Chapter 2: Battery Chemistries

Battery novices often argue that advanced battery systems are now available that offer very high energy densities, deliver 1000 charge/discharge cycles and are paper thin. These attributes are indeed achievable — unfortunately not in the same battery pack. A given battery may be designed for small size and long runtime, but this pack would have a limited cycle life. Another battery may be built for durability, but it would be big and bulky. A third pack may have high energy density and long durability, but would be too expensive for the commercial consumer.

Battery manufacturers are well aware of customer needs and have responded by offering battery packs that best suit the specific application. The mobile phone industry is an example of this clever adaptation. For this market, the emphasis is placed on small size and high energy density. Longevity comes in second.

The mention of NiMH on a battery pack does not automatically guarantee high energy density. A prismatic NiMH battery for a mobile phone, for example, is made for slim geometry and may only have an energy density of 60Wh/kg. The cycle count for this battery would be limited to around 300. In comparison, a cylindrical NiMH offers energy densities of 80Wh/kg and higher. Still, the cycle count of this battery will be moderate to low. High durability NiMH batteries, which are intended for industrial use and the electric vehicle enduring 1000 discharges to 80 percent depth-of discharge, are packaged in large cylindrical cells. The energy density on these cells is a modest 70Wh/kg.

Similarly, Li-ion batteries for defense applications are being produced that far exceed the energy density of the commercial equivalent. Unfortunately, these super-high capacity Li-ion batteries are deemed unsafe in the hands of the public. Neither would the general public be able to afford to buy them.

When energy densities and cycle life are mentioned, this book refers to a middle-of-the-road commercial battery that offers a reasonable compromise in size, energy density, cycle life and price. The book excludes miracle batteries that only live in controlled environments.

Chemistry Comparison

Let's examine the advantages and limitations of today’s popular battery systems. Batteries are scrutinized not only in terms of energy density but service life, load characteristics, maintenance requirements, self-discharge and operational costs. Since NiCd remains a standard against which other batteries are compared, let’s evaluate alternative chemistries against this classic battery type.

Nickel Cadmium (NiCd) — mature and well understood but relatively low in energy density. The NiCd is used where long life, high discharge rate and economical price are important. Main applications are two-way radios, biomedical equipment, professional video cameras and power tools. The NiCd contains toxic metals and is not environmentally friendly.

Nickel-Metal Hydride (NiMH) — has a higher energy density compared to the NiCd at the expense of reduced cycle life. NiMH contains no toxic metals. Applications include mobile phones and laptop computers.

Lead Acid — most economical for larger power applications where weight is of little concern. The lead acid battery is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems.

Lithium Ion (Li-ion) — fastest growing battery system. Li-ion is used where high-energy density and light weight is of prime importance. The Li-ion is more expensive than other systems and must follow strict guidelines to assure safety. Applications include notebook computers and cellular phones.

Lithium Ion Polymer (Li-ion polymer) — a potentially lower cost version of the Li-ion. This chemistry is similar to the Li-ion in terms of energy density. It enables very slim geometry and allows simplified packaging. Main applications are mobile phones.

Reusable Alkaline — replaces disposable household batteries; suitable for low-power applications. Its limited cycle life is compensated by low self-discharge, making this battery ideal for portable entertainment devices and flashlights.

Figure 2-1 compares the characteristics of the six most commonly used rechargeable battery systems in terms of energy density, cycle life, exercise requirements and cost. The figures are based on average ratings of commercially available batteries at the time of publication. Exotic batteries with above average ratings are not included.

  NiCd NiMH Lead
Acid
Li-ion Li-ion
polymer
Reusable

Alkaline
Gravimetric
Energy Density
(Wh/kg)
45-80 60-120 30-50 110-160 100-130 80 (initial)
Internal
Resistance


(includes peripheral circuits) in mW
100 to 2001

6V pack
200 to 3001

6V pack
<1001

12V pack
150 to 2501

7.2V pack
200 to 3001

7.2V pack
200 to 20001

6V pack
Cycle
Life
(to 80% of initial capacity)
15002 300 to 5002,3 200 to

3002
500 to 10003 300 to

500
503

(to 50%)
Fast
Charge Time
1h typical 2-4h 8-16h 2-4h 2-4h 2-3h
Overcharge
Tolerance
moderate low high very low low moderate
Self-discharge
/ Month
(room temperature)
20%4 30%4 5% 10%5 ~10%5 0.3%
Cell
Voltage
(nominal)
1.25V6 1.25V6 2V 3.6V 3.6V 1.5V
Load
Current


