Battery Testing Plan - MSXV

Purpose: Validate that our manufacturing procedure yields a module that can function in all realistic discharge scenarios

Tests:

• Module Resistance

  • See Single Cell Testing - Data Processing Guide for a more detailed overview of the intricacies of DCR measurements

  • Connect the eload to the module with a 10A capable connection

  • Measure the voltage of the pack at the busbars using the Keysight DMM

  • Apply a load of 5A with a slew rate of 0.25A/us with an eload, observe the immediate voltage drop of the pack.

    • Over time, chemical processes will cause the voltage to further sag over time. This must not be accounted for in the DCR measurement, so use the first measurement you observe.

  • Use Keysight multimeter to check resistance on terminals of battery module, with integration time of 20ms.

  • Test: Measure module internal resistance. Follow this tutorial: https://www.youtube.com/watch?v=av38iBxcOgQ

  • The cells in 4S8P configuration alone, based off of the measured internal resistance of ~22mOhm/cell, should have an IR of about 11mOhms

  • Cells should not have greater than a 60mV drop when loaded with 5A

  • Information gained: The measured internal resistance should tell use how much resistance is introduced from out spotwelds and busbars (indication of how good our spotwelds are)

• Nominal Current/Balance Test

  • Insert thermistor into the center of the module, in the top third

  • Measure and record the voltage of each cell to at least 1mV precision

  • Connect the eload with a 30A capable connection (2 thick alligator clips in parallel)

  • Apply a 30A load until the module drops below 12V (empty)

    • The temperature rise should be about 10C after 15 mins

  • Recharge the module to 14.5V

  • Measure the voltage of each cell, the imbalance should be the same as before, within 2mV

• High Current Test

  • Our modules are rated for a max discharge of 58.2A

  • Test: Draw 58.2A from module for 10 min

  • Information gained: Temperature rise from max discharge, visual observations of module in case anything unexpected occurs

Test set-up:

Attach module to module connections with M4 bolts, Belville disc springs, and regular washers in this stack up ( Nut - Washer - Busbar - Contact Grease - Busbar - Washer - Springs - Bolt ). Torque the M4 connections to 2.4 N*m.

This way we can see from the tests how good the connections between modules are (IR introduced) and if the module-module connections introduce any issues.

To validate the "completed" battery modules, it is essential to conduct a full charge-discharge test while monitoring key system parameters such as voltage and temperature. While the continuous discharge of modules at 40+A is unlikely, this validation procedure functions as a stress test.

Initiate the procedure by charging the module to 16.6V (below the maximum to provide a health-span buffer).

Charging

Modules will be charged at 22A to 17V (4.25V per cell). We charge to 4.25V because it yields 105-110% of the rated energy storage https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries . Charging to this higher voltage is allowed because it is still within the spec of our LG M50s (4.2V +- 0.05V). The downside to charging to a high voltage is reduced cyclability, but this is irrelevant to our low cycle use-case.

Another concern is storage. Cell voltage and temperature have a great effect on recoverable capacity.

https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries

At 4.25V (105-110% charge), the recoverable capacity will be even lower.

Therefore, we will discharge test fully charged modules as soon as possible to avoid prolonged storage at high SOC. After a full discharge, modules will be recharged to a storage voltage of 14.8V (3.7V) per cell to minimize capacity loss and self-dischargehttps://batteryuniversity.com/article/bu-702-how-to-store-batteries .

Discharging

Equipment

• BK Precision 8600 E-load

• Rigol DL3031 E-load

• 6, 18AWG wires with banana connectors

• A2D DAQ w/ thermistors

• USB to USB B (Data-Transfer Cable)

• Windows Laptop

Software Setup

https://github.com/kostubhagarwal/module_test_data_acquisition

Procedure

1. Connect A2D DAQ to Windows laptop.

2. Run serial_to_csv.py to check the functionality of the thermistors (only working thermistors will show readings on the terminal).

3. Pause (Ctrl-C) serial_to_csv.py, then complete the physical setup.

4. Connect working 6 thermistors to the battery module busing

   1. use electrical or kapton tape. ensure good contact!

   2. attach to 6 different central locations on the module as shown

5. Connect E-Loads to battery module as shown DONT MESS UP THE POLARITY OF TERMINALS

   1. two wires (one for each terminal) to connect BK Precision 8600 E-load to the battery module

   2. use four wires (double-up for each terminal, as wires are not rated for 30A) to connect Rigol DL3031 E-load to the battery module

6. Set BK Precision 8600 E-Load to 10A CC draw

7. Set Rigol DL3031 E-Load to 30A CC draw

8. Run serial_to_csv.py

9. Start both e-loads

10. Once modules reach 11V (to prevent draining module past safe spot) stop the e-loads

11. Terminate (Ctrl-C >> 2) serial_to_csv.py

12. Save the 'data.csv' file with a different name, as it will be overwritten during the next run of serial_to_csv.py.

13. Follow charging procedure to charge battery pack to 13.2V, this leaves cells at 3.3V which is a good storage voltage.