BMS circuit design

Introduction

This project is for the Formula student competition. I designed this battery management system(BMS) to manage a 12V battery pack on a electrical race car and handle battery overcharge or overdischarge behaviours, which extend battery life and prevent explosion of batteries.

CAD and Circuit

3D model

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Schematic

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Layout

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Chip

Datasheet: S8209A_AN
Product name: S-8209ABB-T8T1U

Spec

Overcharge voltage(Detection)(VCU): 4.35V
Overcharge voltage(Release)(VCL): 4.35V

Cell balance voltage(Detection)(VBU): 4.2V
Cell balance voltage(Release)(VBL): 4.2V

OverDischarge voltage(Detection)(VDL): 2.5V
OverDischarge voltage(Release)(VDU): 2.7V


Operation and Schematic Explaination

This circuit is designed to deal with battery overcharge and overdischarge. It will be explained by a practical example.

The symbol used below can be found here

Normal status

COใ€BOใ€CB of three IC are all low impedance.

Overcharge

If voltage of BAT_3 reached VBU.

  • Step 1

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    CB of IC3 will be pulled up to VDD3 and turn Q6 on.
    Charge current is bypassed since Q6 is on.

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    Bypassing the current doesn't mean BAT_3 is no longer charged. The charge rate is only slower than BAT_1, BAT_2.
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    This mechanism means any VDS >= VBU will trigger CB and turn MOS on.

  • Step 2
    For BAT_3, VDS >= VCU, a series of actions will conduct.
    If VDS >= VCU:
    โ†’ CO = VSS3 (Set to high impedance)
    โ†’ CTLC of IC2 = VSS3
    โ†’ CO of IC2 = VSS2
    โ†’ CTLC of IC1 = VSS2
    โ†’ CO of IC1 = VSS1
    โ†’ Turn off Q2 then turn off Q8
    โ†’ Charging loop is cut.

  • Step 3
    It is in normal status again if all BAT's VDS <= VBL

OverDischarge

If BAT_1, BAT_3 are in normal status and BAT_2 is overdischarged.

  • Step 1

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    VDS of Q5 is below 2.5V

  • Step 2

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    DO of IC2 is set to high impedance, indicate IC1 by CTLD.

  • Step 3

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    The voltage of IC1's CTLD drop will cause DO in High-Z and CB is on high-voltage:
    CB on โ†’ opens Q3 and BAT_1 is short circuit
    DO high โ†’ turns off Q1

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    Heat Problem
    When BAT_1 is short circuit, heat is generated.

    On the PCB layout, it can be found that the pads of R6 R11 R18 are larger than normal ones. This is for heat dissipation and high walts resistors are selected to withstand the temperture rise.

  • Step 4
    Since Q1 is turned off, Q9 will be turned off. The loop is cut.

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Symbol

Overcharge voltage(Detection): VCU
Overcharge voltage(Release): VCL

Cell balance voltage(Detection: VBU
Cell balance voltage(Release): VBL

OverDischarge voltage(Detection): VDL
OverDischarge voltage(Release): VDU

Mosfet Drain to Source voltage: VDS

VSS1: VSS of IC1
VSS2: VSS of IC2
VSS3: VSS of IC3