-    peak

-    best result


20C

1C


5C

0.5C or lower


5C7

0.2C


>2C

1C or lower


>2C

1C or lower


0.5C

0.2C or lower
Operating
Temperature
(discharge only)
-40 to

60°C
-20 to

60°C
-20 to

60°C
-20 to

60°C
0 to

60°C
0 to

65°C
Maintenance
Requirement
30 to 60 days 60 to 90 days 3 to 6 months9 not req. not req. not req.
Typical
Battery Cost


(US$, reference only)
$50

(7.2V)
$60

(7.2V)
$25

(6V)
$100

(7.2V)
$100

(7.2V)
$5

(9V)
Cost
per Cycle
(US$)11
$0.04 $0.12 $0.10 $0.14 $0.29 $0.10-0.50
Commercial
use since
1950 1990 1970 1991 1999 1992
  1. Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of Li-ion and Li-polymer adds about 100mW.
  2. Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
  3. Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
  4. The discharge is highest immediately after charge, then tapers off. The NiCd capacity decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
  5. Internal protection circuits typically consume 3% of the stored energy per month.
  6. 1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference between the cells; it is simply a method of rating.
  7. Capable of high current pulses.
  8. Applies to discharge only; charge temperature range is more confined.
  9. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
  10. Cost of battery for commercially available portable devices.
  11. Derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.

Observation: It is interesting to note that NiCd has the shortest charge time, delivers the highest load current and offers the lowest overall cost-per-cycle, but has the most demanding maintenance requirements.

The Nickel Cadmium (NiCd) Battery

Alkaline nickel battery technology originated in 1899, when Waldmar Jungner invented the NiCd battery. The materials were expensive compared to other battery types available at the time and its use was limited to special applications. In 1932, the active materials were deposited inside a porous nickel-plated electrode and in 1947, research began on a sealed NiCd battery, which recombined the internal gases generated during charge rather than venting them. These advances led to the modern sealed NiCd battery, which is in use today.

The NiCd prefers fast charge to slow charge and pulse charge to DC charge. All other chemistries prefer a shallow discharge and moderate load currents. The NiCd is a strong and silent worker; hard labor poses no problem. In fact, the NiCd is the only battery type that performs best under rigorous working conditions. It does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods. A periodic full discharge is so important that, if omitted, large crystals will form on the cell plates (also referred to as 'memory') and the NiCd will gradually lose its performance.

Among rechargeable batteries, NiCd remains a popular choice for applications such as two-way radios, emergency medical equipment, professional video cameras and power tools. Over 50 percent of all rechargeable batteries for portable equipment are NiCd. However, the introduction of batteries with higher energy densities and less toxic metals is causing a diversion from NiCd to newer technologies. [2.2]

The Nickel-Metal Hydride (NiMH) Battery

Research of the NiMH system started in the 1970s as a means of discovering how to store hydrogen for the nickel hydrogen battery. Today, nickel hydrogen batteries are mainly used for satellite applications. They are bulky, contain high-pressure steel canisters and cost thousands of dollars each.

In the early experimental days of the NiMH battery, the metal hydride alloys were unstable in the cell environment and the desired performance characteristics could not be achieved. As a result, the development of the NiMH slowed down. New hydride alloys were developed in the 1980s that were stable enough for use in a cell. Since the late 1980s, NiMH has steadily improved, mainly in terms of energy density.

The success of the NiMH has been driven by its high energy density and the use of environmentally friendly metals. The modern NiMH offers up to 40 percent higher energy density compared to NiCd. There is potential for yet higher capacities, but not without some negative side effects.

Both NiMH and NiCd are affected by high self-discharge. The NiCd loses about 10 percent of its capacity within the first 24 hours, after which the self-discharge settles to about 10 percent per month. The self-discharge of the NiMH is about one-and-a-half to two times greater compared to NiCd. Selection of hydride materials that improve hydrogen bonding and reduce corrosion of the alloy constituents reduces the rate of self-discharge, but at the cost of lower energy density.

