In this Instructable the design of the BMS345 will be explained. The design is fully open-source, the design files can be found in the GitHub link in the last step. There is also a limited supply available on Tindie.

The BMS345 is a BatteryManagementSystem that supports 3, 4 and 5 cells lithium-ion packs. If you build/buy a pack with unprotected cells, this PCB can be added to handle protection and charging. This includes:

Under/overvoltage protection
Overcurrent (/shortcircuit) protection
Cell balancing
MPPT charging

The documentation will be divided into:

Protection
Charging
Configuration
The end product

Enjoy :)

Step 1: Protection

The protection is handled by the TI BQ77915.

The input resistors are 1K, which sets balancing current to 4mA/cell
The header is the commonly used JST-XH 4/5/6P depending on the configuration
A NTC can be connected to header J5, but this features is disabled by default by R26
The negative connection is switched by a dual N-channel mosfet (NVMFD5C466NL)
The current sense resistor is 2x8m (4m equivalent) ohm, setting the current protection to 15A

Step 2: Charging

The charging is handled by the TI BQ24650

D1 shows the charging status, an external LED can be connected via J4
R30 sets the charge current to 1A
Temperature sensing is disabled by R13/R14/C14
MPPT voltage is set to 17.2V by R22 and R28
The mosfets are the same dual-package type as is used in the protection circuit
The default charge voltage is 4.2V, which barely trips the overvoltage protection of the BQ77915. It is recommended to populate R36 with 22M to lower the charge voltage to 4.05V/cell. This avoids false over-voltage triggering.

Charging can be done from a 24V 1A supply or even a solar panel (only for 3/4S configurations).