Why Do AA and AAA Rechargeable Batteries Use 1.2V Rather Than 1.5V?

When it comes to rechargeable batteries, specifically AA and AAA sizes, the voltage of 1.2V seems to dominate. This is in contrast to the 1.5V provided by non-rechargeable alkaline batteries. This article delves into the reason behind this choice and explores the chemistry that makes it possible.

The Chemistry Behind 1.2V vs 1.5V

The choice of 1.2V for rechargeable AA and AAA batteries is rooted in the chemistry of the cells themselves. When a voltage is generated, it is the direct result of a chemical reaction taking place between different materials. This is why we have differences in voltage between various types of batteries.

NiMH (Nickel Metal Hydride) Batteries: NiMH batteries have a nominal voltage of 1.2V, which is significantly lower than the 1.5V of alkaline batteries. This is because the chemical reactions involved in these batteries do not produce the same high voltage as those in alkaline cells. For instance, the reaction between zinc and carbon in carbon-zinc batteries results in a higher voltage. NiCd (Nickel Cadmium) Batteries: Similar to NiMH, NiCd batteries also have a nominal voltage of 1.2V. This is due to the different chemical reactions that occur within the cell, which produce a different voltage than alkaline batteries. Alkaline Batteries: Alkaline batteries, while still rechargeable, generally offer a nominal voltage of 1.2V initially, but they can provide a higher instantaneous voltage (1.54V) during the first few minutes after charging. However, their voltage drops significantly over time.

It is important to understand that while non-rechargeable alkaline batteries provide a stable 1.5V, rechargeable alkaline cells typically do not offer this same level of consistency due to the chemical processes involved.

Why Not 1.5V?

The question often arises: why not make rechargeable batteries with a 1.5V output similar to alkalines? The answer lies in the chemistry and the limitations of current technology.

One of the key factors is the nature of the materials used in rechargeable cells. For instance, in NiMH and NiCd batteries, the materials used cannot be processed in the same way as those in alkaline batteries. Zinc in carbon-zinc batteries can be reverse processed, but the same process cannot be applied to nickel or lithium materials used in rechargeable cells. This difference in processing results in a lower voltage output.

Rechargeability and Voltage

The rechargeability of different battery types is also related to their voltage output. Rechargeable cells can be designed to provide specific voltage outputs, but they are always lower than their non-rechargeable counterparts. This is because the chemical reactions involved in the recharging process do not produce as high a voltage.

For instance, NiMH cells have a nominal voltage of 1.2V, but they can be charged to a slightly higher voltage (around 1.4V to 1.42V). However, this higher voltage is not typically sustainable over multiple cycles, and the cell's voltage will naturally drop back to its nominal level.

Rechargeable alkaline cells, lithium cells, and NiMH batteries all have their unique charging requirements and limitations. For example, rechargeable alkaline cells use a pulsed current charger due to their tendency to overheat and potentially destruct during a standard fixed current charge. In contrast, lithium and NiMH cells have different charging needs, requiring specialized chargers to ensure optimal performance and longevity.

Alternatives and Considerations

For those who need exactly 1.5V for their applications, while price is a consideration, there are options on the market that offer better energy density and performance. Lithium cells, in particular, offer a higher energy density, making them a better choice for devices that require a consistent and reliable voltage output. Rechargeable lithium cells and their preferred chargers are more expensive but provide a more predictable and higher voltage.

It is also worth noting that rechargeable NiMH cells can suffer from a 'memory effect' if not fully discharged and recharged. This is similar to NiCd cells, which are also more prone to this issue if not properly managed.

Conclusion

The choice of 1.2V for rechargeable AA and AAA batteries is a result of the underlying chemistry and the unique requirements of the materials used in these cells. While this may seem less compatible with devices requiring 1.5V, it reflects the fundamental principles of battery technology. For users seeking a consistent 1.5V, alkaline batteries remain the go-to choice, though they are non-rechargeable. For those willing to invest in more advanced technology, rechargeable lithium cells are a more future-proof solution.