How to Choose the Right Battery for your Application – Part 2

In part 1 of this 2 part blog series we discussed performance, design and form factor considerations in selecting the right battery for your application. In this blog post, we talk about battery classifications in primary and secondary batteries with a discussion on common chemistries and their size and design restrictions. The goal of this 2 part series is to help you evaluate the many stock battery choices that you have, when developing an innovative product. Only when you have exhausted stock battery options, should you consider going the application specific battery route.

Battery classifications

In our last blog post, we discussed that batteries may be broadly classified as primary and secondary. Each of these may be subdivided further based on form factor and chemistry.

Primary Batteries

Cylindrical primary batteries that are readily available at retail are all 1.5V systems. Most widely distributed are aqueous alkaline Zn/MnO2 batteries in cylindrical sizes D, C, AA, AAA, and 4A, and the boxy multi-cell 9V battery. Alkaline batteries suffer considerable capacity (energy) losses at higher power (current) levels, and may give significantly less than rated capacity when discharged at high power or current levels.

Also well distributed are non-aqueous primary Li/FeS2 batteries in cylindrical AA and AAA sizes. Because of their internal high-area wound construction, they can deliver nearly full capacity at much higher rates and lower temperatures than alkaline batteries. The Li/FeS2 batteries are higher cost than the alkaline Zn/MnO2 batteries, but can be more economical in high-power-consumption applications.  They are also considerably lighter in weight than alkaline batteries and perform better at low temperatures, so are favorites with outdoors enthusiasts.

Primary button cell (aqueous) types with voltage range of 1.4V-1.5V, while not as thin as non-aqueous coin cells, are significantly cheaper and generally support higher currents. “Silver-zinc” (Zn/Ag2O or Zn/AgO) button cells support higher pulse currents than zinc-air batteries.

 

Primary coin cell (non-aqueous) types (3V) are wider and thinner than aqueous button cells because of the lower conductivities of non-aqueous electrolytes. A higher electrode interface area and thinner electrode structure are needed to enhance the current-carrying capability of a non-aqueous cell. The common coin cells are Li/MnO2, with electrolyte salts dissolved in organic solvents. Such cells carry a CR number, such as CR2016 or CR2032. The first two digits are the diameter in mm, and the last two are height in tenths of mm. These examples are 20mm (d) by 1.6mm (h) or 3.2mm (h).

Rechargeable Batteries

“Household” rechargeable nickel metal hydride (NiMH) batteries (and their chargers) intended as replacements for frequently-used primary batteries, are widely available at retail. Most common are AA and AAA sizes, but C and D sizes can also be found.  The initial investment is higher (more expensive batteries plus the cost of the charger), but for heavy use, these replacement batteries can be much more economical,

Rechargeable battery form factors Rechargeable batteries intended to be built into devices are available in standard cylindrical sizes (different from the primary battery sizes) for larger devices and a host of standard and custom prismatic (flat, rectangular) form factors for thinner devices.

Aqueous rechargeable cells are dominated by the nickel metal hydride system (NiMH), replacing the older nickel-cadmium (NiCad). Cadmium is banned in batteries in some locales, due to its carcinogenic effects. NiMH batteries can support higher currents than other types of rechargeables, and so are the systems of choice for devices requiring very high discharge pulses or currents. The NiMH system is well-proven and highly robust. Most batteries can be cycled 500-1000 times before their performance is seriously degraded. Charge retention (retained capacity with time after termination of charging) is limited, and quite variable, depending on manufacturer and usage pattern. NiMH batteries are usually left on charge when not in use. Only a relatively low cost voltage limited charger is needed, as NiMH batteries can tolerate extended periods of “trickle” charging to maintain full charge capacity.

Non-aqueous rechargeable systems are “lithium ion” systems. There is never any metallic lithium present in a lithium ion battery. Lithium ions are shuttled back and forth between negative and positive electrodes, each of which serves as a redox host for lithium ions.

Serious safety problems and permanent loss of capacity can result from even brief periods of overcharge, so sophisticated (and more expensive) electronic circuits for recharging are needed. There can be fire hazards associated with overheating of most lithium ion batteries, whether from environmental abuse, overcharging, or internal battery defects.

“Lithium ion” batteries have better charge retention than NiMH batteries, so do not need to be left on trickle charge to maintain capacity when not in use. “Lithium ion” batteries use flammable organic solvents for electrolyte solutions.

Lithium polymer batteries use solvent-swollen polymer films as separators, which immobilize the liquid electrolyte to make battery assembly simpler and to capture some of the characteristics of solid electrolyte batteries. Since polymer electrolytes are still flammable, these systems share many of the safety hazards of “lithium ion” batteries.

“Solid electrolyte” lithium ion batteries use solids in which lithium ions are quite mobile as electrolytes. Solid electrolytes have lower conductivities than liquid electrolytes, so solid electrolyte batteries are useful only in low-power applications, or for applications in higher temperatures that increase the conductivities of the electrolytes. Solid electrolytes are less subject to parasitic degradation reactions, so tend to have much longer cycle lives than corresponding liquid electrolyte systems.

Clearly, there are many battery options that are readily available. You may often find that a specific chemistry is not optimized in the form factor that you are looking for. Contact us to learn more about our application specific battery development services. If you are looking for more information about batteries, check out our battery resources or battery fundamentals sections.

About the Author

John

Hi, I'm John, editor-in-chief of an Flexel Battery online magazine!

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