Lithium-ion polymer batteries, polymer lithium ion, or more commonly lithium polymer batteries (abbreviated Li-poly, Li-Pol, LiPo, LIP, PLI or LiP) are rechargeable batteries which have technologically evolved from lithium-ion batteries. Ultimately, the lithium-salt electrolyte is not held in an organic solvent as in the lithium-ion design, but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile. The advantages of Li-poly over the lithium-ion design include lower cost manufacturing and being more robust to physical damage. Lithium-ion polymer batteries started appearing in consumer electronics around 1996.

Overview

Cells sold today as polymer batteries have a different design from the older lithium-ion cells. Unlike lithium-ion cylindrical, or prismatic cells, which have a rigid metal case, polymer cells have a flexible, foil-type (polymer laminate) case, but they still contain organic solvent. The main difference between commercial polymer and lithium-ion cells is that in the latter the rigid case presses the electrodes and the separator onto each other, whereas in polymer cells this external pressure is not required because the electrode sheets and the separator sheets are laminated onto each other.

Since no metal battery cell casing is needed, the battery can be lighter and it can be specifically shaped to fit the device it will power. Because of the denser packaging without intercell spacing between cylindrical cells and the lack of metal casing, the energy density of Li-poly batteries is over 20% higher than that of a classical Li-ion battery and they store more energy than nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries of the same volume.

The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination. Overcharging a Li-poly battery will likely result in explosion and/or fire. During discharge on load, the load has to be removed as soon as the voltage drops below approximately 3.0 V per cell (used in a series combination), or else the battery will subsequently no longer accept a full charge and may experience problems holding voltage under load.

Early in its development, lithium polymer technology had problems with internal resistance. Other challenges include longer charge times and slower maximum discharge rates compared to more mature technologies. Li-poly batteries typically require more than an hour for a full charge. Recent design improvements have increased maximum discharge currents from two times to 15 or even 30 times the cell capacity (discharge rate in amps, cell capacity in amp-hours). In March 2005 Toshiba announced a new design offering a much faster (about 1–3 minutes) rate of charge. These cells have yet to reach the market but are expected to have a dramatic effect on the power tool and electric vehicle industries, and a major effect on consumer electronics.[citation needed]

When compared to the lithium-ion battery, Li-poly has a greater life cycle degradation rate. However, in recent years, manufacturers have been declaring upwards of 500 charge-discharge cycles before the capacity drops to 80% (see Sanyo). Another variant of Li-poly cells, the "thin film rechargeable lithium battery", has been shown to provide more than 10,000 cycles.
http://en.wikipedia.org/wiki/Li-poly

Charging Li-Poly Batteries

Lithium Polymer or LiPo batteries have very specific charging requirements and MUST only be changed by specific chargers designed to charge lithium polymer batteries.

A 1s or 1 cell LiPo battery has a nominal voltage of 3.7v. When fully charged it has a maximum voltage of 4.2v and when fully discharged, it should never go below 3.0v without risking cell damage.

A 5s4p battery pack means that the pack contains 5 cells in a series circuit and 4 cells in a parallel circuit.

Since each cell is 3.7v (nominal) a 5s LiPo battery has a nominal voltage of 18.5v, a fully charged voltage of 21.0v and a maximum discharged voltage of 15.0v before damage occurs.

When charging LiPo batteries, they must be charged at the voltage of the number of cells in series, therefore a 5s4p pack must be charged as a 5 cell pack.

The LiPo charger you’re using must be able to handle the cell count of the battery you are charging.

Most of the more expensive LiPo chargers will automatically detect the cell count of the battery being charged while the cheaper ones will require a manual setting. Wile some of the really good ones will allow you to manually select the cell count and then will double check it automatically for you.

Please also note that some chargers (such as those used for toys or cell phones) are made to charge a specific cell count and are not configurable for other cell counts. It is very important that these chargers only be used to charge the batteries they are designed for.

