Battery Management System
(Project finished in 2015)

Description
Communication protocol
Fast charging procedure
Central unit (prototype)
Host protocol
Setup utility program for PC
Downloads

The central unit has been built using an dsPIC30F4011 DSP microcontroller. This unit uses serial port (UART1) for communicating with the battery via optocouplers as well as for communicating with external computer (e.g. PC) via same port.

As the same communication port is shared between two devices, the communication protocol with the external computer is bit more complicated...

The central unit uses LCD display (4*20 or 2*16) for displaying the status of the battery, charge progress, etc.

The first prototype has been built with the 4*20L CD display and has following features:

• Display input voltage

• Display output voltage

• Isolated serial port for communication with the battery

• Isolated serial port for communication with the external computer

• Display the temperature of the power supply (7-12V) power stage

• Display cell No., the voltage and the status of the cell with highest voltage

• Display  cell No., the voltage and the status of the cell with lowest voltage

• Display the battery pack status (any cell bleeding, all cells bleeding, etc.)

• Displays the PWM duty cycle of the charger

• Displays time

• Has integrated charger for battery with two LiPo cells, current up to 1A

• Accepts input power in voltage range from 12V, up to 30V (28kHz buck converter)

• Connects to external computer via isolated RS232 port, which is shared with cell modules

With this prototype are developed all programs needed for

• LC display (alphanumeric display, 4 bit data bus)

• Math library

• Serial communication service routines

• PWM service routine

• Communication protocols for battery and the external computer

The firmware is written in ASM30, i.e. all software has been developed without any third party libraries and it is written directly in machine code, using ASM30 assembler.

The module accepts battery packs up to 255 cells and automatically detects the number of cells in the battery as well as it reads all calibration constants from cell modules during the init procedure.

The module uses internal R/C oscillator (~7.19MHz in PLL*16 mode, ~34ns enabling instruction cycle) and is running in interrupt mode (the microcontroller is held in idle mode while waits for an interrupt) in order to reduce energy consumption.

The prototype has connected two LiPo cells (18650) with cell modules and polls the cells every half second (per cell) when the external power is not present and, when charging (external power is present), it polls the cells every 30ms per cell.
Next to do is to change polling system in such way that central module sends commands to cells without waiting for answer, but leaving enough time to first cell module to pass the message through.

The charging procedure is made as it is described in  Fast charging procedure, the difference is that the charging is controlled by voltage, not current all the time.
The battery is connected through 1 Ohm resistor (on it's negative side), which is used to measure the charging current. In moment when the external voltage is connected, the controller records first initial voltage of battery and then increases it in steps of 0.01V every second (this one second is initial time constant for increasing voltage) until:

• the charging current is above 1A
• the temperature of the power stage is above 50 deg. C
• any cell is bleeding

If any cell goes into high voltage alarm, the voltage is decreased by 0.01V, the time is increased by one (e.g. from one second to two seconds) but max to 32 seconds and charging is continued monitoring cells every 30ms per cell (total time 60ms to read both cells).
If both cells are found bleeding, same procedure as for high voltage alarm is applied.
If one cell is found bleeding, the timer has been reset.
If no one cell is found bleeding and timer expired time constant as well as the temperature of the power stage is below 50 deg. C, the voltage is increased again for 0.01V.
If temperature of the power stage increases above 56 deg. C, the output voltage is decreased for 0.01V, but the timer constant is left unchanged.

During the charging, the voltages of both cells are displayed on the display as well as the temperature of the power stage, charging current, PWM duty cycle and the input voltage (external power supply). The status of the battery is shown on the display with three characters, used to indicate presence of low alarm, high alarm and bleeding:

• b - one cell is bleeding
• B - both cell are bleeding
• l - one cell has activated low voltage alarm
• L - both cell have activated low voltage alarm
• h - one cell has activated high voltage alarm
• H - both cell have activated high voltage alarm

The central module uses same port (which is used for communication with the cells) for communication with the host computer (e.g. PC). The host communication protocol is explained here.