The battery capacity is the current capacity of the battery and is expressed in Ampere-hours, abbreviated Ah.
- Chemical Capacity – full storage capacity of the chemistry when measured from full to empty or empty to full. This is normally defined at a given C-rate and maximum and minimum voltages.
- Designed Capacity – the storage capacity allowed to be used by the application. Often called the Usable SoC Window for the battery pack. This is a reduced window designed to ensure safety, performance and longevity.
- Available Capacity – this is the capacity that can be accessed taking into account the temperature, age, health and use of the cell.
Battery capacity is expressed in ampere-hours. Battery capacity is effected by:
- Temperature
- Discharge rate – normally the higher the discharge rate the lower the capacity.
- Ageing – capacity will decrease will calendar life and based on the useage history.
Under well defined conditions this is often referred to as the Rated Capacity as the battery capacity is likely to be different under different temperature, discharge rates and prior use.
Ampere-Hour
An alternative unit of electrical charge. Product of the current strength (measured in amperes) and the duration (in hours) of the current. The quantity of electricity (capacity) of a battery or cell is usually expressed in ampere hours.
Symbol: Ah
One ampere-hour = 3,600 coulombs.
Batteries have an Ampere-Hour (Ah) rating. A discharge rate is normally included with this to signify the maximum current that the battery can be discharged at and achieve the rated capacity.
As an example a battery with 60Ah C/20 has a 60Ah capacity when discharged at the capacity divided by 20 which equals 3 Amps in this case.
milliampere hour (mAh) – One thousandth of an Ampere-hour (Ah), this is commonly used when stating the capacity of rechargeable batteries used in mobile phones. 1 mAh = 3.6 Coulombs
Open Circuit Voltage
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.
Cell Capacity and Pack Size
There are very good reasons for selecting a battery cell and using it for multiple applications, thus leveraging the maximum buying opportunity for one cell rather than splitting this across 2 or 3 different cells.
This means that the specifications of the cell will be fixed. Let us suppose we select a 50Ah cell with a nominal cell voltage of 3.6V
A 400V pack would be arranged with 96 cells in series, 2 cells in parallel would create pack with a total energy of 34.6kWh
Changing the number of cells in series by 1 gives a change in total energy of 3.6V x 2 x 50Ah = 360Wh.
Increasing or decreasing the number of cells in parallel changes the total energy by 96 x 3.6V x 50Ah = 17,280Wh.
Usable Energy
In the simplest terms the usable energy of a battery is the Total Energy multiplied by the Usable SoC Window. The total energy is the nominal voltage multiplied by the nominal rated capacity.
However, if you have been through the Battery Basics you will have realised that the battery cell and pack do not have a linear performance and this is true for the usable energy.