A Look at Cell Formats and how to Build a good Battery

Early batteries of the 1700s and 1800s were mostly encased in glass jars. As the batteries grew in size, jars shifted to sealed wooden containers and composite materials. There were few size standards, except perhaps the No. 6 Dry Cell named after its six inches of height. Other sizes were hand-built for specific uses. With the move to portability, sealed cylindrical cells emerged that led to standards. In around 1917, the National Institute of Standards and Technology formalized the alphabet nomenclature that is still used today. Table 1 summarizes these historic and current battery sizes.

Standardization included primary cells, mostly in carbon-zinc; alkaline emerged only in the early 1960s. With the popularity of the sealed nickel-cadmium in the 1950s and 1960s, new sizes appeared, many of which were derived from the “A” and “C” sizes. Manufacturers of Li-ion departed from conventional sizes and invented their own.

The International Electrochemical Commission (IEC), a non-governmental standards organization founded in 1906, developed standards for most rechargeable batteries under the number of 600086. The relevant US standards are the ANSI C18 series developed by the US National Electrical Manufacturers Association (NEMA).

A successful standard for a cylindrical cell is the 18650. Developed in the mid-1990s for lithium-ion, these cells power laptops, electric bicycles and even electric vehicles, as with the Tesla cars. The first two digits designate the diameter in millimeters; the next three digits are the length in tenths of millimeters. The 18650 is 18mm in diameter and 65.0mm in length.

Prismatic cells use the first two digits to indicate the thickness in tenth of millimeters. The next two digits designate the widths and the last two provide the length of the cell in millimeters. The 564656P prismatic cell, for example, is 5.6mm thick, 46mm wide and 56mm long. P stands for prismatic. Because of the large variety of chemistries and their diversity within, battery cells do not mark the chemistry.

Looking at the batteries in mobile phones and laptops one sees a departure of established standards. This is in part due to the manufacturer’s inability to agree on a standard. Most consumer devices come with a custom-made battery. Compact design and tailoring to market demands are swaying manufacturers away from standards. High volume tolerates unique sizes that are often short-lived.

In the early days, a battery was perceived as “big” and this reflects in the sizing convention. While the “F” nomenclature may have been chosen as a middle-of-the-road battery in the late 1800s, our forefathers did not forestall that a tiny battery could do computing, serve as telephone and shoot pictures in a smartphone. Running out of letters towards the smaller sizes led to the awkward of AA, AAA and AAAA designation.

Since the introduction of the 9V battery in 1956, no new format emerged. Meanwhile portable devices lowered the operating voltages and 9V is overkill. The battery has six cells in series and is expensive to manufacture. A 3.6V alternative would serve well. This pack should have a coding system to prevent charging primaries and selecting the correct charge algorithm for secondary chemistries.

Starter batteries for vehicles also follow battery norms, which consist of the North American BCI, the European DIN and the Japanese JIS standards. These batteries are similar in footprint to allow swapping. To standardize, American car manufacturers are converting to the American DIN size batteries. Deep-cycle and stationary batteries have no standardized norms and the replacement packs must be sourced from the original maker. The attempt to standardize electric vehicle batteries may not work either and follow the failed attempt of common laptop batteries in the 1990s.