The current and voltage produced by batteries are determined by their different designs and chemistries. To develop a thorough understanding of batteries, it is important to examine the advantages and disadvantages of various battery shapes and sizes. Today, battery constructions fall into four main categories including cylindrical, button, prismatic and sachet designs.
In cylindrical batteries, the negative and positive electrodes are rolled together in a metal cylinder. The anode (negative electrode) is coated to the inside of the battery casing. The porous separator, usually made of paper or cardboard, keeps the anode away from the cathode (the positive electrode), which is on the inner side of it. The separator is soaked with the electrolyte solution to provide the ionic connection between electrodes. Almost all cylindrical cells contain a venting system to release excess gases when needed.
The ease of manufacturing, high energy density and mechanical stability of the cylindrical cell contribute to the wide use of this battery construction type. Additionally, cylindrical batteries most often are reasonably priced. Power tools, medical instruments, toys and communications equipment usually rely on cylindrical batteries for power generation. The biggest weaknesses of the cylindrical battery are the bulky size and less than maximum use of space due to voids or pockets that form when cells of this shape are stacked together.
Many applications exist where a small, very compact battery is useful and the power demands are low. For certain products, button cells meet this need. Unfortunately, their advantages in small size and ease in stacking also mean they are slow to charge and have a tendency to swell if this process occurs too rapidly. Although initially used in cordless phones, medical devices and industrial applications, the button cell is now more common in its non-rechargeable form for hearing aids and watches.
Consumer demand for smaller, lighter portable products generated the need for smaller, lighter batteries, and prismatic cells were introduced in the early 1990s. In this type of cell construction, the anode and cathode are inserted into a rectangular shaped case, with separators between them. This offers improved flexibility when designing battery packs and better use of space than the cylindrical construction method.
Prismatic cells have slightly lower energy densities than cylindrical cells and are more expensive to manufacture. The design of their venting systems may cause them to bulge if the battery is not used or charged correctly.
The sachet or pouch cell is the latest packaging design to become available on the market. In this construction, the typical hard casing that surrounds the electrodes is replaced by flexible foil packaging, maximizing use of space and reducing weight. The foil pouch can be shaped to fit a specific space, and therefore is frequently custom manufactured for special applications. One potential issue with this construction is that the gas generated during charging and discharging may cause the pouch to swell. However, if the battery is used properly, this will not occur.
Whereas certain sachet cells are simply materials wound into the "jelly-roll" shape, flattened and enclosed in a foil pouch, others are truly stacked, allowing the thinnest construction, greatest flexibility and best use of space. Valence uses "stacked" technology, designed with an electrolyte that doubles as a separator for additional weight and space advantages.