BU-106: Advantages of Primary Batteries

Primary batteries, also known as non-rechargeable batteries, tend to get overshadowed by the media attention secondary or rechargeable batteries receive. Heavy focus on one product over another may convince folks that primary batteries are old technology on the way out. Not so.

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Primaries play an important role, especially when charging is impractical or impossible, such as in military combat, rescue missions and forest-fire services. Regulated under IEC , primary batteries also service pacemakers in heart patients, tire pressure gauges in vehicles, smart meters, intelligent drill bits in mining, animal-tracking, remote light beacons, as well as wristwatches, remote controls, electric keys and children’s toys.

Most implantable pacemaker batteries are lithium-based, draw only 10–20 microamperes (µA) and last 5–10 years. Many hearing aid batteries are also primary with a capacity from 70–600mAh, good for 5–14 days before a replacement is needed. The rechargeable version offers less capacity per size and lasts for about 20 hours. Cost-saving is the major advantage.

High specific energy, long storage times and instant readiness give primary batteries a unique advantage over other power sources. They can be carried to remote locations and used instantly, even after long storage; they are also readily available and environmentally friendly when disposed.

The most popular primary battery is alkaline. It has a high specific energy and is cost effective, environmentally friendly and leak-proof even when fully discharged. Alkaline can be stored for up to 10 years, has a good safety record and can be carried on an aircraft without being subject to UN Transport and other regulations. The negative is low load currents, limiting its use to light loads such as remote controls, flashlights and portable entertainment devices.

Moving into higher capacities and better loading leads to lithium-metal batteries. These have very strict air shipping guidelines and are subject to Dangerous Good Regulations involving Class 9 hazardous material. (See BU-704a: Shipping Lithium-based Batteries by Air)

Figure 1 compares the specific energy of lead acid, NiMH and Li-ion as secondary, as well as alkaline and lithium-metal as primary batteries.

Specific energy only indicates the capacity a battery can hold and does not include power delivery, a weakness with most primary batteries. Manufacturers of primary batteries publish specify specific energy; specific power is seldom published. While most secondary batteries are rated at a 1C discharge current, the capacity on consumer-grade primary batteries is measured with a very low current of 25mA. In addition, the batteries are allowed to discharge from the nominal 1.5V for alkaline to 0.8V before deemed fully discharged. This provides impressive readings on paper, but the results are less flattering when applying loads that draw higher currents.

Figure 2 compares the performance of primary and secondary batteries as “Rated” and “Actual.” Rated refers to the specific energy when discharging at a very low current; Actual discharges at 1C, the way most secondary batteries are rated. The figure clearly demonstrates that the primary alkaline performs well with light load typical to entertainment devices, while the secondary batteries represented by lead acid, NiMH and Li-ion have a lower rated capacity (Rated) but are better when being loaded with a 1C discharge (Actual).

One of the reasons for low performance under load conditions is the high internal resistance of primary batteries, which causes the voltage to collapse. Resistance determines how well electrical current flows through a material or device and is measured in ohms (Ω). As the battery depletes on discharge, the already elevated resistance increases further. Digital cameras with primary batteries are borderline cases — a power tool on alkaline would be impractical. A spent alkaline in a digital camera often leaves enough energy to run the kitchen clock for two years.

Table 3 illustrates the capacity of standard alkaline batteries with loads that run typical personal entertainment devices or small flashlights.

AA and AAA are the most common cell formats for primary batteries. Known as penlight batteries for pocket lights, the AA became available to the public in and was used as a spy tool during World War I; the American National Standards Institute standardized the format in . The AAA was developed in to reduce the size of the Kodak and Polaroid cameras and shrink other portable devices. In the s, an offshoot of the 9V battery produced the AAAA for laser pointers, LED penlights, computer styli and headphone amplifiers. (The 9V uses six AAAA in series.)

Table 4 compares common primary batteries. (See BU-301: A look at Old and New Battery Packaging)

The AA cell contains roughly twice the capacity of the smaller AAA at a similar price. This doubles the energy cost of the AAA over the AA. Energy cost often takes second stage in preference to downsizing. This is the case with bicycle lights where the AA format would only increase the size of the light slightly but could deliver twice the runtime for the same cost.