The NiMH has been replacing the NiCd in markets such as wireless communications and mobile computing. In many parts of the world, the buyer is encouraged to use NiMH rather than NiCd batteries. This is due to environmental concerns about careless disposal of the spent battery.

The question is often asked, “Has NiMH improved over the last few years?” Experts agree that considerable improvements have been achieved, but the limitations remain. Most of the shortcomings are native to the nickel-based technology and are shared with the NiCd battery. It is widely accepted that NiMH is an interim step to lithium battery technology.

Initially more expensive than the NiCd, the price of the NiMH has dropped and is now almost at par value. This was made possible with high volume production. With a lower demand for NiCd, there will be a tendency for the price to increase. [2.3]

The Lead Acid Battery

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Today, the flooded lead acid battery is used in automobiles, forklifts and large uninterruptible power supply (UPS) systems.

During the mid 1970s, researchers developed a maintenance-free lead acid battery, which could operate in any position. The liquid electrolyte was transformed into moistened separators and the enclosure was sealed. Safety valves were added to allow venting of gas during charge and discharge.

Text Box: The SLA and VRLA can never be charged to their full potential.Driven by diverse applications, two designations of batteries emerged. They are the sealed lead acid (SLA), also known under the brand name of Gelcell, and the valve regulated lead acid (VRLA). Technically, both batteries are the same. No scientific definition exists as to when an SLA becomes a VRLA. (Engineers may argue that the word ‘sealed lead acid’ is a misnomer because no lead acid battery can be totally sealed. In essence, all are valve regulated.)

The SLA has a typical capacity range of 0.2Ah to 30Ah and powers portable and wheeled applications. Typical uses are personal UPS units for PC backup, small emergency lighting units, ventilators for health care patients and wheelchairs. Because of low cost, dependable service and minimal maintenance requirements, the SLA battery is the preferred choice for biomedical and health care instruments in hospitals and retirement homes.

The VRLA battery is generally used for stationary applications. Their capacities range from 30Ah to several thousand Ah and are found in larger UPS systems for power backup. Typical uses are mobile phone repeaters, cable distribution centers, Internet hubs and utilities, as well as power backup for banks, hospitals, airports and military installations.

Unlike the flooded lead acid battery, both the SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging would cause gassing and water depletion. Consequently, the SLA and VRLA can never be charged to their full potential.

Among modern rechargeable batteries, the lead acid battery family has the lowest energy density. For the purpose of analysis, we use the term ‘sealed lead acid’ to describe the lead acid batteries for portable use and ‘valve regulated lead acid’ for stationary applications. Because of our focus on portable batteries, we focus mainly on the SLA.

Text Box: The SLA must always be stored in a charged state.The SLA is not subject to memory. Leaving the battery on float charge for a prolonged time does not cause damage. The battery’s charge retention is best among rechargeable batteries. Whereas the NiCd self-discharges approximately 40 percent of its stored energy in three months, the SLA self-discharges the same amount in one year. The SLA is relatively inexpensive to purchase but the operational costs can be more expensive than the NiCd if full cycles are required on a repetitive basis.

The SLA does not lend itself to fast charging — typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state. Leaving the battery in a discharged condition causes sulfation, a condition that makes the battery difficult, if not impossible, to recharge.

Unlike the NiCd, the SLA does not like deep cycling. A full discharge causes extra strain and each discharge/charge cycle robs the battery of a small amount of capacity. This loss is very small while the battery is in good operating condition, but becomes more acute once the performance drops below 80 percent of its nominal capacity. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger SLA battery is recommended.

Depending on the depth of discharge and operating temperature, the SLA provides 200 to 300 discharge/charge cycles. The primary reason for its relatively short cycle life is grid corrosion of the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at higher operating temperatures. Applying charge/discharge cycles does not prevent or reverse the trend.

There are some methods that improve the performance and prolong the life of the SLA. The optimum operating temperature for a VRLA battery is 25°C (77°F). As a rule of thumb, every 8°C (15°F) rise in temperature will cut the battery life in half. VRLA that would last for 10 years at 25°C would only be good for 5 years if operated at 33°C (95°F). The same battery would endure a little more than one year at a temperature of 42°C (107°F). [2.4]

The SLA has a relatively low energy density compared with other rechargeable batteries, making it unsuitable for handheld devices that demand compact size. In addition, performance at low temperatures is greatly reduced.

