The cylindrical cell

The cylindrical cell continues to be the most widely used packaging. It is easy to manufacture, offers high energy density and provides good mechanical stability. The cylinder has the ability to withstand high internal pressures. Typical applications are wireless communication, mobile computing, biomedical instruments, power tools and applications that do not demand ultra-small size.

Most nickel cadmium systems come in cylindrical cells. Other chemistries also make use of the cylindrical design. The 18650 is among the most popular lithium-ion cells ('18' denotes the diameter and '650' the length in millimeters). Lead-based systems are also available in cylindrical design of which the Cyclone by Hawker is the most common.

Cylindrical cells are equipped with a resealable venting mechanism to release pressure under extreme conditions such as excessive overcharge. nickel-based cells can sustain a pressure of about 13.5 Bar or 200 pounds per square inch (psi). Venting occurs between 10-13.5 Bar or 150-200 psi.

The drawback of the cylindrical cell is poor space utilization. Because of fixed cell size, a battery pack must be designed around available cell sizes.

The button cell
The button cell was developed to reduce packs size and improve stacking. Non-rechargeable cells and are found in watches, hearing aids and memory backup.
(Photo courtesy of Sanyo; design courtesy of Panasonic)

The rechargeable button cells are mostly nickel-based and are found in older cordless telephones, biomedical devices and industrial instruments. Although inexpensive to manufacture, the main drawback is charge times of 10-16 hour and swelling if charged too rapidly. New designs claim faster charge capabilities. Button cells have no safety vent.

The prismatic cell
The prismatic cell was developed in the early 1990 to response to consumer demand for thinner geometry. Prismatic cells are commonly reserved for the lithium battery family. The polymer version is exclusively prismatic.

The prismatic cell comes in various sizes with capacities from 400mAh to 2000mAh and higher. No standard cell size exists; rather, prismatic cells are custom-made for cell phones and other high volume items.

The negative attributes of the prismatic cell are slightly lower energy densities and higher manufacturing costs than the cylindrical cell. In addition, the prismatic cell does not provide the same mechanical stability enjoyed by the cylindrical cell.
Prismatic cells have no venting system. To prevent bulging on pressure build up, heavier gauge metal is used for the container. Some degree of bulging must be considered in equipment design.

The pouch cell
The introduction of the pouch cell in 1995 made a profound advancement in cell design. Rather than using expensive metallic enclosures and glass-to-metal electrical feed-troughs, a heat-sealable foil is used. The electrical contacts consist of conductive foil tabs that are welded to the electrode and sealed to the pouch material.

The pouch cell concept allows tailoring to exact cell dimensions. It makes the most efficient use of available space and achieves a packaging efficiency of 90 to 95 percent, the highest among battery packs. Because of the absence of a metal can, the pouch pack is light. The main application is cell phones. No standardized pouch cells exist, each manufacturer builds to a special application.

The pouch cell is exclusively used for lithium-based chemistries. Manufacturing cost is still higher than conventional systems and its reliability has not been fully proven. In addition, the energy density and load current are slightly lower. The cycle life is not well documented but remains less than that of other packaging systems.

A critical issue with the pouch cell is the swelling that occurs when gas is generated during charging or discharging. Allowance must be made for some expansion, even though battery manufacturers insist that the cells do not generate gas if correctly charged. It is best not to stack pouch cells, but lay them side-by-side.

The pouch cell is highly sensitive to twisting. Point pressure must also be avoided. The protective housing must be designed to protect the cell from mechanical stress.

2:42:06 PM    

Lithium-ion-polymer has not caught on as quickly as some analysts had expected. Its superiority to other systems and low manufacturing costs has not been realized. No improvements in capacity gains are achieved - in fact, the capacity is slightly less than that of the standard lithium-ion battery. lithium-ion-polymer finds its market niche in wafer-thin geometries, such as batteries for credit cards and other such applications.

Advantages
· Very low profile - batteries resembling the profile of a credit card are feasible.
· Flexible form factor - manufacturers are not bound by standard cell formats. With high volume, any reasonable size can be produced economically.
· Lightweight - gelled electrolytes enable simplified packaging by eliminating the metal shell.
· Improved safety - more resistant to overcharge; less chance for electrolyte leakage.

