Appreciate the importance of non-rechargeable (primary) batteries.
Rechargeable batteries are gaining such high media attention that some might consider 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. Other applications for primaries are pacemakers for 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. High specific energy, long storage times and instant readiness give the primary battery a unique advantage over other power sources. Primary batteries can be carried to remote locations and used instantly, even after long storage. They are also readily available and environmentally friendly.
The implantable pacemaker battery has one of the highest energy densities. Most are primary lithium-metal that holds about twice the capacity of a rechargeable lithium-ion and has very low self-discharge. Pacemaker batteries draw 10–20 microamperes and last 5–10 years. Most 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.
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. 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 either. 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 a small amount of gas on discharge and device using them must have provision for venting.
Primary batteries have one of the highest energy densities. A regular household alkaline provides about 40 percent more energy than the average Li-ion. The most energy-dense primary is the lithium battery that comes in many versions and includes lithium iron disulfide (Li-FeS2), lithium manganese dioxide and lithium-thionyl chloride, by which lithium-thionyl chloride has the highest specific energy of more than 500Wh/kg. Figure 1 compares the gravimetric energy densities of lead acid, NiMH, Li-ion, alkaline and lithium primary batteries. (See also BU-701: How to Prime 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 energy a battery can hold. This, however, does not guarantee delivery and/or loading capabilities. Primary batteries tend to have high internal resistance, which limits the discharge to light loads such as remote controls, flashlights and portable entertainment devices. Digital cameras are borderline cases – a power tool on alkaline would be impracticable.
Manufacturers of consumer primary batteries only specify specific energy; the specific power (ability to deliver power) is not published. While most secondary batteries are rated at a 1C discharge current, the capacity of primary batteries is measured with a very low current of 25mA, a fraction of a C. In addition, the batteries are allowed to go down to a very low voltage of 0.8 volts per cell before they are deemed fully discharge. This evaluation method provides impressive readings on paper, but the results are deceiving under a more demanding load.
Figure 2 compares the performance of primary and secondary batteries as “Rated” and “Actual.” Rated is the Wh/kg when discharging at a very low current; Actual is the Wh/kg derived when discharging at 1C. The graph clearly shows that the primary alkaline performs well with a load that is typical to an entertainment device, while the secondary batteries are more resilient under loading. 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
The reason for the sharp performance drop on primary batteries is high internal resistance, which causes a voltage collapse under load. The already high resistance increases further as the battery depletes on discharge. For example, when the battery goes flat on a digital camera, much usable capacity is left behind at a reduced discharge rate. A spent alkaline can power a kitchen clock for two years.
Table 3 illustrates the capacity of standard alkaline batteries with loads that are typical to personal entertainment devices or small flashlights. Discharging at fractional C-rates produces high capacities, skewing a comparison with rechargeable batteries.
Table 3: Alkaline specifications. The discharge resembles entertainment devices with low loads.
Courtesy of Panasonic
Primary batteries are practical for applications that draw power occasionally for a short time but they can get expensive when in continuous use. Price becomes an 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 will need its own look-up table as the resistive characteristics differ. A more accurate method is counting 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.
Last Updated 2015-06-19
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