The SLA is rated at a 5-hour discharge or 0.2C. Some batteries are even rated at a slow 20 hour discharge. Longer discharge times produce higher capacity readings. The SLA performs well on high pulse currents. During these pulses, discharge rates well in excess of 1C can be drawn.

In terms of disposal, the SLA is less harmful than the NiCd battery but the high lead content makes the SLA environmentally unfriendly. Ninety percent of lead acid-based batteries are being recycled.

The Lithium Ion Battery

Pioneer work with the lithium battery began in 1912 under G.N. Lewis but it was not until the early 1970s that the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the 1980s, but failed due to safety problems.

Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density per weight. Rechargeable batteries using lithium metal anodes (negative electrodes) are capable of providing both high voltage and excellent capacity, resulting in an extraordinary high energy density.

After much research on rechargeable lithium batteries during the 1980s, it was found that cycling causes changes on the lithium electrode. These transformations, which are part of normal wear and tear, reduce the thermal stability, causing potential thermal runaway conditions. When this occurs, the cell temperature quickly approaches the melting point of lithium, resulting in a violent reaction called ‘venting with flame’. A large quantity of rechargeable lithium batteries sent to Japan had to be recalled in 1991 after a battery in a mobile phone released flaming gases and inflicted burns to a person’s face.

Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, the Li-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first Li-ion battery. Other manufacturers followed suit. Today, the Li-ion is the fastest growing and most promising battery chemistry.

The energy density of the Li-ion is typically twice that of the standard NiCd. Improvements in electrode active materials have the potential of increasing the energy density close to three times that of the NiCd. In addition to high capacity, the load characteristics are reasonably good and behave similarly to the NiCd in terms of discharge characteristics (similar shape of discharge profile, but different voltage). The flat discharge curve offers effective utilization of the stored power in a desirable voltage spectrum.

The Li-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery’s life. In addition, the self-discharge is less than half compared to NiCd and NiMH, making the Li-ion well suited for modern fuel gauge applications.

The high cell voltage of Li-ion allows the manufacture of battery packs consisting of only one cell. Many of today’s mobile phones run on a single cell, an advantage that simplifies battery design. Supply voltages of electronic applications have been heading lower, which in turn requires fewer cells per battery pack. To maintain the same power, however, higher currents are needed. This emphasizes the importance of very low cell resistance to allow unrestricted flow of current.

Chemistry variations — During recent years, several types of Li-ion batteries have emerged with only one thing in common — the catchword 'lithium'. Although strikingly similar on the outside, lithium-based batteries can vary widely. This book addresses the lithium-based batteries that are predominantly used in commercial products.

Sony’s original version of the Li-ion used coke, a product of coal, as the negative electrode. Since 1997, most Li-ions (including Sony’s) have shifted to graphite. This electrode provides a flatter discharge voltage curve than coke and offers a sharp knee bend at the end of discharge (see Figure 2-5). As a result, the graphite system delivers the stored energy by only having to discharge to 3.0V/cell, whereas the coke version must be discharged to 2.5V to get similar runtime. In addition, the graphite version is capable of delivering a higher discharge current and remains cooler during charge and discharge than the coke version.

For the positive electrode, two distinct chemistries have emerged. They are cobalt and spinel (also known as manganese). Whereas cobalt has been in use longer, spinel is inherently safer and more forgiving if abused. Small prismatic spinel packs for mobile phones may only include a thermal fuse and temperature sensor. In addition to cost savings on a simplified protection circuit, the raw material cost for spinel is lower than that of cobalt. [2.5]

As a trade-off, spinel offers a slightly lower energy density, suffers capacity loss at temperatures above 40°C and ages quicker than cobalt. Figure 2-6 compares the advantages and disadvantages of the two chemistries. [2.6]

Based on present generation 18650 cells. The energy density tends to be lower for prismatic cells.

The choice of metals, chemicals and additives help balance the critical trade-off between high energy density, long storage time, extended cycle life and safety. High energy densities can be achieved with relative ease. For example, adding more nickel in lieu of cobalt increases the ampere/hours rating and lowers the manufacturing cost but makes the cell less safe. While a start-up company may focus on high energy density to gain quick market acceptance, safety, cycle life and storage capabilities may be compromised. Reputable manufacturers, such as Sony, Panasonic, Sanyo, Moli Energy and Polystor place high importance on safety. Regulatory authorities assure that only safe batteries are sold to the public.