Also, chargers that auto detect the cell count of a LiPo battery can sometimes be wrong. They use the current voltage of the battery to determine the cell count and if the battery is fully charged or at a lower voltage than it should be, it may read the cell count incorrectly. This is why it is very important to double check that it reads he right cell count which is typically displayed on the LCD display.

For example a 5 cell 18.5 volt LiPo battery that’s been depleted to less than 15 volts may be confused with a 4 cell, 14.8v battery and thus charged as such. Also, a fully charged 5 cell battery at 21.0v may be confused as a 6 cell 22.0v battery and charged as such.

Charging a lithium polymer battery at a higher voltage than it’s rated for, or overcharging it, can lead to a fire or an explosion (see video below).

LiPo Battery Charging Tips

• Always use a charger made to charge LiPo packs.
• Double check that the settings for the lithium polymer charger are correct for the pack being charged – this includes the cell count as well as the current settings.
• In general, most lithium polymer batteries should be charged to no more than 4.2 volts per cell or depleted to less than 3.0 volts per cell. There are new generation batteries available that can handle higher / lower voltages, but they are still new and thus are the exception to the rule.
• Ensure that charging leads are connected correctly. Reverse charging can lead to cell damage or a fire or explosion.
• Always charge LiPo batteries on surfaces that won’t catch on fire such as cement, steel, ceramic or stone. Wooden tables and carpeted floors are not recommended charging surfaces.
• Do not charge batteries near flammable products or liquids.
• Never charge a LiPo battery while inside your model or other electronic device. If it catches fire it can lead to total destruction of the item it is being charged in.
• LiPo batteries should be charged within a temperature range of 0C to 50C. Batteries charged outside this temperature range may experience leakage, heat generation or cell damage.
• Never leave a charging lithium polymer battery pack unattended.
• Do not charge inside an automobile, especially while driving.
• Do not store batteries inside an automobile.
• Do not charge a lithium polymer battery pack at a rate over 1C.
• Never charge a LiPo pack that has ballooned or swelled due to over / under charging or from a crash.
• Never charge a lithium polymer battery pack that has been punctured or damaged in a crash.
• Never, under ANY circumstances let the positive and negative battery leads touch. It can lead to cell ballooning, cell damage or fire or an explosion.
• Have a fire extinguisher near the charging area or a large bucket of dry sand. Do not try to distinguish with water.
• If you notice your LiPo battery pack is swelling, stop the charging process immediately, put the battery in a safe container and observe it for 15 minutes.

http://www.rchelisite.com/lipo_battery_charging_and_safety_guide.php

Prolonging life in multiple cells through cell balancing

Analog front ends that balance cells and eliminate mismatches of cells in series or parallel significantly improve battery efficiency and increase the overall pack capacity. As the number of cells and load currents increase, the potential for mismatch also increases. There are two kinds of mismatch in the pack: State-of-Charge (SOC) and capacity/energy (C/E) mismatch. Though the SOC mismatch is more common, each problem limits the pack capacity (mAh) to the capacity of the weakest cell.

It is important to recognize that the cell mismatch results more from limitations in process control and inspection than from variations inherent in the Lithium Ion chemistry. The use of cell balancing can improve the performance of series connected Li-ion Cells by addressing both SOC and C/E issues.[3] SOC mismatch can be remedied by balancing the cell during an initial conditioning period and subsequently only during the charge phase. C/E mismatch remedies are more difficult to implement and harder to measure and require balancing during both charge and discharge periods.

Cell balancing is defined as the application of differential currents to individual cells (or combinations of cells) in a series string. Normally, of course, cells in a series string receive identical currents. A battery pack requires additional components and circuitry to achieve cell balancing. However, the use of a fully integrated analog front end for cell balancing[4] reduces the required external components to just balancing resistors.

This type of solution eliminates the need for discrete capacitors, diodes and most other resistors to achieve balance.