To cut cost, cities often consolidate purchases and this includes bulk acquisitions of alkaline batteries. A city the size of Vancouver, Canada, with about 600,000 citizens would buy roughly 33,000 AA, 16,000 AAA, 4,500 C and 5,600 D-size alkaline cells for general use.

Retail prices of the alkaline AA vary, so does performance. Exponent Inc. a US engineering firm, checked the capacity of eight brand-name alkaline batteries in AA packages and discovered an 800 percent discrepancy between the highest and lowest performers. The test standard was based on counting the shots of a digital camera until the batteries were depleted, a test that considered capacity and loading capability of a battery.

Figure 5 illustrates the number of shots a digital camera can take with discharge pulses of 1.3W using alkaline, NiMH and Lithium Li-FeS2 in an AA format. (With two cells in series at 3V, 1.3W draws 433mA.) The clear winner was Li-FeS2 (Lithium AA) with 690 pulses; the second was NiMH with 520 pulses, and the distant third was standard alkaline, producing only 85 pulses. Internal resistance rather than capacity governs the shot count. (See BU-801a: How to Rate Battery Runtime)

The relationship between battery capacity and current delivery is best illustrated with the Ragone Chart. Named after David V. Ragone, the Ragone chart evaluates an energy storage device on energy and power. Energy in Ah presents the available storage capacity of a battery that is responsible for the runtime; power in watts governs the load current.

Figure 6 illustrates the Ragone chart with the 1.3W load of a digital camera (indicated by the red arrow and dotted line) using lithium (Li-FeS2), NiMH and alkaline. The horizontal axis displays energy in Wh and the vertical axis provides power in watts. The scale is logarithmic to allow a wide selection of battery sizes.

Digital camera loads NiMH, Li-FeS2 and alkaline with 1.3W pulses according to ANSI C18.1 (dotted line). The results are:

• Li- FeS2 690 pluses
• NiMH 520 pulses
• Alkaline 85 pulses

Energy = Capacity x V
Power = Current x VSource: Quinn Horn, Exponent Inc.

The performance of the battery chemistries varies according to the position of the Ragone line. NiMH delivers the highest power and works well at high loads but it has the lowest specific energy. Lithium Li-FeS2 has the highest specific energy and satisfies moderate loading conditions, and alkaline offers an economic solution for lower current drains.

Summary

Primary batteries are practical for applications that draw occasional power, but they can get expensive when in continuous use. Price is a further issue when the packs are replaced after each mission, regardless of length of use. Discarding partially used batteries is common, especially in fleet applications and critical missions as it is convenient to simply issue fresh packs with each assignment rather than estimating the usage. At a battery conference a US Army general said that half of the batteries discarded still have 50 percent energy left.

The state-of-charge of primary batteries can be estimated by measuring the internal resistance. Each battery type needs its own look-up table as the resistive characteristics may differ. A more accurate method is coulomb counting that observes out-flowing energy, but this requires a more expensive circuit and is seldom done. (See BU-903: How to Measure State-of-charge – Coulomb Counting). This requires a more expensive circuit and is seldom done

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References

Although the progress of technology moves fast and batteries are more popular for supporting mobile devices, it should be noted that the actual battery chemistries haven’t changed much over the last several years. The shift in hand-held devices are for rechargeable solutions with more energy and lighter weight which targets lithium chemistry.

However, there is still a big demand for alkaline batteries because they provide a higher discharge, are very economical, and have a long storage life. We are still producing some unique alkaline battery packs that are used in commercial and military applications. Some for ocean data collection, tracking devices, and other various uses.

An alkaline battery is a type of a dry cell primary battery that uses the chemical reaction of zinc and magnesium oxide and an alkaline electrolyte of potassium hydroxide to generate electric current. It is currently the most popular type of disposable battery in the market. Other common applications include small electronic devices such as clocks and flashlights, as well as portable radios and electronic toys.

Using Alkaline Batteries in Your Application

Note that an alkaline battery is a primary battery. It is intended for disposal upon single use. However, manufacturers have come up with specially designed battery cells to produce rechargeable alkaline batteries.

How Does Alkaline Work?