Li-ion cells cause less harm when disposed of than lead or cadmium-based batteries. Among the Li-ion family, the spinel is the friendliest in terms of disposal.

Despite its overall advantages, Li-ion also has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the maximum charge and discharge current is limited and the cell temperature is monitored to prevent temperature extremes. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.

Aging is a concern with most Li-ion batteries. For unknown reasons, battery manufacturers are silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. Over two or perhaps three years, the battery frequently fails. It should be mentioned that other chemistries also have age-related degenerative effects. This is especially true for the NiMH if exposed to high ambient temperatures.

Storing the battery in a cool place slows down the aging process of the Li-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should only be partially charged when in storage.

Extended storage is not recommended for Li-ion batteries. Instead, packs should be rotated. The buyer should be aware of the manufacturing date when purchasing a replacement Li-ion battery. Unfortunately, this information is often encoded in an encrypted serial number and is only available to the manufacturer.

Manufacturers are constantly improving the chemistry of the Li-ion battery. Every six months, a new and enhanced chemical combination is tried. With such rapid progress, it becomes difficult to assess how well the revised battery ages and how it performs after long-term storage.

Cost analysis — The most economical lithium-based battery in terms of cost-to-energy ratio is a pack using the cylindrical 18650 cell. This battery is somewhat bulky but suitable for portable applications such as mobile computing. If a slimmer pack is required (thinner than 18 mm), the prismatic Li-ion cell is the best choice. There is little or no gain in energy density per weight and size over the 18650, however the cost is more than double.

If an ultra-slim geometry is needed (less than 4 mm), the best choice is Li-ion polymer. This is the most expensive option in terms of energy cost. The Li-ion polymer does not offer appreciable energy gains over conventional Li-ion systems, nor does it match the durability of the 18560 cell. [2.7]

Caution: Li-ion batteries have a high energy density. Exercise precaution when handling and testing. Do not short circuit, overcharge, crush, drop, mutilate, penetrate, apply reverse polarity, expose to high temperature or disassemble. Only use the Li-ion battery with the designated protection circuit. High case temperature resulting from abuse of the cell could cause physical injury. The electrolyte is highly flammable. Rupture may cause venting with flame.

The Lithium Polymer Battery

The Li-polymer differentiates itself from other battery systems in the type of electrolyte used. The original design, dating back to the 1970s, uses a dry solid polymer electrolyte only. This electrolyte resembles a plastic-like film that does not conduct electricity but allows an exchange of ions (electrically charged atoms or groups of atoms). The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolyte.

The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile geometry. There is no danger of flammability because no liquid or gelled electrolyte is used.

With a cell thickness measuring as little as one millimeter (0.039 inches), equipment designers are left to their own imagination in terms of form, shape and size. It is possible to create designs which form part of a protective housing, are in the shape of a mat that can be rolled up, or are even embedded into a carrying case or piece of clothing. Such innovative batteries are still a few years away, especially for the commercial market.

Unfortunately, the dry Li-polymer suffers from poor conductivity. Internal resistance is too high and cannot deliver the current bursts needed for modern communication devices and spinning up the hard drives of mobile computing equipment. Although heating the cell to 60°C (140°F) and higher increases the conductivity to acceptable levels, this requirement is unsuitable in commercial applications.

Research is continuing to develop a dry solid Li-polymer battery that performs at room temperature. A dry solid Li-polymer version is expected to be commercially available by 2005. It is expected to be very stable; would run 1000 full cycles and would have higher energy densities than today’s Li-ion battery.

In the meantime, some Li-polymers are used as standby batteries in hot climates. One manufacturer has added heating elements that keeps the battery in the conductive temperature range at all times. Such a battery performs well for the application intended because high ambient temperatures do not affect the service life of this battery in the same way it does the VRLA, for example.

To make a small Li-polymer battery conductive, some gelled electrolyte has been added. Most of the commercial Li-polymer batteries used today for mobile phones are a hybrid and contain gelled electrolyte. The correct term for this system is ‘Lithium Ion Polymer’. For promotional reasons, most battery manufacturers mark the battery simply as Li-polymer. Since the hybrid lithium polymer is the only functioning polymer battery for portable use today, we will focus on this chemistry.

