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
CO
ใBO
ใCB
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