DIY Lithium Battery Health Monitor (SoC/SoH LED Display)

🔧 What You'll Build:

A compact device that displays State of Charge (SoC) and State of Health (SoH) of lithium batteries using LED bars. Perfect for salvaging old cells or DIY power banks!

📊 Key Features:

✅ Real-time monitoring via BQ27441 fuel gauge IC

✅ Dual LED bars for SoC/SoH (WS2812B NeoPixels)

✅ USB-C charging (BQ24075 IC)

✅ Open-source design (PCB/code/3D files included)

🎥 Demo: See it in action 👇🏻

Supplies

Core main Components

1. BQ27441 Fuel Gauge IC (Texas Instruments)
2. BQ24075 Charger IC (Texas Instruments)
3. ATmega328P MCU (Arduino-compatible)
4. WS2812B LED Bars (10x2 configuration)
5. 18650 Battery Holder
6. 0.01Ω Current-Sense Resistor (1206 package)
7. USB-C Connector (TYPEC-304-BCP16)

Full BOM list here ➡️ (GitHub repo)

PCB & Assembly

1. Custom PCB → Order from JLCPCB
2. Solder Paste (ChipQuik SMD291AX)
3. MHP50 Hot Plate
4. Flux Remover (Chemtronics Flux-Off)

Tools

1. Soldering Station (Hakko FX888D)
2. LINKMICRO Digital Microscope
3. ESD-Safe Tweezers (5-piece set)
4. ANENG AN8008 Multimeter

3D Printing

1. PLA Filament (Overture Black)
2. M2 Threaded Inserts

Software

1. Altium Designer → Full Project design files (Altium 365)
2. Arduino IDE

🔌 Schematic Overview

I designed this system around three core ICs:

1. BQ27441 Fuel Gauge - For accurate SoC/SoH measurement via I²C
2. BQ24075 Charger - Handles USB-C power input and battery charging
3. ATmega328P MCU - Processes data and drives LED indicators

Key Design Challenges Solved:

1. Precision Sensing: Added a 0.01Ω current-sense resistor (R10) for coulomb counting
2. Thermal Safety: Designed dual NTC support (onboard and external battery sensors)
3. Noise Reduction: Used star grounding and 100nF decoupling caps near all ICs

🛠️ Design Tools Used

1. Schematic Capture: Altium Designer
2. Component Sourcing: Selected JLCPCB-compatible parts for easy assembly

Pro Tip:

Always run a Design Rule Check (DRC) before finalizing! My schematic passed with zero errors thanks to Altium's real-time validation.

📁 Design Files:

1. Full Schematic PDF
2. Altium 365 Project

🛠️ Tools You’ll Need

1. Solder Paste (ChipQuik SMD291AX)
2. MHP50 Hot Plate
3. LINKMICRO Microscope
4. PCB Stencil (Included in GitHub repo’s Gerber files)

🔬 Assembly Process

1. Apply Solder Paste

1. Secure the PCB stencil over the board using tape.
2. Spread solder paste evenly with a squeegee (or old credit card).
3. Pro Tip: Chill the paste for 5 mins first for better viscosity!
4. I 3D printed a support to help me apply solder paste → (3D STL File here)

2. Place Components

1. Use ESD tweezers to position tiny parts (0402 resistors, ICs).
2. Start with the smallest components (resistors/caps) → larger ICs last.

3. Reflow with Hot Plate

1. Preheat the MHP50 hot plate to 150°C.
2. Place the PCB on the plate and heat until paste turns shiny (~220°C, 2-3 mins).
3. Critical: Let it cool naturally, no fans!

4. Inspect Under Microscope

1. Check each joint with the LINKMICRO microscope:
2. Good joint: Smooth, concave fillet.
3. Bad joint: Dull or ball-shaped (reflow with hot air).
4. Pay special attention to:
5. BQ27441’s tiny pins (SON-12 package)
6. USB-C connector (easy to bridge)

⚠️ Troubleshooting

1. Bridged Pads? Use copper braid w/ flux to wick excess solder.
2. Tombstoning? Redo paste application, uneven amounts cause this.
3. Cold Joints? Reheat with hot air gun at 250°C.

📌 Pro Tips

1. Stencil Alignment: Use PCB fiducials for perfect paste deposits.
2. Double-Sided Boards: Assemble bottom side first (larger components last).
3. No Hot Plate? A skillet works in a pinch (monitor with IR thermometer).

🛠️ What You’ll Need

1. 3D-Printed Parts (STL Files)
2. M2 Threaded Inserts and screws
3. Assembled PCB (from Step 2)
4. Soldering Iron (for insert installation)

🔧 Assembly Steps

1 Install Threaded Inserts

1. Heat the inserts with a soldering iron at 200°C and press into housing holes.
2. Pro Tip: Use an M2 screw to align inserts vertically while cooling.

2 Mount the PCB

1. Secure the board with M2 screws, ensuring USB-C port aligns with the housing cutout.
2. Connect the 18650 holder/JST connector.

3 Attach LED Diffusers

1. Snap the 3D-printed light guides over the WS2812B LEDs for even glow.

💻 Programming Setup

1. Tools Needed:
2. USBASP Programmer (for bootloader)
3. Arduino IDE (for firmware)

⚡ Flashing Process

1. Upload Bootloader (ISP Port)
2. Connect USBASP to the PCB’s 6-pin ISP header.
3. Select ATmega328P and burn Arduino Nano Bootloader.

1. Upload Firmware
2. Open Battery_health.ino in Arduino IDE.
3. Set board to "Arduino Nano" (same chip).
4. Adjust BATTERY_CAPACITY in code to match your cell (e.g., 3000 for 3000mAh).
5. You can get the code from here → GitHub direct link

🔋 Testing with Used Batteries

1. Insert Cells (try 3 scenarios):
2. Healthy cell: SoH ≈100%, SoC updates smoothly.
3. Aged cell (e.g., laptop pull): SoH <80%, LED bar shows degradation.
4. Dead cell: SoH <20%, verify charger blocks over-discharge.

1. Validate Accuracy
2. Compare SoC readings to a bench power supply at known voltages.

📌 Pro Tips

1. Bootloader Issues? Double-check ISP pin connections with a continuity tester.
2. No USBASP? Use an Arduino-as-ISP (tutorial in here).
3. Calibration: For lab-grade accuracy, log data in BQStudio (Texas Instruments’ tool).