With gelled electrolyte added, what then is the difference between Li-ion and Li-ion polymer? Although the characteristics and performance of the two systems are very similar, the Li-ion polymer is unique in that it uses a solid electrolyte, replacing the porous separator. The gelled electrolyte is simply added to enhance ion conductivity.

Technical difficulties and delays in volume manufacturing have deferred the introduction of the Li-ion polymer battery. This postponement, as some critics argue, is due to ‘cashing in’ on the Li-ion battery. Manufacturers have invested heavily in research, development and equipment to mass-produce the Li-ion. Now businesses and shareholders want to see a return on their investment.

In addition, the promised superiority of the Li-ion polymer has not yet been realized. No improvements in capacity gains have been achieved — in fact, the capacity is slightly less than that of the standard Li-ion battery. For the present, there is no cost advantage in using the Li-ion polymer battery. The thin profile has, however, compelled mobile phone manufacturers to use this promising technology for their new generation handsets.

One of the advantages of the Li-ion polymer, however, is simpler packaging because the electrodes can easily be stacked. Foil packaging, similar to that used in the food industry, is being used. No defined norm in cell size has been established by the industry. [2.8]

Reusable Alkaline Batteries

The idea of recharging alkaline batteries is not new. Although not endorsed by manufacturers, ordinary alkaline batteries have been recharged in households for many years. Recharging these batteries is only effective, however, if the cells have been discharged to less than 50 percent of their total capacity. The number of recharges depends solely on the depth of discharge and is limited to a few at best. With each recharge, less capacity can be reclaimed. There is a cautionary advisory, however: charging ordinary alkaline batteries may generate hydrogen gas, which can lead to explosion. It is therefore not prudent to charge ordinary alkaline unsupervised.

In comparison, the reusable alkaline is designed for repeated recharge. It too loses charge acceptance with each recharge. The longevity of the reusable alkaline is a direct function of the depth of discharge; the deeper the discharge, the fewer cycles the battery can endure.

Tests performed by Cadex on ‘AA’ reusable alkaline cells showed a very high capacity reading on the first discharge. In fact, the energy density was similar to that of a NiMH battery. When the battery was discharged, then later recharged using the manufacturer’s charger, the reusable alkaline settled at 60 percent, a capacity slightly below that of a NiCd. Repeat cycling in the same manner resulted in a fractional capacity loss with each cycle. In our tests, the discharge current was adjusted to 200mA (0.2 C-rate, or one fifth of the rated capacity); the end-of-discharge threshold was set to 1V/cell.

An additional limitation of the reusable alkaline system is its low load current capability of 400mA (lower than 400mA provides better results). Although adequate for portable AM/FM radios, CD players, tape players and flashlights, 400mA is insufficient to power most mobile phones and video cameras.

The reusable alkaline is inexpensive but the cost per cycle is high when compared to the nickel-based rechargeables. Whereas the NiCd checks in at $0.04 per cycle based on 1500 cycles, the reusable alkaline costs $0.50 based on 10 full discharge cycles. For many applications, this seemingly high cost is still economical when compared to the non-reusable alkaline that has a one-time use. If the reusable alkaline battery is only partially discharged before recharge, an improved cycle life is possible. At 50 percent depth of discharge, 50 cycles can be expected.

To compare the operating cost between the standard and reusable alkaline, a study was done on flashlight batteries for hospital use. The low-intensity care unit using the flashlights only occasionally achieved measurable savings by employing the reusable alkaline. The high-intensity unit that used the flashlights constantly, on the other hand, did not attain the same result. Deeper discharge and more frequent recharge reduced their service life and offset any cost advantage over the standard alkaline battery.

