BMS

Introduction

PCM (Protection Circuit Module) designed for 8 series Lithium battery pack, with the state of art battery gas gauge controller IC (integrated Circuit) which constantly monitors cell temperature, individual cell voltages, pack current, remaining capacity, varies protection status and other critical battery parameters, generates control signals that may be used with a BMS (Battery Management System), manages and protects the battery, takes appropriate actions, makes the battery free from damage, maximize the effective battery life; it also communicates SOC (State of Charge) & SOH (State of Health) with host via the SMBus (System Management Bus) or RS232 (Radio Sector 232) interface, displaying remaining capacity, cell voltages and other useful information via LED module and OLED panel.

Functional descriptions

Measurements

Individual cell voltages, current flowing in and out of the battery & battery temperature are measured via on‐chip ADC (Analog-Digital Convertor).

NTC

NTC (Negative Temperature Coefficient) thermistor is used for battery temperature measurement and should be attached tightly to the battery surface.

Protection function

Pack over voltage protection

Protection will be triggered when total pack voltage reaches the preset value for a preset time period during Charging, the charge FET (Field Effect Transistor) and discharge FET will then be shut off. The protection will be released if the pack voltage is returned to the preset value or if discharge current is detected.

Cell over voltage protection

Protection will be triggered when any cell’s voltage reaches the preset value for a preset time period, the charge FET and discharge FET will then be shut off; The protection will be released if all cell’s voltage is returned to the preset value or if discharge current is detected.

Over charge current protection

Protection will be triggered when charge current reaches the preset value for a preset time period, the charge FET will then be shut off; The protection will be released after preset time period or if discharge current is detected.


High/low temperature protection in charging

Protection will be triggered when the pack temperature reaches the preset value for a preset time period during charging, the charge FET will then be shut off; the protection will be released if the pack temperature is returned to the preset value (range).

Pack under voltage protection

Protection will be triggered when the pack voltage below the preset value for a preset time period, the discharge FET will then be shut off; the protection will be released if the pack voltage is returned to the preset value or if charge current is detected.

Cell under voltage protection

Protection will be triggered when the pack voltage below the preset value for a preset time period, the discharge FET will then be shut off; The protection will be released if all cell’s voltage are returned to the preset value or if charge current is detected.

Over discharge current protection

Protection will be triggered when discharge current reaches the preset value for a preset time period, the charge FET will then be shut off; The protection will be released if average current below the preset value after present time period, or if charge current is detected.


High/low temperature protection in discharging

Protection will be triggered when the pack temperature reaches the preset value for a preset time period during discharging, the discharge FET will then be shut off; the protection will be released if the pack temperature is returned to the preset value (range).

Short circuit protection

Protection will be triggered when current reaches the preset short circuit protection value for a preset time period, the charge and discharge FET will then be shut off; the protection will be released after preset time period.



Cell balancing

It’s always necessary to connect multiple Lithium Polymer cells in series in order to increase the total pack voltage, however even go through serious cell matching processing, still there is difference between cells, main reasons are,

  • Tiny difference of characteristic (self-discharge rate, impedance, temperature coefficient,) between cells, even under same charging current, but different cell takes in different amount of charge and has different cell voltage.
  • Cells with almost same characteristics will behavior differently after number of charge/discharge cycles.
  • Such a pack undergoes certain cycle of operation, the tiny difference between cells will be maximized to be reflected as imbalance of cell voltage between series, the core of BMS such as protection and control is relied on cell voltages as a basic reference, when there is imbalance condition, some cell’s voltage will be raised faster during charging or be dropped faster during discharging, these will trigger over or under voltage protections, as a result causing the battery pack stop charging or discharging earlier, reducing the effective capacity and finally reduce the battery life.

    The cell balancing function of BMS helps to smooth out the high and low voltage between series, when the cell balancing function is enabled, whenever the cell voltage reaches the “Cell balancing start voltage”, and also the voltage difference between the highest and lowest cell reaches the “Cell balancing window”, the cell balancing will be started.

    Other Uses

    SMBus communication

    BMS supports SMBus 2.0 communication protocol for smart battery system (SBS), firmware changes to the parameter settings can be done by connecting the BMS Smart interface device. So battery can be monitored remotely.


    RS232 communication

    The RS232 port on the PCM is used to communicate with host computer system to exchange different kinds of battery information and later the parameters if desired by the field use. This change can be affected immediately and the host device can be deployed with the new settings.

    LED display

    An external LED (Light Emitting Diode)display module can be connected to the on board LED interface to display the remaining capacity and battery status, a push button on the LED module is necessary to turn the display on, in active and idle mode, LED will be turned on for 5 seconds when the push button is pressed.

    OLED display

    An external OLED (Organic Light Emitting Diode) display module can be connected to the on board OLED interface to display the remaining capacity and almost all kinds of useful battery status.

    Event count & and misc. Information

    BMS supports data logging function, while in active mode and idle mode, connecting the PCM to computer via BMS Smart Interface, user can review or to reset the usage data logged inside the PCM.