Battery pack cells are balanced when all the cells in the battery pack meet two conditions:

If all cells have the same capacity, then they are balanced when they have the same relative State of Charge (SOC.) In this case, the Open Circuit Voltage (OCV) is a good measure of the SOC. If, in an out of balance pack, all cells can be differentially charged to full capacity (balanced), then they will subsequently cycle normally without any additional adjustments. This is mostly a one shot fix.
 

If the cells have different capacities, they are also considered balanced when the SOC is the same. But, since SOC is a relative measure, the absolute amount of capacity for each cell is different. To keep the cells with different capacities at the same SOC, cell balancing must provide differential amounts of current to cells in the series string during both charge and discharge on every cycle.

http://en.wikipedia.org/wiki/Li-poly

LiPo Battery Handling & Storage

• Keep LiPo battery packs WELL out of reach of children.
• Do not put battery packs in pockets or bags where they can short circuit.
• Do not store or transport or store batteries where they can come into contact with sharp or metallic objects.
• Do not store your LiPo pack in extreme temperatures below 0C or above 50C.
• Always store your LiPo pack in a safe and non flammable container away from flammable objects. A LiPo Sack or metal / ceramic storage container is best.
• Always store your LiPo’s partially charged. They will maintain their performance levels over time and there’s no need to cycle them unless stored for periods longer than 3-6 months.

Other LiPo Battery Tips

• Do not immerse the battery in water or allow the battery to get wet.
• Do not short circuit the battery.
• Do not pierce the lithium polymer battery with a sharp object – it will lead to ignition or an explosion.
• Do not short circuit the battery.
• Do not solder directly to the battery.
• Do not hit the battery with a hard object such as a hammer or rock.
• Do not dispose of in fire or heat.
• Do not use the battery with the positive and negative terminals reversed.
• Do not disassemble or modify the battery.
• Do not fully discharge your LiPo battery pack. Discharging a LiPo beyond it’s critical minimum voltage (often 3.0v) can cause damage to the battery.
• Do not breath in the smoke fumes of a LiPo battery that is on fire. They are toxic.
• When mailing or shipping LiPo batteries, always ship them at a 30% charged state for safety reasons.
• When storing batteries for extended periods, store at a half charged state.
• To dispose of a LiPo battery, discharge it fully then place it in a bucket of salt water for one week. To dispose of, follow your municipal battery disposal guidelines.
• If your battery becomes damaged, do not place it on a flammable surface - it’s possible that a chemical reaction can take place which could cause a fire. Put the battery in a safe and non flammable place and observe it for at least an hour.
• If the electrolyte in the cells gets on your skin, thoroughly wash with soap and water. If it gets in your eyes, rinse thoroughly with cool water and seek immediate medical attention.
• Finally, always follow the manufacturer’s safety instructions and charging guidelines for lithium polymer battery packs.

http://www.rchelisite.com/lipo_battery_charging_and_safety_guide.php

How to store batteries

Batteries are perishable products that start deteriorating right from the moment they leave the factory. There are simple preventive measures that battery users can apply to slow the aging process. This paper provides guidelines to reduce age-related capacity losses and how to prime new and stored batteries.

The recommended storage temperature for most batteries is 15°C (59°F). While lead-acid batteries must always be kept at full charge, nickel and lithium-based chemistries should be stored at 40% state-of-charge (SoC). This level minimizes age-related capacity loss, yet keeps the battery in operating condition even with some self-discharge. While the open terminal voltage of nickel-based batteries cannot be used to determine the SoC accurately, voltage fuel gauging works well for lithium-ion cells. However, differences in the electrochemistry of the electrodes and electrolyte between manufacturers vary the voltage profile slightly. A SoC of 50% reads about 3.8V; 40% is 3.75V. Store lithium-ion at an open terminal voltage of 3.75-3.80V. Allow the battery to rest 90 minutes after charge before taking the voltage reading.