Limitations
· Lower energy density and decreased cycle count compared to lithium-ion.
· Expensive to manufacture.
· No standard sizes. Most cells are produced for high volume consumer markets.
· Higher cost-to-energy ratio than lithium-ion

2:38:14 PM    

The most economical lithium-ion battery in terms of cost-to-energy ratio is the cylindrical 18650 (18 is the diameter and 650 the length in mm). This cell is used for mobile computing and other applications that do not demand ultra-thin geometry. If a slim pack is required, the prismatic lithium-ion cell is the best choice. These cells come at a higher cost in terms of stored energy.

Advantages
· High energy density - potential for yet higher capacities.
· Does not need prolonged priming when new. One regular charge is all that's needed
· Relatively low self-discharge - self-discharge is less than half that of nickel-based batteries.
· Low Maintenance - no periodic discharge is needed; there is no memory

Limitations
· Requires protection circuit to maintain voltage and current within safe limits.
· Subject to aging, even if not in use - storing the battery in a cool place and at 40% charge reduces the aging effect.
· Moderate discharge current - not suitable for heavy loads.
· Transportation restrictions - shipment of larger quantities may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.
· Expensive to manufacture - about 40 percent higher in cost than nickel-cadmium.
· Not fully mature - metals and chemicals are changing on a continuing b

2:36:57 PM    

The energy density of lithium-ion is typically twice that of the standard nickel-cadmium. There is potential for higher energy densities. The load characteristics are reasonably good and behave similarly to nickel-cadmium in terms of discharge. The high cell voltage of 3.6 volts allows battery pack designs with only one cell. Most of today's mobile phones run on a single cell. A nickel-based pack would require three 1.2-volt cells connected in series.

2:35:47 PM    

Here is a summary of the advantages and limitations of nickel-metal hydride batteries.

Advantages
-30 to 40% higher capacity over standard nickel-cadmium. - nickel-metal hydride has potential for yet higher energy densities.
-Less prone to memory than nickel-cadmium - fewer exercise cycles are required.
-Simple storage and transportation - transport is not subject to regulatory control.
-Environmentally friendly - contains only mild toxins; profitable for recycling.

Limitations
-Limited service life - the performance starts to deteriorate after 200 to 300 cycles if repeatedly deeply cycled
-Limited discharge current - although nickel-metal hydride is capable of delivering high discharge currents, heavy load reduces the battery's cycle life.
-More complex charge algorithm needed - nickel-metal hydride generates more heat during charge and requires slightly longer charge times than nickel-cadmium. Trickle charge settings are critical because the battery cannot absorb overcharge.
-High self-discharge - typically 50% higher than nickel-cadmium. New chemical additives have improved self-discharge but at the expense of lower energy density.
-Performance degrades if stored at elevated temperatures - nickel-metal hydride should be stored in a cool place at 40% state-of-charge.
-High maintenance - nickel-metal hydride requires regular full discharge to prevent crystalline formation. nickel-cadmium should be exercised once a month, nickel-metal hydride once in every 3 months.

2:34:47 PM    

Advantages
-Fast and simple charge, even after prolonged storage.
-High number of charge/discharge cycles - if properly maintained, nickel-cadmium provides over 1000 charge/discharge cycles.
-Good load performance - nickel-cadmium allows recharging at low temperatures.
-Long shelf life - five-year storage is possible. Some priming prior to use will be required.
-Simple storage and transportation - most airfreight companies accept nickel-cadmium without special conditions.
-Good low temperature performance.
-Forgiving if abused - nickel-cadmium is one of the most rugged rechargeable batteries.
-Economically priced - nickel-cadmium is lowest in terms of cost per cycle.
-Available in a wide range of sizes and performance options - most nickel-cadmium cells are cylindrical.

Limitations
-Relatively low energy density.
-Memory effect - nickel-cadmium must periodically be exercised to prevent memory.
-Environmentally unfriendly - nickel-cadmium contains toxic metals. Some countries restrict its use.
-Relatively high self-discharge - needs recharging after storage

2:33:47 PM    

Table 1 summarizes the characteristics of the common batteries. The figures are based on average ratings at time of publication. Note that nickel-cadmium has the shortest charge time, delivers the highest load current and offers the lowest overall cost-per-cycle but needs regular maintenance.


Table 1: Characteristics of commonly used rechargeable batteries.

1) Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of lithium?ion and lithium-ion-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, and then tapers off. The capacity of nickel-cadmium 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 as 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.

