This capacity refers to the amount of energy that the user can access. Also known as net capacity, it is basically the energy the car can effectively utilize to propel itself. It is the primary concern for drivers, as it directly determines the range achievable on a single charge.
The reduced capacity is also referred to as the usable capacity or available kilowatt-hours (kWh). This represents approximately 95% to 99% of the total available capacity. Take, for example, a battery with a capacity of 65 kWh. However, only 62 kWh will be accessible and ready for the car to utilize. What is happening with the additional 3 kilowatt-hours of capacity? There is a portion of energy that is considered “unusable” and is set aside for both the high and low sections of the charge curve.
Moreover, calculating this capacity can be quite intricate as it relies on various operational parameters. The usable energy of a battery can be determined by multiplying the total energy by the usable state of charge (SoC) window. To calculate the overall energy, you simply need to multiply the nominal voltage by the nominal rated capacity.
Also See: How to Convert Gas Units to kWh
Factors Affecting Usable Capacity
Every battery pack is characterized by two distinct capacity values – the Installed/Theoretical Capacity and the Usable/Actual Capacity. The installed capacity is determined by the nominal specifications found in the datasheet, while the usable capacity is calculated considering various conditions.
Batteries are not only intricate, but the methods used to measure and express their capacities and usability are equally complex.
1. BMS Electronics Error: Every electronic component and chip possesses tolerances and measurement accuracies. Differences and errors in SoC calculation can lead to significant errors and reduced capacity if not calibrated and adjusted correctly.
2. Capacity Fade Compensation: The usable capacity of an electric vehicle (EV) battery degrades over time due to the number of cycles. As the battery ages, its capacity gradually diminishes. Failing to consider this reduction could lead to a decrease in the vehicle’s range, which may result in warranty concerns.
3. Cutoff Voltages: It adjusts the Depth of Discharge (DoD) with hysteresis and also impacts the battery’s usable capacity.
4. Depth of Discharge (DoD): The number of cycles directly depends on it. Limiting the depth of discharge (DoD) within upper and lower limits is crucial in order to extend the lifespan of the battery. This significantly reduces the battery pack’s usable capacity.
5. Discharge Rate: The higher the C rate is, the lower will be the usable capacity
6. Temperature: The life of the cell is greatly impacted by the operating temperature, resulting in a faster degradation of its usable capacity. The operating temperature is determined by several factors, including:
- Ambient temperature
- C rate
- Cooling system efficiency
- Current duration along with additional factors
7. Storage Loss: It also impacts the deterioration of the cell and, consequently, its usable capacity.
Total Battery Capacity Vs. Usable Battery Capacity
The available capacity and the total capacity may differ in relation to the battery chemistry since certain types of lithium-ion batteries are more suitable for being charged to 100%, whereas other batteries degrade more quickly with frequent full charges.
The total capacity of a battery pack refers to the maximum amount of energy it can store when fully charged to 100%. On the other hand, the usable capacity represents the amount of energy that the user can actually access.