The above are defects under normal conditions, and there may also be short circuits caused by poor IC and MOS equipment. For example, in the BK-901 that appeared in the previous period, the delay time in the IC with the model number ‘312D’ was too long, which caused the MOS or other components to be damaged before the IC took the corresponding action. Note: The easiest and most direct way to confess whether the IC or MOS is malfunctioning is to replace the suspected components.


3. short circuit protection without self-recovery
1. The IC used in planning has no self-recovery function, such as G2J, G2Z, etc.
2. The short-circuit recovery time of the instrument is too short, or the load is not removed during the short-circuit test. For example, the test leads are not removed from the test end after the short-circuit test leads are short-circuited with the multimeter voltage file (the multimeter is equivalent to a load of several megabytes).
3. Leakage between P+ and P-, such as impurity rosin between the pads, impurity yellow glue or P+, P- capacitance breakdown, ICVdd to Vss breakdown. (The resistance is only a few K to a few hundred K).
4. Assuming that the above is no problem, maybe the IC is broken down, you can check the resistance between the pins of the IC.


4. large internal resistance
1. Because the internal resistance of MOS is relatively stable, the internal resistance is large, the first suspect should be the relatively simple components such as FUSE or PTC.
2. Assuming that the resistance of FUSE or PTC is normal, check the resistance of the via hole between the P+ and P- pads and the component surface depending on the structure of the protection board. Perhaps the via has a micro-break phenomenon and the resistance is relatively large.
3. Assuming that there are no problems with the above, it is necessary to doubt whether the MOS is abnormal: firstly, confirm whether there is a problem with the welding; secondly, the thickness of the board (whether it is simply bent), because the bending may cause the pin welding to be abnormal; then the MOS tube Put it under a microscope to observe whether it is broken; after all, check the resistance of the MOS pin with a multimeter to see if it is broken down.

5. ID malfunction
1. The ID resistance itself is abnormal due to virtual welding, cracking or the resistance material is not closed: the two ends of the resistance can be welded from the beginning, if the ID is normal after the rewelding, the resistance is virtual welding, if it is cracked, the resistance will be cracked after the rewelding open.
2. ID via is not conductive: use a multimeter to check both ends of the via.
3. The internal circuit has problems: you can scrape the solder mask to see if the internal circuit is disconnected or short-circuited.