Cell Testing

Cell testing and the data thereof underpins the fundamental design of a battery pack from the initial sizing through to control system parameterization and final sign-off of the system. These tests come under a few high level

  • Diagnostics
  • Performance data
    • electrical
    • thermal
    • mechanical
  • BMS data
  • Ageing performance
  • Abuse tests
  • Legislative testing

Diagnostics

There are some measurements that can be made to check for internal faults in cells.

LG Energy solutions Relaxation delta Voltage (RdV)

Relaxation delta Voltage can be used to find micro internal short circuits in cells.

Performance Data

Battery Capacity

The amount of energy available from a battery. Battery capacity is expressed in ampere-hours.

The battery is soaked at a nominal temperature and then charged at a defined rate until it reaches the maximum cell voltage. The cell is then discharged at a defined C-rate until it reaches a lower defined cell voltage. The current versus time (coulomb counting) is then used to establish the Ah capacity of the cell or pack.

Open Circuit Voltage (OCV)

The Open Circuit Voltage (OCV) is a fundamental parameter of the cell. The OCV of a battery cell is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.

open circuit voltage table of values

Measurement of OCV

The measurement of Open Circuit Voltage can be time consuming as it needs to be established versus the State of Charge (SoC) of the cell. Doing this in fine steps from 0% to 100%, with the measurement being made “at rest”, means we have to charge/discharge and then let it rest before making the voltage measurement.

Hybrid Pulse Power Characterization (HPPC)

Battery test used to determine the dynamic performance characteristics of a battery, in particular the DC Internal Resistance of the cell. The battery is pulse discharged typically at 1C for 10s. The voltage and current profile is then used to determine the internal resistance of the cell. These measurements are repeated for charge and over different temperatures, States of Charge, different pulse lengths and at different rates. In this way a complete map of the internal resistance of the cell can be created and using this a power map versus SoC and temperature.

Heat Capacity

You can initially use a generic specific heat capacity for a cell to do some initial thermal modelling. However, the heat capacity of a cell varies enough with chemistry and design that it is worth measuring it for the specific cell you are using. This will allow you to develop more accurate thermal models of the cell.

What is Calorimetry?

Calorimetry is a branch of thermodynamics that deals with heat transfer quantification. Heat can be transferred during a variety of processes, such as chemical reactions, phase changes, or dissolution of solutes in a solvent. Calorimetry follows the first law of thermodynamics, which states that energy cannot be created or destroyed, only transferred. The direction that the heat takes will determine the type of process: if heat is absorbed, the process will be endothermic, whereas if heat is released, the process will be exothermic.

specific heat capacity measurement schematic

Measuring Specific Heat Capacity

  • Sample Container: At the heart of the chamber is a sample container where the material under investigation is placed. 
  • Temperature Sensors: The chamber is equipped with fast response thermocouples to monitor the temperature of the sample continuously.
  • Heating System: A resistive heating system surrounds the chamber, capable of precisely controlling the temperature of the sample according to the experimental protocol.
  • Insulation: The entire assembly is heavily insulated to ensure adiabatic conditions, meaning that once the external heating is stopped, no heat should enter or leave the system.

Thermal Conductivity

Mechanical

Structural

A number of fundamental structural tests are performed at cell sub-component and component level.

zwickroell tensile testing of 6 um thick copper foil

Cell Component Mechanical Tests

Cell component mechanical tests are extensive and required in order that a more detailed understanding of how the cells behave can be established. The battery cell is a complex: chemistry, electrical, thermal and mechanical system. Cycle ageing is seen as a deteriation in the chemistry of the cell, but should also be considered as a mechanical system. Expanding and contracting through charging and discharging that leads to fracturing.

Image shows tensile testing of 6µm thick copper foil.

Vibration

Mechanical Shock

BMS data

In order to design and calibrate the BMS to work with a particular battery cell there is a set of measured data that will be required.

Ageing Performance

The ageing performance of a cell is initially broken down into two types of testing:

  1. Cycle ageing
  2. Calendar ageing

Results from this data can then be used to estimate the lifetime performance of a battery pack.

As more data becomes available on the use cycles, battery pack design and control limits the ageing performance will then be repeated against specific parameters.

Abuse Tests

It is important to establish how a battery cell performs at the limits and beyond.

  • Nail Test – perhaps the most severe cell test with the highest rate of heat production.

Legislative Testing

There are a number of legislative tests that apply to a single cell and are required to prove for safe transport and use.

  • UN/DOT 38.3 – if you want to ship any cells in the world they need to meet this standard.
  • IEC 62133 – mandated by many IEC end-device standards
  • UL 2054 – mandated by a number of U.S. end device standards.