Battery Over/Under-Voltage Window Comparator PCB Design

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1. Project Overview

In modern Battery Management Systems (BMS), maintaining battery cell voltages within a safe operational window is vital to prevent deep-discharge degradation or overcharge thermal runaway.

This project implements an autonomous hardware-level Window Comparator using dual precision LM741 Operational Amplifiers. It continuously senses the battery terminal voltage in real-time and provides immediate visual alerts through dedicated fault LEDs without requiring a microcontroller.

Key Features:

• Under-Voltage Indicator (Low Battery): Red LED 1 turns ON when battery voltage falls below 5.18 V.
• Normal Operating Window (Safe Zone): Both LEDs remain OFF while the battery voltage stays between 5.18 V and 12.94 V.
• Over-Voltage Indicator (Over-Charge): Red LED 2 turns ON when battery voltage exceeds 12.94 V.
• Decoupling Protection: Integrated 100 nF high-frequency ceramic capacitor to suppress power rail switching transients.

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2. Circuit Schematic

The schematic was designed and captured in eSim Eeschema with complete Electrical Rules Check validation (0 Errors).

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3. Mathematical Calculations & Working Principle

A. Reference Voltage Ladder (Supply V_CC = 12 V):

• Total Resistance:
  R_total = R1 (4.7 k) + R2 (3.3 k) + R3 (2.2 k) = 10.2 k Ohm

• Upper Threshold Reference (V_high_ref):
  V_high_ref = 12 V * (R2 + R3) / R_total = 12 V * (5.5 k / 10.2 k) = 6.47 V

• Lower Threshold Reference (V_low_ref):
  V_low_ref = 12 V * R3 / R_total = 12 V * (2.2 k / 10.2 k) = 2.59 V

B. Battery Voltage Sensing Divider (R4 = R5 = 10 k Ohm):

• Scaled Sense Voltage (V_sense):
  V_sense = V_battery * R5 / (R4 + R5) = V_battery * (10 k / 20 k) = 0.50 * V_battery

C. Threshold Mapping to Actual Battery Voltages:

• Low-Battery Fault Cutoff:
  V_batt_low = V_low_ref / 0.50 = 2.59 V / 0.50 = 5.18 V

• Over-Voltage Fault Cutoff:
  V_batt_high = V_high_ref / 0.50 = 6.47 V / 0.50 = 12.94 V

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4. Operational Truth Table

Battery Voltage (V_batt)	Scaled Sense (V_sense)	Comparator U1 (Low-Batt)	Comparator U2 (Over-Volt)	LED 1 (Low-Batt)	LED 2 (Over-Volt)	Operating Status
< 5.18 V	< 2.59 V	HIGH (~11.5 V)	LOW (0 V)	ON (Red)	OFF	Under-Voltage Fault
5.18 V - 12.94 V	2.59 V - 6.47 V	LOW (0 V)	LOW (0 V)	OFF	OFF	Normal (Safe Window)
> 12.94 V	> 6.47 V	LOW (0 V)	HIGH (~11.5 V)	OFF	ON (Red)	Over-Voltage Fault

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5. Bill of Materials (BOM) & Footprint Assignment

RefDes	Qty	Value / Component	Function	KiCad Footprint (CvPcb)
X1, X2	2	LM741	Operational Amplifier ICs	Housings_DIP:DIP-8_W7.62mm
R1	1	4.7 k Ohm, 0.25 W	Reference Ladder (Top)	Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal
R2	1	3.3 k Ohm, 0.25 W	Reference Ladder (Middle)	Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal
R3	1	2.2 k Ohm, 0.25 W	Reference Ladder (Bottom)	Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal
R4, R5	2	10 k Ohm, 0.25 W	Battery Sense Divider	Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal
R6, R7	2	330 Ohm, 0.25 W	LED Current Limiters	Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal
D1, D2	2	5mm Standard LED	Status Indicator LEDs	LEDs:LED_D5.0mm
C1	1	100 nF (0.1 uF)	HF Decoupling Capacitor	Capacitors_THT:C_Disc_D3.0mm_W1.6mm_P2.50mm
v1, v2	2	2-Pin Screw Terminal	Power & Sense Ports	Pin_Headers:Pin_Header_Straight_1x02_Pitch2.54mm

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6. 2-Layer PCB Design Specifications

The PCB was laid out and routed in Pcbnew adhering to IPC standards:

• Copper Layers: 2 Layers (Top Layer F.Cu in Red, Bottom Layer B.Cu in Green).
• Track Width: 0.250 mm (9.84 mils) signal traces with 45 degree mitered bends.
• Routing Strategy: Orthogonal trace routing to eliminate track collisions without external jumpers.
• DRC Verification: 0 Errors, 0 Unconnected Pads (20/20 Connections Complete).
• Manufacturing Outputs: Industrial standard RS-274X Gerber and Excellon drill files generated in ./Gerber/.

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7. How to Run the Project in eSim

1. Clone the repository:

   git clone https://github.com/krisanthM/eSim_PCB_Design_Project_Files.git

2. Open in eSim:

   • Launch eSim v2.3 / v2.5.
   • Click Open Project -> Select the battery_comparator directory.

3. Verify Schematic:

   • Open Eeschema -> Click Tools > Perform Electrical Rules Check (ERC) -> Verify 0 Errors.

4. Run Simulation:

   • In eSim, click Convert KiCad to Ngspice -> Click Simulation.

5. Inspect PCB Layout:

   • Click PCB Layout to launch Pcbnew -> Click Tools > Perform Design Rules Check (DRC).
   • Press Alt + 3 to view the 3D board render.

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8. Repository File Structure

├── battery_comparator.sch              # Schematic file (Eeschema)
├── battery_comparator.kicad_pcb         # 2-Layer PCB Layout (Pcbnew)
├── battery_comparator.net              # PCB Netlist
├── battery_comparator.cir              # Ngspice Netlist
├── battery_comparator.pro              # Project Configuration
├── Gerber/                             # Gerber & Drill Manufacturing Files
├── report_esim.pdf                     # Detailed Project Report
├── battery_comparator.png              # Circuit Schematic Image
└── README.md                           # Project Documentation

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9. Author & Acknowledgements

• Author: Krisanth M (@krisanthM)
• Email: krisanth2006@gmail.com
• Organization: FOSSEE, Indian Institute of Technology Bombay (IIT Bombay)
• Program: eSim Semester-Long Internship -- Autumn 2026 (Task 7 Submission)