You may have an innovative product with a specific set of challenging battery requirements. As you look for the right battery for your application, custom battery development may come to mind. However before you go the custom battery route, it is very important for you to answer this question: is there a stock battery that meets my needs? There are a lot of readily available solutions for you to choose from. Understanding the characteristics of different types of batteries as well as knowing the power and energy requirements of your device can help you make the most suitable battery choice for your application. In this blog which constitutes a two part series, we discuss some considerations when scouting for a battery for your application. In part 2 we discuss common battery classifications and delve into different chemistry and form factors.
Here are 5 considerations in helping you find the right battery for your application.
Primary vs Secondary – One of the first questions is whether to use primary (non-chargeable) batteries or secondary (rechargeable) batteries. An important consideration is the expected usage pattern. If you expect most users to use up the battery capacity only intermittently or only once every month, or longer, a primary battery may be the battery of choice. Another case where a primary battery is a good choice is when you have a disposal product that is intended to work for several weeks or a few months. If the expected battery life usage is daily or weekly and the product operating lifetime is a year or more, then a rechargeable battery may be the right choice.
If the logical choice is a primary (non-rechargeable) battery, then cost, size, voltage, capacity or energy, and power requirements come into play. Since they are intended to be replaced by consumers during the life of the device, availability at retail is also a consideration.
Rechargeable batteries sold within the device are usually available only from the device supplier or specialty retailers, typically online. For example, Li-Ion packs for electronics such as laptops are normally custom-made for the device and are sold as packs for the device.
Built-in secondary batteries require charging circuitry designed for the particular battery or battery pack (multi-cell battery). Secondary batteries can usually be recharged and discharged several hundred times with only modest loss of discharge capacity, so are more economical and have less environmental impact overall. Primary batteries are less expensive than secondary ones on a per unit basis, but need to be purchased numerous times over the life of the device application.
If the logical choice is a secondary (rechargeable) battery, you may want to work with a battery pack supplier that supplies charger technology appropriate for the battery or batteries chosen for the application. Since rechargeables are not usually intended to be replaced by consumers during the life of the device, availability at retail is less important.
Chemistries – One of the most common battery chemistry classifications is non-aqueous and aqueous electrolytes. Aqueous electrolytes are water based, while non- aqueous electrolytes are organic based. Non-aqueous electrolytes have lower ionic conductivities than their aqueous counterparts. This means that non-aqueous cells must have larger electrode areas and thinner electrode structures to maintain lower internal current densities and lower cell impedances. Cylindrical non-aqueous cells have long, thin electrodes wound into a “jellyroll” structure, while cylindrical alkaline zinc primary cells are built with an outer shell electrode and a thick center core electrode. Flat prismatic rechargeable cells can have an elliptically wound or stacked plate internal configuration, or a single pair of thin electrodes.
Form Factors – Cylindrical form factors generally cost less to manufacture than the thin, flat prismatic cell form factors needed for very thin devices. Device dimensions and product cost can influence the choice of form factor. Primary alkaline chemistry is available only in cylindrical and button cell form factors, but rechargeable aqueous (nickel-based) and non-aqueous (lithium ion based) cells are available in cylindrical, coin, and prismatic form factors.
Design Priorities and Usability – The size and shape of the application device, its energy and power requirements, and the desired user experience of battery life limit the battery choices for the application. Often times you have to trade one or more battery metrics to gain in another. A good example is the constant complaint of short battery life in cell phones. This is the result of placing device size and thinness higher in priority than battery life. Where miniaturization and physical profile is not such a high priority, the device can easily be designed to provide much longer battery life.
Another example of design priorities influencing battery selection is wearable fitness trackers designed to be used on a continuous basis. While the Fitbit Flex model uses a rechargeable battery that needs to be charged every few days, the Garmin Vivofit uses a primary battery which does not need to be recharged, but is replaced every year or so.
Power and Energy Requirements – The basic starting point for making a rational selection of battery is to characterize the power and energy requirements of the application, and then to balance the desired user experience of battery life against device dimensional priorities. (If you are unfamiliar with battery terms or need a little more detail see our blog post on battery terms explained.)
The battery life energy requirement will be the number of joules or watt-hours consumed between battery replacement (primaries) or recharging (secondaries). Most batteries are rated in capacity (amp-hours) rather than in energy (watt-hours or joules), the difference being in battery operating voltage under the power demands of the application. (Energy = power x time = voltage x capacity. Check out our blog post on power density vs energy density to determine which is more important for your application).
Electrical properties of battery systems influence the choice of power and voltage conditioning components needed in the application circuitry to make good use of the battery capacity. Battery makers supply typical voltage vs. depth of discharge curves in their product data sheets.
Primary Zinc-MnO2 batteries have a “sloping discharge curve,” meaning that the output voltage declines as the battery is discharged. The range of 1.5V (very light load, fully charged) to 0.9V (near end of life) is usually cited, a reflection of the voltage tolerance of common analog circuits. Aqueous rechargeable NiMH batteries have a “flat discharge curve,” with little deviation in voltage as the battery is discharged. Lithium ion battery discharge curves are typically in between these two examples, experiencing some loss of voltage as the battery is discharged, but less so than with Zn-MnO2 batteries. Consult the manufacturer for the details of voltage vs. load and depth of discharge for a particular battery type. (Check out this blog post from Electropaedia for discharge curve slopes of some common battery chemistries)
Clearly, there are a lot of factors to consider when selecting the right battery for your application, If you are looking for more information about batteries, check out our battery resources or battery fundamentals sections.