In summary, the standard alkaline offers maximum energy density whereas the reusable alkaline provides the benefit of allowing some recharging. The compromise of the reusable alkaline is loss of charge acceptance after the first recharge. [2.9]

The Supercapacitor

The supercapacitor resembles a regular capacitor with the exception that it offers very high capacitance in a small size. Energy storage is by means of static charge. Applying a voltage differential on the positive and negative plates charges the supercapacitor. This concept is similar to an electrical charge that builds up when walking on a carpet. Touching an object at ground potential releases the energy. The supercapacitor concept has been around for a number of years and has found many niche applications.
Whereas a regular capacitor consists of conductive foils and a dry separator, the supercapacitor is a cross between a capacitor and an electro-chemical battery. It uses special electrodes and some electrolyte. There are three kinds of electrode materials suitable for the supercapacitor, namely: high surface area activated carbons, metal oxide and conducting polymers. The one using high surface area activated carbons is the most economical to manufacture. This system is also called Double Layer Capacitor (DLC) because the energy is stored in the double layer formed near the carbon electrode surface.

The electrolyte may be aqueous or organic. The aqueous electrolyte offers low internal resistance but limits the voltage to one volt. In contrast, the organic electrolyte allows two and three volts of charge, but the internal resistance is higher.

To make the supercapacitor practical for use in electronic circuits, higher voltages are needed. Connecting the cells in series accomplishes this task. If more than three or four capacitors are connected in series, voltage balancing must be used to prevent any cell from reaching over-voltage.
The amount of energy a capacitor can hold is measured in microfarads or µF. (1µF = 0.000,001 farad). Small capacitors are measured in nanofarads (1000 times smaller than 1µF) and picofarads (1 million times smaller than 1µF). Supercapacitors are rated in units of 1 farad and higher. The gravimetric energy density is 1 to 10Wh/kg. This energy density is high in comparison to the electrolytic capacitor but lower than batteries. A relatively low internal resistance offers good conductivity.

The supercapacitor provides the energy of approximately one tenth that of the NiMH battery. Whereas the electro-chemical battery delivers a fairly steady voltage in the usable energy spectrum, the voltage of the supercapacitor is linear and drops from full voltage to zero volts without the customary flat voltage curve characterized by most chemical batteries. Because of this linear discharge, the supercapacitor is unable to deliver the full charge. The percentage of charge that is available depends on the voltage requirements of the application.

If, for example, a 6V battery is allowed to discharge to 4.5V before the equipment cuts off, the supercapacitor reaches that threshold within the first quarter of the discharge time. The remaining energy slips into an unusable voltage range. A DC-to-DC converter can be used to increase the voltage range but this option adds costs and introduces inefficiencies of 10 to 15 percent.

The most common supercapacitor applications are memory backup and standby power. In some special applications, the supercapacitor can be used as a direct replacement of the electrochemical battery. Additional uses are filtering and smoothing of pulsed load currents.
A supercapacitor can, for example, improve the current handling of a battery. During low load current, the battery charges the supercapacitor. The stored energy then kicks in when a high load current is requested. This enhances the battery's performance, prolongs the runtime and even extends the longevity of the battery. The supercapacitor will find a ready market for portable fuel cells to compensate for the sluggish performance of some systems and enhance peak performance.

If used as a battery enhancer, the supercapacitor can be placed inside the portable equipment or across the positive and negative terminals in the battery pack. If put into the equipment, provision must be made to limit the high influx of current when the equipment is turned on.

Low impedance supercapacitors can be charged in seconds. The charge characteristics are similar to those of an electro-chemical battery. The initial charge is fairly rapid; the topping charge takes some extra time. In terms of charging method, the supercapacitor resembles the lead acid cell. Full charge takes place when a set voltage limit is reached. Unlike the electro-chemical battery, the supercapacitor does not require a full-charge detection circuit. Supercapacitors can also be trickle charged.

Limitations Unable to use the full energy spectrum - depending on the application, not all energy is available. Low energy density - typically holds one-fifth to one-tenth the energy of an electrochemical battery. Cells have low voltages - serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series. High self-discharge - the self-discharge is considerably higher than that of an electrochemical battery. [2.10]

By nature, the voltage limiting circuit compensates for the self-discharge. The supercapacitor can be recharged and discharged virtually an unlimited number of times. Unlike the electrochemical battery, there is very little wear and tear induced by cycling.

The self-discharge of the supercapacitor is substantially higher than that of the electro-chemical battery. Typically, the voltage of the supercapacitor with an organic electrolyte drops from full charge to the 30 percent level in as little as 10 hours.

Other supercapacitors can retain the charged energy longer. With these designs, the capacity drops from full charge to 85 percent in 10 days. In 30 days, the voltage drops to roughly 65 percent and to 40 percent after 60 days.