Can you explain why batteries with the same capacity have different usable capacities?

Batteries are essential components of most modern devices, from smartphones to electric cars. One of the most important specifications of a battery is its usable capacity, which is the amount of energy that can be stored and delivered by the battery under normal conditions. The usable capacity of a battery is influenced by several factors, including its chemistry, construction, and usage patterns. In this blog post, we will explore why the usable capacity of batteries can vary widely, even for batteries with the same nominal capacity.

Battery Chemistry

The chemistry of a battery is one of the most important factors that determines its usable capacity. Different types of batteries use different chemical reactions to store and deliver energy, and these reactions have different efficiencies and limitations. For example, lead-acid batteries, which are commonly used in automobiles and backup power systems, have a relatively low usable capacity compared to other types of batteries with the same nominal capacity. This is because lead-acid batteries have a relatively low energy density and are prone to sulfation, which can reduce their capacity over time.

On the other hand, lithium-ion batteries, which are widely used in portable devices and electric vehicles, have a much higher usable capacity compared to lead-acid batteries of the same nominal capacity. This is because lithium-ion batteries have a higher energy density and are less prone to degradation over time. However, even among lithium-ion batteries, there can be significant differences in usable capacity depending on the specific chemistry used. For example, lithium-iron-phosphate (LiFePO4) batteries have a lower energy density compared to other lithium-ion batteries, but they have a longer lifespan and are more stable under high temperatures.

Construction and design

The construction and design of a battery can also affect its usable capacity. Batteries can be designed with different electrode materials, electrolytes, separators, and packaging, and these components can influence the battery’s efficiency, durability, and safety. For example, nickel-metal-hydride (NiMH) batteries, which are commonly used in hybrid vehicles and portable devices, have a lower usable capacity compared to lithium-ion batteries of the same nominal capacity. This is because NiMH batteries have a higher self-discharge rate and a lower energy density compared to lithium-ion batteries.

In addition, the size and shape of a battery can affect its usable capacity. For example, cylindrical lithium-ion batteries, which are commonly used in laptops and power tools, have a higher usable capacity compared to prismatic lithium-ion batteries of the same nominal capacity. This is because cylindrical batteries have lower internal resistance and can dissipate heat more effectively, which reduces the risk of thermal runaway.

Usage patterns

The usage patterns of a battery can also impact its usable capacity. Batteries are designed to operate under certain conditions, such as a specific temperature range, discharge rate, and charging voltage. If a battery is used outside of these specifications, its usable capacity can be reduced. For example, if a lithium-ion battery is repeatedly discharged to a very low state of charge, it can cause irreversible damage to the electrodes, which can reduce the battery’s usable capacity over time. Similarly, if a battery is charged at a voltage that is too high or too low, it can cause damage to the electrodes or electrolyte, which can reduce the battery’s usable capacity or even cause a safety hazard.

Conclusion

In summary, the usable capacity of a battery can vary widely depending on several factors, including its chemistry, construction, and usage patterns. Battery manufacturers must carefully balance these factors to optimize the performance, durability, and safety of their products. Consumers should also be aware of these factors when choosing and using batteries, as improper usage can reduce the usable capacity and lifespan of a battery. As battery technology continues to evolve, we can expect to see new types of batteries with higher usable capacities and improved performance in the future.