Appreciate the importance of non-rechargeable (primary) batteries.
Rechargeable batteries get such high media attention that some folks might consider primaries or non-rechargeable batteries as old technology. Primaries play an important role, especially when charging is impractical or impossible, such as in military combat, rescue missions and forest-fire services. These batteries also service as pacemakers in heart patients, tire pressure gauges in vehicles, intelligent drill bits in mining, animal-tracking, light beacons, not to forget wristwatches, remote controls, electric keys and children’s toys.
Most implantable pacemaker batteries are lithium-based; draw 10–20 microamperes and last 5–10 years. Many hearing aid batteries are also primary with a capacity from 70 to 600mAh, good for 5 to 14 days before replacement. The rechargeable version offers less capacity for its size and lasts for about 20 hours between charges. The ability to recharge will save money in the long run.
High specific energy, long storage times and instant readiness give the primary battery 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 disposing.
The most popular primary battery is alkaline. It has high specific energy, 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 board an aircraft without subject to UN Transport and other regulations. The negative is low load currents. This limits their 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 of Dangerous Good Regulation involving Class 9 hazardous material. (See BU-704a: Shipping Lithium-based Batteries by Air.) Figure 1 compares specific energy of lead acid, NiMH, Li-ion as rechargeable and alkaline and lithium as primary batteries.
Figure 1: Specific energy comparison of secondary and primary batteries
Secondary batteries are typically rated at 1C; alkaline uses much lower discharge currents.
Courtesy of Cadex
Specific energy indicates the capacity a battery can hold. This does not include power delivery, a weakness with most primary batteries that also stretches to lithium-based units.
Manufacturers of primary batteries only specify specific energy; specific power is not 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 drawing currents.
Figure 2 compares the performance of primary and secondary batteries as “Rated” and “Actual.” Lead acid, NiMH and Li-ion are secondary batteries, while alkaline and lithium are primary. Rated refers to the specific energy when discharging at a very low current; Actual is with a 1C discharging, the way most secondary batteries are rated. The graph clearly shows that the primary alkaline performs well with load requirements for most entertainment devices, while the secondary batteries are more resilient for industrial use. A long-life alkaline (not shown) will provide better results.
Figure 2: Energy comparison under load. ”Rated” refers to a mild discharge; “Actual” is a load at 1C. High internal resistance limits alkaline battery to light loads.
Courtesy of Cadex
One of the reasons for low performance under load conditions is the high internal resistance of primary batteries, which causes a voltage collapse under load. The already elevated resistance increases further as the battery depletes on discharge. Digital cameras powered by primary batteries are borderline cases – a power tool on alkaline is impractical. When an alkaline in a digital camera goes flat, it leaves enough energy to run the kitchen clock for two years.
Table 3 illustrates the capacity of standard alkaline batteries with loads that power typical personal entertainment devices or small flashlights.
Table 3: Alkaline specifications. The discharge resembles entertainment devices with low loads.
Courtesy of Panasonic
The 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 Standard 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. (See BU-301: A look at Old and New Battery Packaging) Table 4 compares carbon-zinc, alkaline, lithium, NiCd, NiMH and nickel-zinc and the AA and AAA cell sizes.
|Discharge Rate||Very low||Low||Medium||Very high||High|
|Shelf life||1-2 years||7 years||10-15 years||5 years||5 years|
|Not in all stores||
|Not in all stores||
Table 4: Summary of batteries available in AA and AAA format
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 larger AA. In an effort to downsize, energy cost often takes second stage as device manufacturers prefer to use the smaller sizes. This is the case with bicycle lights where the AA format would only increase the size of the lighting slightly but could deliver twice the energy for the same cost.
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 best and lowest performers. The test standard was based on counting the shots of a digital camera until the batteries were depleted. This allowed testing the runtime under moderate load conditions.
Figure 5 illustrates the number of shots a digital camera can take with discharge pulses of 1.3 watts on 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)
Figure 5: Number of shots a digital camera can take with Alkaline NiMH and Lithium
Li-FeS2, NiMH and Alkaline have similar capacities; the internal resistance governs the shot count on a digital camera.
Li-FeS2, 3Ah, 690 pulses
NiMH, 2.5Ah, 520 pulses
Alkaline, 3Ah, 85 pulses
Test: ANSI C18.1
Courtesy of Exponent
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 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.
Figure 6: Ragone chart illustrates battery performance with various load conditions.
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 V
Courtesy Quinn Horn, Exponent Inc.
The performance of the battery chemistries varies according to the position of the Ragone line. NiMH delivers the highest power but has the lowest specific energy and works well at high loads. Lithium Li-FeS2 has the highest specific energy and satisfies moderate loading conditions, and alkaline offers an economic solution for lower current drains.
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. It is convenient to simply issue fresh packs with each assignment rather than estimating 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 applying a brief load and checking the voltage drop. Each battery type needs its own look-up table as the resistive characteristics differ. A more accurate method is counting the out-flowing energy, a measurement also known as coulomb counting. (See BU-903: How to Measure State-of-charge – Coulomb Counting). This requires a more expensive circuit and is seldom done.
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
Last Updated 2015-08-06
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