Battery Capacity

The amount of energy available from a battery.

  • 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

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