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
- Performance data
- BMS data
- Ageing performance
- Abuse tests
- Legislative testing
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.
Hybrid Pulse Power Characterization (HPPC)
Battery test used to determine the dynamic performance characteristics of a battery. 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.
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.
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.
The ageing performance of a cell is initially broken down into two types of testing:
- Cycle ageing
- 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.
It is important to establish how a battery cell performs at the limits and beyond.
There are a number of legislative tests that apply to a single cell and are required to prove for safe transport and use.