Examining Loading Characteristics on Primary and Secondary Batteries

Rechargeable batteries are gaining such high media attention that some consider non-rechargeables as old technology. Primaries still play an important role, especially when charging is impractical or impossible such as in military combat, rescue missions and forest-fire services. Other applications for primaries are pacemakers for heart patients, tire pressure gauges in cars and trucks, transmitters for bird tracking, intelligent drill bits in mining, light beacons in oceans, not to forget our wristwatches, remote controls, electric keys and children’s toys. High specific energy, long storage times and instant readiness give the primary battery a unique advantage over other power sources. Primary batteries are generally inexpensive, readily available and environmentally friendly.

Carbon-zinc, also known as the Leclanché battery, is one of the least expensive primary batteries and often comes with consumer devices when the batteries are included. Alkaline-manganese, known as Alkaline, is an improved version of the old carbon-zinc. Lewis Urry invented it in 1949 while working with the Eveready Battery Company Laboratory in Parma, Ohio. Alkaline delivers more energy at higher load currents than carbon-zinc and does not leak when depleted, although it is not totally leak-proof. A discharging Alkaline generates hydroxide gases. Pressure buildup can rupture the seal and cause corrosion in form of a feathery crystalline structure that can spread to neighboring parts and cause damage. All primary batteries produce gas on discharge and the portable device must have provision for venting.

Lithium Iron Disulfide (Li-FeS2) is a newcomer to the primary battery family and offers improved performance. Lithium batteries normally deliver 3 volts and higher, but Li-FeS2 produces 1.5 volts to be compatible with the AA and AAA formats. It has a higher capacity and a lower internal resistance than Alkaline. This enables moderate to heavy loads and is ideal for digital cameras. Further advantages are improved low temperature performance, superior leakage resistance and low self-discharge, allowing 15 years of storage at ambient temperatures. Low weight and minimal toxicity are added benefits.

The disadvantages of the Li-FeS2 are a higher price and transportation issues because of the lithium metal content in the anode. This causes restriction in air shipment. In 2004, the US DOT and the Federal Aviation Administration (FAA) banned bulk shipments of primary lithium batteries on passenger flights, but airline passengers can still carry them on board or in checked bags. Each AA-sized Li-FeS2 contains 0.98 grams of lithium; the air limitation of primary lithium batteries is 2 grams (8 grams for rechargeable Li-ion). This restricts each passenger to two cells but exceptions have been made in which 12 sample batteries can be carried. Read more about How to Transport Batteries.

The Li-FeS2 includes safety devices in the form of a resettable PTC thermal switch that limits the current at high temperature. The Li-FeS2 cell cannot be recharged as is possible with NiMH in the AA and AAA formats. Recharging, putting in a cell backwards or mixing with used or other battery types could cause a leak or explosion. Read more about Safety Concerns with Li-ion.

Figures 1 and 2 compare the discharge voltage and internal resistance of Alkaline and Li-FeS2 at a 50mA pulsed load. Of interest is the flat voltage curve and the low internal resistance of Lithium; Alkaline shows a gradual voltage drop and a permanent increase in resistance with use. This shortens the runtime, especially at an elevated load.

The AA and AAA are the most common cell formats. 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. Table 3 compares carbon-zinc, alkaline, lithium, NiCd, NiMH and nickel-zinc and the AA and AAA cell sizes.

Table 3: Summary of batteries available in AA and AAA format. The capacity on the AA is double that of the AAA at similar price, making the energy storage on the AAA twice than of the AA.

* In mAh; discharge current is less than 500mA; ** estimated prices in $US (2012)

The AAA cell contains roughly half the capacity of the larger AA at a similar price. In essence, the energy cost of the AAA is twice that of the AA. In an effort to downsize, energy cost often takes second stage and device manufacturers prefer using the smaller AAA over the larger AA. This is the case with bicycle lights where the AA format would only increase the device slightly but deliver twice the energy for the same battery expense. Proper design considerations contribute protecting the environment.

Retail prices of the Alkaline AA vary, so does performance. Exponent, a US engineering firm, checked the capacity of eight brand-name Alkaline batteries in AA packages and discovered a discrepancy between the best and lowest performers of 800 percent. An easy gauge to test batteries is counting the shots a digital camera can take with a set of cells. The elevated current of the digital camera stresses the battery more than a remote control or a kitchen clock would. When a regular Alkaline stops functioning in a digital camera, the remaining energy can still power a remote control and run a kitchen clock for up to two years.

Figure 4 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 AA packages. (With two cells in series at 3V, 1.3W draws 433mA.) Although the three battery chemistries tested have similar capacities, the results vary largely. The clear winner is Li-FeS2 with 690 pulses; the second is NiMH with 520 pulses and the distant third is standard Alkaline producing only 85 pules. Internal resistance rather than capacity governs the shot count.

Figure 4: 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 rated capacity as a performance indicator is most useful at low discharge currents; at higher loads the power factor begins to play an important role. The relationship between capacity and the ability to deliver current can best be 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 and is responsible for the runtime; power in watts governs the load current. These two attributes are important for digital applications that require long runtimes but must also accommodate current pulses. The Ragone chart can be moved up and down depending on power demands.

Figure 5 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 Watt/hours and the vertical axis provides power in Watts. The scale is logarithmic to allow a wide selection of battery sizes.

Figure 5: 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 of Exponent

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 such as power tools. The Lithium Li-FeS2 has the highest specific energy and satisfies moderate loading conditions, such as digital cameras, medical instruments and similar portable devices. Alkaline offers an economic solution for lower current drains such as flashlights, remote controls and wall clocks.

References

• 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
• Batteries in a Portable World, 3^rd edition (2011)