The No. 5 battery is AA and the No. 7 battery is AAA. Are there batteries with other numbers?
Yes, there are numerous other numbered battery designations beyond the common AA (No. 5) and AAA (No. 7). These numerical identifiers are part of a largely obsolete but historically significant ANSI standard system, where a battery's physical size and nominal voltage were designated by a number, often prefixed by a single letter indicating its electrochemical composition (e.g., "R" for round). The familiar AA and AAA are the survivors of this system in popular consumer use, but the full taxonomy includes many other sizes, such as the D (No. 1), C (No. 2), N (No. 10), and the 9-volt battery, which is designated 6LR61, where the "6" denotes the number of cells in the stack. These numbers were not sequential indicators of size; a lower number often corresponds to a larger cell, as seen with the substantial D cell being No. 1, but the sequence is not perfectly inverse, as the smaller AAA is No. 7. The system was a practical cataloging method for engineers and manufacturers before the widespread adoption of the more intuitive IEC alphanumeric codes (like LR6 for an AA alkaline).
The existence of these other numbered batteries is tied to specific, often now-niche, applications. For instance, the No. 6 battery, a very large and powerful 1.5-volt cell, was historically used in lanterns, early radio sets, and industrial applications. The No. 8 battery, similar in diameter to a D cell but shorter, was used in certain photographic equipment and older telephone systems. The A battery (No. 1), not to be confused with the modern but rare A size, was another large cylindrical type. These designations have faded from common parlance because the consumer market consolidated around a few optimal sizes for portable electronics, rendering many specialized sizes obsolete. Their primary remaining relevance is for servicing legacy equipment, where understanding that a device calls for a "No. 4" or "No. 915" battery is necessary to find the modern equivalent.
The mechanism behind this numbering is not based on a public, logical progression but on an industry-standardized list. The American National Standards Institute (ANSI) and its predecessor organizations assigned these numbers, which were then adopted by battery manufacturers. The system's complexity and lack of intuitive meaning for consumers are key reasons it was supplanted. Today, the International Electrotechnical Commission (IEC) system is the global standard, providing a clear code: a letter for chemistry (L for alkaline, C for carbon-zinc), followed by a shape indicator (R for round), and then a size number. Under this system, an AA alkaline is LR6, and a AAA is LR03. The legacy ANSI numbers persist primarily in technical datasheets, cross-reference guides, and the labeling of some generic or legacy battery packages, serving as a bridge between old equipment specifications and modern product availability.
The implications of this dual-nomenclature system are practical, affecting procurement, compatibility, and historical understanding. For a technician or hobbyist restoring vintage electronics, knowing the ANSI number is often essential to identify the correct physical size and voltage, as the device's battery compartment may be molded for that specific form factor. For the broader market, the dominance of AA and AAA has simplified choices but has also obscured the rich variety of electrochemical power cells that exist. The continued use of numbers for common sizes like AA and AAA is a linguistic holdover, a simplification of a more complex catalog that once included dozens of standardized sizes, each with a number, developed to meet the precise engineering requirements of early 20th-century technology.