The process of bringing an innovative electronic device to market requires difficult decisions. One of these decisions is what will power the device. Off-the-shelf batteries are available in standard sizes; their size and performance are a given. Custom batteries, however, present an array of possibilities in terms of performance, form factor, and other capacities that off-the-shelf batteries cannot match. This is true even when a custom battery uses a conventional battery chemistry. The key to the decision of using an off-the-shelf or custom battery involves weighing factors such as form factor, performance cost, time, and technical risk.
The goal of this post is to give the reader an understanding of what battery chemistries are available. The general outline provided below can direct readers in the right direction by highlighting a battery chemistry that is right for the product. Further, after they research the options available, they will know if a market option exists that fits their need, or if a custom battery can be made for them, using a conventional battery chemistry.
One of the first questions you need to ask when selecting a battery is, does the device need a primary or a secondary battery? A primary battery is one that does not recharge (single use only), and a secondary battery can be recharged multiple times. While this would seem to indicate that you would always want a secondary battery, primary batteries have the advantage in terms of cost, shelf life, and the reduced complexity of not needing charging circuitry. There a number of different battery chemistries available, both primary and secondary. Each battery chemistry has its own advantages and disadvantages. Some of the most common primary battery chemistries are described below.
Alkaline Electrolyte Batteries: Alkaline, Mercury, and Silver Oxide
Alkaline batteries are the most common primary battery today. They have a high energy density and a shelf life of up to 10 years. Alkaline batteries are available in various cylindrical and coin cell form factors, and are found in watches, remote controls, radios, and many other devices. These batteries use a caustic electrolyte, normally potassium hydroxide with some additives, a powered zinc anode, and a manganese dioxide cathode. Some alkaline batteries incorporate or replace some or all of the manganese dioxide with nickel oxyhydroxide; this enables the battery to provide higher power at a minimal cost increase, particularly in the case of some special high power designs. With that in mind, however, alkaline batteries are hard to seal, so a pouch cell form factor is not a design option for these batteries.
Like alkaline batteries, mercury and silver oxide batteries use a caustic electrolyte and a zinc electrode; the other electrode is either mercuric oxide or sliver oxide, respectively. Historically, mercury batteries offered a flat discharge curve and high capacity compared to contemporary zinc-carbon dry cells. They were used for applications such as CPU clocks and photographic light meters. Because of mercury’s toxicity and environmental hazard, however, mercury batteries have been largely phased out. These uses have been largely replaced by silver oxide batteries. Silver oxide batteries have an improved run time compared to alkaline batteries and power low-current devices. Because of their high capacity and track record of excellent reliability, the military also uses silver oxide batteries for large-scale technology, such as submarines and torpedoes. Because of the caustic electrolyte, silver oxide cells have the same sealing issue as alkaline batteries, so a pouch cell is not a possible design for these chemistries.
Zinc-carbon batteries are the least expensive primary cells, and products that use zinc-carbon batteries often include them for free when the product is sold. These batteries are typically used for low drain applications such as flash lights, portable radios, and toys with low power output requirements; their shelf life, power, energy density, and temperature range are inferior to alkaline batteries. The battery electrodes are zinc and manganese dioxide, which are the same electrode materials used in alkaline batteries. Zinc-carbon batteries, however, use a zinc salt solution, as opposed to the caustic electrolytes of the previous two families; pouch cells are a design option because the electrolyte does not creep the way that caustic electrolytes do. The zinc salt solution is much easier to contain than a caustic electrolyte, so zinc-carbon cells are useful for pouch cells where lithium is not an option. These batteries are biocompatible and can be used for microbattery fabrication for biomedical applications. A weakness of this battery family, however, is that its performance drops dramatically at low temperatures.
Not to be confused with lithium-ion batteries, lithium batteries have many different chemistries, all of which use a metallic lithium anode. Lithium batteries tend to have a long shelf life and higher capacity, but a high cost per unit. They provide voltages ranging from 1.5-3.7 Volts, and a shelf life of up to 20 years. Lithium-MnO2 cells are the most common lithium batteries; they are often sold as coin cells for watches and other portable consumer electronics, and they have a voltage of 3V. Lithium-FeS2 batteries offer a voltage of 1.5V and are a higher performing drop-in replacement for alkaline batteries. Some chemistries lend themselves to implantable electronics, like pacemakers, which use lithium-iodine batteries, or implantable defibrillators, which use lithium-silver vanadium oxide batteries. Other batteries include lithium-CFx batteries, which have very high capacity, and lithium-thionyl chloride batteries, which have exceptional capacity and shelf life but are very unsafe if mishandled The electrolytes used for lithium batteries are nonaqueous (they do not contain water) and can be liquid or solid.
