5 string 21700 battery protection board

With BYD's charge and discharge management IC BM3451 as the core, it is suitable for charging and discharging integrated protection boards for 5-cell 21700 lithium battery packs connected in series.
The function has been verified and is part of the future IP5389 mobile power supply project. Due to the size limitations of the case, the battery pack composed of this PCB cannot be placed in the aluminum case. A smaller protective board needs to be redesigned, so this project will not be implemented in the future. Update again.
If you have a cheap and easy-to-use MOS tube, please leave a message in the comment area.
 
 
 
Device description:
The MOS tubes for protection (U2~U5) can be 30V. The smaller the internal resistance, the better.
The current sensing resistor is selected according to the size of the protection current. This design is 20A, so a 5mΩ resistor is welded.
The current sharing MOS transistors (Q1~Q5) can use NMOS in the same package.
The current sharing resistor (such as R17) has an indicator light reserved, and the LED can be soldered to check the battery balancing status.
If the battery capacity gap in the battery pack is too large, it will affect the overall discharge performance of the battery pack. It is recommended to replace the deteriorated batteries.
The power of a 1206 resistor is 1W. It is recommended that 2-3 resistors be connected in parallel to share the power.
PCB thickness can be selected as 1.0mm, taking into account strength and volume.
 
Welding precautions:
The 8.4V and 16.8V battery connection holes have reserved nickel ribbon pads. Using nickel ribbon connections can reduce the size of the battery pack.
The P-pad is close to the protective MOS tube. When welding, clean the silicone wire and solder to be careful of short circuit.
There are window opening areas reserved at P+ and P-, which can lead to power supply through copper strips to reduce the size of the battery pack. It is recommended to delete them if they are not needed.
 
Debugging instructions:
It is recommended to cooperate with the "5-string 21700 Protection Board-Protection Function Verification Module" to verify the functionality of the protection board before welding it to the battery.
The verification board uses an electrolytic capacitor to simulate battery charging and discharging. It simulates battery overcharging by adjusting the input voltage and measures whether the overcharge protection voltage of 4.20V is correct. The output is connected to a load to simulate battery discharge and overdischarging and measures whether the overdischarge voltage is correct.
 
Fixing methods for reference: