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.

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 60086, 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 1915 and was used as a spy tool during World War I; the American National Standards Institute standardized the format in 1947. The AAA was developed in 1954 to reduce the size of the Kodak and Polaroid cameras and shrink other portable devices. In the 1990s, 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.

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

References

[1] Source: Panasonic
[2] Source: Exponent
Presentation by Dan Durbin, Energizer Applications support, Medical Device & Manufacturing (MD&M) West, Anaheim, CA, 15 February 2012
Presentation by Quinn Horn, Ph.D., P.E. Exponent, Inc. Medical Device & Manufacturing (MD&M) West, Anaheim, CA, 15 February 2012