Battery Pack Testing

Project Description

This project measures the energy capacity of battery packs by sampling the voltage across a load resistor over time. The total watt-hours (Wh) are calculated to evaluate battery performance. Data is collected using a LabJack T7 and analyzed through custom-developed software.

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Development

🔍 Lead Error Analysis

When performing battery tests, lead wires (positive and negative) connect the battery to a known resistor. Voltage is measured across the resistor to calculate energy output. However, some energy is dissipated in the lead wires themselves, introducing a slight measurement error.

• Wire Specs: 18-gauge copper, 15 inches each (30 inches total).
• Expected Resistance (25–65°C): ~16.275–18.775 mΩ.
• Measured Example:
  • Lead voltage drop: 14.77 mV
  • Load voltage: 5.11109 V
  • Load resistor: 5.13 Ω
  • Current: ≈ 0.996 A
  • Lead resistance: ≈ 14.824 mΩ (single wire), ≈ 29.6 mΩ (both wires)

Findings:

• Measured resistance was ~1.82× higher than theoretical—likely due to alligator clip connections.
• This introduces a 0.58% error in energy measurements, which is considered minor for most practical tests.

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🧪 Resistor Error Analysis

• Long-Term Testing: Resistors were tested daily over several months; no degradation was found.

• Thermal Behavior:

  • Resistance increases with temperature (linear trend).
  • Power increases cause temperature increases, which in turn raise resistance.

• Thermal Performance:

  • Resistor temperatures were logged under >5W loads.

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🧑‍💻 Code Development

Custom C++ code was developed to:

• Perform continuous voltage reads at a set interval.
• Save readings to a .csv file.
• Automatically stop and compute total watt-hours when voltage falls below a threshold (battery drained).

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🧪 Testing Procedure

1. Hardware Setup:

   • Connect the LabJack T7 to your computer via USB.
   • Connect the battery to a known resistor using 18-gauge wire.
   • Wire as follows:
     • Negative side of resistor → GND
     • Positive side of resistor → AIN0

2. Software Setup:

   • Download and unzip the BatteryPackTesting project folder.
   • Open VS Code and select:

     File → Open → CMakeLists.txt (in BatteryPackTesting folder)
     

   • Build the project:

     Build → Build All
     

   • Press the green Run arrow labeled battery.exe.

3. Running the Test:

   • Follow the on-screen prompts to input test details.
   • Connect the battery when prompted. The program will begin printing voltage readings (~5V typical).
   • When the battery is fully discharged, the test ends automatically and a .csv is generated.

4. Data Upload & Review:

   • Upload the .csv file to the shared Google Drive folder (organized by battery pack).
   • Open the file with Google Sheets:

     Right-click → Open With → Google Sheets
     

   • Review and save the following values:
     • Average Voltage
     • Total Watt-Hours (Wh)
     • Total Milliamp-Hours (mAh)

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📊 Battery Pack Results

Across multiple tests, actual Wh output was consistently higher than advertised values—indicating conservative manufacturer estimates or better-than-expected performance.

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📝 Acknowledgments

• Developed as part of a project at LabJack Corporation
• Additional contributions by Josh Kempe (LabJack Intern)
• Libraries & Tools:
  • LabJack T7
  • LJM Library
  • 200W Load Resistors
  • C++ / CMake
  • Visual Studio Code