Remember that batteries have a negative electrode, or cathode, and a positive electrode, or anode. In an alkaline battery, the anode is the zinc and the cathode are the magnesium oxide. Modern alkaline batteries also have carbon in the cathode mix. Understanding the chemical reaction of zinc and magnesium oxide is essential to understanding how alkaline battery works.

Chemical reactions transpire in the anode and cathode due to their individual interactions with the ions from the alkaline electrolyte solution of potassium hydroxide.

In the zinc anode, an interaction with ions of potassium hydroxide causes a buildup of excess electron. This buildup results in an electrical difference between the anode and cathode. Furthermore, because of the buildup of electrons in the zinc anode, they would have the natural tendency to move somewhere else. In an alkaline battery, these excess electrons should move in the magnesium oxide cathode. This is impossible by default because there is no direct connection between the anode and the magnesium cathode.

Allowing the excess electrons to move requires creating a closed circuit, particularly by placing the alkaline battery into a device. The movement of the electrons from the anode to the cathode via the closed circuit creates an electric current. This is how alkaline batteries power electronic devices.

It is also important to note that the magnesium oxide cathode able to receive the excess or free electrons due to its interaction with the ions from the alkaline electrolyte solution of potassium hydroxide. To be specific, the reaction ions from the electrolyte reacting with free electrons to form compounds.

Advantages

One of the advantages of alkaline batteries over other primary batteries and rechargeable batteries is that it has higher energy density. For example, this battery has double the energy density of a Leclanché cell and zinc-carbon batteries. This allows the battery to produce the same energy while lasting longer than other batteries.

The rechargeable variant of this battery also has four times the capacity of an equivalent nickel cadmium or nickel metal hydride batteries.

Longevity is another advantage of alkaline battery. It has longer shelf life than batteries with chloride-type electrolyte. It could last to up to seven years unused, losing about five percent of its energy every year. This means that it does not easily run out of power while not in use. This battery also functions even at very low temperatures. Susceptibility to leakage is also low compared to a Leclanché cell battery.

Safety is also another advantage of alkaline battery. Compared with acid-based and lead-based counterparts, this battery has lesser environmental impact. It does not require any special disposal methods. The compounds inside an alkaline battery do not pose serious health issues except from mild irritations.

Alkaline chemistry is manufactured in your standard size models which makes it easier for engineers to design to and access cells to validate proof of concepts and prototype beta builds. Alkaline batteries can be stung in series to create high voltage packs and can be configured in many options to fit many types of requirements.

Alkaline batteries are composed primarily of common metals such as steel, zinc, and manganese and do not pose a health or environmental risk during normal use or disposal. Since the early ’s mercury has been removed from the chemistry makeup, so they can safely be disposed with normal household waste, everywhere but California.

In Vermont, a law now requires primary battery producers to fund a statewide collection and recycling program. Several cell producers have partnered with “Call2Recycle”, a nonprofit battery recycling program to manage a statewide program for the collection and recycling of household batteries You will also find several home improvement stores nationwide have free recycling as well

An Alkaline battery pack is also a very good candidate for potting, which we do for several custom applications. Being a primary chemistry, no charging/cycling would need to be performed so the cells don’t experience the typical heating, internal temperature changes, or potential gassing that a rechargeable battery sees.

Disadvantages

Compared to other batteries, alkaline batteries have some disadvantages. For example, compared with a lithium ion battery, an alkaline battery is bulkier and heavier. Note that li-ion batteries have higher energy density.

Another disadvantage of alkaline battery is high internal resistance. Remember that internal resistance serves as a gatekeeper for determining runtime. A high internal resistance reduces the power output of a battery.

Leakage is also possible in alkaline batteries although Leclanché cell and zinc-carbon batteries have higher susceptibility. When left in devices for too long, the battery can leak, and the leaked materials can corrode circuits.

Summary

When choosing your battery type, the alkaline chemistry has been shown to have many positive advantages such as low cost, easily accessible, standard sizes, high energy, etc. Of course, there are always some drawbacks, but the advantages often outweigh those disadvantages in many common applications, so the alkaline battery remains a popular choice throughout many market segments.

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Key Takeaways