The options for different battery chemistries go beyond those listed above, and even within the parameters listed, there are many subtle chemical and device variations that could alter the battery’s performance. For example, by making electrodes thinner you can increase battery power, but the larger amount of supporting materials, such as current collectors and separators, reduces the energy density.
While primary batteries are still commonly in use, many devices use secondary (rechargeable) batteries, particularly in applications with heavy drains. Mobile phones, handheld gaming devices, and laptops are just some of the many devices that use secondary batteries. There are several chemistries that should be considered when choosing a secondary battery for a device.
These batteries have a low energy density but are cheap to manufacture and can be designed to tolerate deep cycling. The applications of these batteries include automobile ignition systems and back-up power for servers and computers. A more recent and fast-growing application of these batteries is solar energy storage. Due to their low energy density and highly corrosive sulfuric acid electrolyte, they are generally not used for portable consumer electronics.
This family of batteries is known for its high voltage and energy density; these batteries have attracted the most attention of any secondary battery chemistry. It also has low self-discharge, and this factor coupled with steadily decreasing prices allows it to displace lead-acid batteries in some applications. The list of portable electronics that use lithium-ion batteries is extensive, including cellular phones, smart watches, power tools, computers, and many others. These batteries are currently the best option for fully electric cars, and they are sometimes used in hybrid cars as well.
There are many different chemistries that exist within the family of lithium-ion batteries, using different electrode materials. In all of these chemistries, the lithium ion inserts (or intercalates) into both the positive and negative electrode; the difference in voltage during the insertion/removal reaction enables the battery to store energy. The most common lithium-ion battery for small form factor applications uses graphite as the negative electrode and lithium cobalt oxide as the positive electrode; these batteries have a voltage of 3.7-4.2V. Other noteworthy electrode materials include silicon and lithium titanate for the anode (negative electrode), and lithium manganese oxide, lithium iron phosphate, and lithium nickel cobalt aluminum oxide for the cathode (positive electrode). While conventional lithium-ion batteries use a nonaqueous organic electrolyte, lithium-polymer batteries, which use a gelled polymer electrolyte, are also common. Some pose safety issues, so specific and appropriate pack design and charging protocols are important. Because of the required protective circuitry when using lithium-ion batteries, they are generally manufactured as a device-specific pack, rather than sold as individual cells like primary batteries.
Nickel Metal Hydride and Nickel Cadmium
Nickel-metal hydride batteries use a nickel oxide cathode, a metal hydride anode, and a caustic electrolyte. They have largely replaced nickel-cadmium batteries, which use cadmium in place of the metal hydride; they offer higher power than nickel metal hydride batteries, but are being phased out due to cadmium’s toxicity. Nickel metal hydride batteries are most commonly sold as small form factor cells, which are drop-in replacements for alkaline or zinc-carbon batteries (AA, AAA, C, D, 9V, ect.). They are more tolerant of varying charging and discharging protocols than lithium-ion batteries, so they are often sold as individual cells directly to consumers. Large form factor nickel metal hydride batteries have also been used in hybrid-electric and full electric cars, including the Prius and the EV1. Nickel metal hydride cells have a nominal voltage of 1.25V, and the energy density is below lithium-ion energy density.
There are many different battery chemistries, all of which have different aspects that make them attractive options for different applications. This list presents the options that are available in off-the-shelf batteries, and which can be tweaked to match your application in custom batteries. Hopefully this blog can direct you towards a chemistry that is right for your electronic device.
Of course, if an off-the-shelf battery can meet all of your needs then there is no need to even think about custom battery development. If, however, off-the-shelf batteries do not meet your needs, then a custom battery may be right for you. Custom battery design is a significant undertaking, but if it is handled correctly, the custom battery can do things which an off the shelf battery cannot.