Battery Module - Production Testing

Owned by Micah Black
Last updated: Jul 29, 2021
3 min read

Module Details

25P 2S modules (7.4V nominal, 8.4V max, 5V fully discharged)
25 * 3.45Ah = 86.25Ah per module
29 modules to test in total, 18 will be used in the car

Test Requirements

Test Equipment

Electronic Load - Keysight N7909A x2?
Power Supply - Keysight N7000 series power supply?
Data Acquisition Unit - (DAQ970A with a plug-in module, model DAQ970A)

Outputs

  1. Number for cell capacity

  2. Number for DC and AC internal resistance

  3. Log of temperature and voltage over the entire discharge

    1. This will give us a Differential Voltage / Incremental Capacity curve which can be used to estimate a lot of parameters.

Actual Testing

 

Equipment

  1. Charge and discharge equipment:

    1. Multicomp Pro MP710256 30V 30A 300W Power Supply with remote sense

    2. Rigol DL3031 60A 150V 350W Electronic Load with remote sense

  2. Voltage Measurement Equipment:

    1. Keysight 34410A multimeter

    2. Rigol DM3068 multimeter

  3. Temperature Measurement Equipment:

    1. A2D 64 channel DAQ module

  4. Computer to control the testing

Procedure

Ideally, we would be able to discharge each parallel group of the 2S module separately. This would allow us to ‘balance charge’ both cells. This would be fairly complicated and would require more equipment, so we will not pursue this option for now. This does leave the testing open to increase risk of over-voltage and under-voltage, as well as degrading the cells a little more than if properly balancing them.

Since we’re charging at 30A, we will need some decent balancing capability. We will have measurements of the 2 cell voltages from the 2 multimeters. We can burn power from the high cell with a resistor, and control it with a transistor - this technique is called passive balancing and is the simplest way to implement cell balancing. We can control this charge burning mechanism from the A2D 64 channel DAQ module. We would ideally want all of the voltage, current, temperature, and balancing functions to happen from the same python code.

 

 

We will charge and discharge each module, assuming that the modules have the same capacity and are at the same voltage.

During the entire test, log voltage, current, and temperature for each cell. Multimeters to log single cell voltages (XXs interval (fast as possible)), and DAQ module to measure the thermistors (5s interval).

  1. Charge the module with proper CC/CV curve

    1. 8.4V max

    2. 30A CC portion

      1. standard charge for these cells is 1.7A/cell, so 42.5A is standard charge. Power supply is limited to 30A

    3. 1.25A End current

      1. 50mA * 25 cells = 1.25A

    4. 1s measurement interval

  2. 15 minute rest

  3. Discharge the module at constant current

    1. 5V end voltage

    2. 40A discharge

      1. Highest our DC load supports to keep constant current over the entire discharge

    3. 1s measurement interval

  4. 15 minute rest

  5. Storage Charge to maintain cell safety

    1. 7.6V max voltage (~3.8V/cell)

    2. 30A CC portion

    3. 25A end current

      1. We don’t want the storage to take a ton of time - we don’t need to wait for the current to drop all the way - voltage will sag a bit after finishing charge, but we have increased the voltage to 3.8V/cell instead of the standard 3.7V/cell to account for this.

After the above procedure is followed, we will calculate the capacity of each of the parallel groups to ensure there are no major discrepancies. Any voltage limits that were exceeded or not hit correctly, we will be able to create a model of the MJ1 cells and use that to extrapolate the data to get a full capacity measurement. We will also examine the temperature graphs and the IC/DV graphs to ensure uniformity of the cells.