2:32:38 PM    

Below is a summary of the strength and limitations of today's popular battery systems. Although energy density is paramount, other important attributes are service life, load characteristics, maintenance requirements, self-discharge and operational costs. Since nickel-cadmium remains a standard against which batteries are compared, we evaluate alternative chemistries against this classic battery type.

-Nickel-cadmium - mature but has moderate energy density. nickel-cadmium is used where long life, high discharge rate and extended temperature range is important. Main applications are two-way radios, biomedical equipment and power tools. nickel-cadmium contains toxic metals.

-Nickel-metal-hydride - has a higher energy density compared to nickel-cadmium at the expense of reduced cycle life. There are no toxic metals. Applications include mobile phones and laptop computers.

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

-Lithium-ion - fastest growing battery system; offers high-energy density and low weight. Protection circuit are needed to limit voltage and current for safety reasons. Applications include notebook computers and cell phones.

-Lithium-ion-polymer - Similar to lithium-ion, this system enables slim geometry and simple packaging at the expense of higher cost per watt/hours. Main applications are cell phones.

-Reusable Alkaline - Its limited cycle life and low load current is compensated by long shelf life, making this battery ideal for portable entertainment devices and flashlights.
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Howstuffworks "How Digital Jewelry Will Work"
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Howstuffworks "How Power Paper Will Work"
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Power Paper will work exactly like a traditional battery, but it will be nearly as thin as a piece of paper. A Power Paper cell can generate 1.5 volts of electricity, which is about the same output as a watch or calculator battery. A Power Paper cell will be 0.5 millimeters thick, and several cells can be used in combination to provide more power. Here's how the Power Paper cell will work:

• A zinc and manganese dioxide (MnO₂) -based cathode and anode are fabricated from proprietary inks. In a battery, the cathode refers to the positive terminal and the anode refers to the negative terminal.

• Standard silkscreen printing presses are used to print the batteries onto paper and other substrates.

• Power Paper batteries are integrated into production and assembly processes of thin electronic devices.

Image courtesy Power Paper A Power Paper cell integrated with a sheet of paper

Power Paper batteries are printed directly onto thin substrates, such as paper, so they are far more flexible than any other battery. Because ink is used to produce Power Paper, the batteries are considered dry, and don't need the metal casing that conventional batteries do to contain harmful, toxic chemicals. This lack of casing allows electronics manufacturers to utilize the power source in many shapes and sizes. Since it doesn't require special production equipment, Power Paper can be made outside of clean- or dry-room conditions, which lowers production costs. Power Paper batteries can be produced for a mere 1 cent per square inch.

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Howstuffworks "How Printable Computers Will Work"
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Howstuffworks "How Printable Computers Will Work"
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Howstuffworks "How Computerized Clothing Will Work"
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Howstuffworks "How Batteries Work"
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Yes, it does exist. And, yes, your batteries could possibly have the effects of it. It's the memory effect. The term "memory" basically is described as the battery "remembers" its usual discharge point and superficially "needs" a charge whenever it hits that point. In other words, if you have a NiCd that always gets discharged to only 50% of its capacity, it will eventually not run below that 50% mark if you ever wanted to discharge it to a lower point. Many people who do not know about this effect just throw away the battery because they think it is dead. More than likely, the battery can revived providing that the battery isn't completely damaged (i.e. from years of memory buildup). The most simple way to get rid of memory is to discharge the battery to 1.0 volts per cell (VPC) on a minimal load, and then charge it fully. Repeat this procedure until you notice the battery lasting longer and longer on the drain, until it holds its correct capacity and not the "memorized" one. Unfortunately, unless you have good equipment, it is hard to discharge to 1.0 VPC without accidentally "reversing" a cell. (See the Universal Camcorder Battery Charger Page) Now, if you were only working on one cell at a time, discharging to 1.0 VPC would be easy, but most batteries nowadays for cellular phones and such are multiple cells in a plastic case. This makes it hard to get every cell to 1.0 VPC. No batteries are created equal, and what will most likely happen in a multi-cell battery is that one or more of the cells will "reverse" because they are weaker than the other cells. The reversed cell begins to accept a "backwards" charge from the other better charged cells around it. This is really bad for a battery if you don't catch it, because chances are it won't charge again while in the pack.
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Howstuffworks "How Batteries Work"
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Howstuffworks "How Batteries Work"
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