Figure 1 illustrates the recoverable capacity at various storage temperatures and charge levels over one year.


Figure 1: Non-recoverable capacity loss on lithium-ion and nickel-based batteries after storage. High charge levels and elevated temperatures hasten the capacity loss.

Among the lithium-ion family, cobalt has a slight advantage over manganese (spinel) in terms of storage at elevated temperatures. nickel-based batteries are also affected by elevated temperature but to a lesser degree than lithium-ion.

Lithium-ion powers most of today's laptop computers. The battery compartment on many laptops rises to about 45°C (113°F) during operation. The combination of high charge level and elevated ambient temperature presents an unfavorable condition for the battery. This explains the short lifespan of many laptop batteries.

Nickel-metal-hydride can be stored for about three years. The capacity drop that occurs during storage is permanent and cannot be reversed. Cool temperatures and a partial charge slows aging. Nickel-cadmium stores reasonably well. Field test reveled that NiCd batteries stored for five years still performed well after priming cycles. Alkaline and lithium batteries (primary) can be stored for up to 10 years. The capacity loss is minimal.

The sealed lead-acid battery can be stored for up to two years. A periodic topping charge, also referred to as 'refresh charge', is required to prevent the open cell voltage from dropping below 2.10V. (Some lead-acid batteries may allow lower voltage levels.) Insufficient charge induces sulfation, an oxidation layer on the negative plate that inhibits the current flow on charge and discharge. Topping charge and/or cycling may restore some of the capacity losses in the early stages.

Priming new batteries

Manufacturers recommend to trickle charge a nickel-based battery for 24 hours when new and after long storage. This service brings all cells to equal charge level and redistributes the electrolyte to remedy dry spots on the separator brought on by gravitation of the electrolyte. It is advisable to verify the capacity with a battery analyzer before use. This is especially important in critical applications.

Cycling (priming) is recommended to regain lost capacity after a nickel-based battery has been stored for 6 months or longer. A slow charge followed by one or several discharge/charge cycles will do this. The recovery rate is governed by the condition under which the battery was stored. The longer and warmer the storage temperature, the more cycles will be required. The Prime program of the Cadex battery analyzers automatically applies the number of cycles needed to regain full capacity.

Nickel-based batteries are not always fully formed when leaving the factory. Applying several charge/ discharge cycles through normal use or with a battery analyzer completes the forming. The number of cycles needed to attain full capacity differs between cell manufacturers. Quality cells perform to specification after 5-7 cycles. Those lacking formation may need 50 or more cycles to reach acceptable capacity levels.

What is the difference between priming and forming? For the user, both symptoms manifest themselves as insufficient capacity. The difference may be explained in that forming needs to be done only once when the battery is new, while priming must be repeated after each prolonged storage.

Lithium-ion batteries deliver full power after the initial charge. Manufacturers of lithium-ion cells insist that no priming is required. However, priming is beneficial as an initial start and to verify battery performance. Excessive cycling should be avoided because of wear-down effect.

The internal protection circuit of lithium-based batteries is known to cause some problems after a long storage. If the battery is left discharged after use, the self-discharge will further drain the pack and eventually drip the protection circuit at about 2.5 volts per cell. At this point, the charger will no longer recognize the battery and the pack appears dead. Advanced battery analyzers (Cadex) feature the Boost program that activates the protection circuit to enable a recharge. If the cell voltage has fallen below 1.5V/cell and has remained in that state for a few days, a recharge should be avoided for safety reasons.

To reduce the self-discharge on newly manufactured batteries, advanced lithium-ion packs feature a sleep mode that keeps the protection circuit off until activated by a brief charge. Once engaged, the battery remains operational and the advantage of the sleep mode no longer applies.

Lead-acid batteries should be primed by applying a full charge, followed by a discharge and recharge. Verifying the capacity through a discharge is important, especially if the battery is engaged in critical applications such as powering medical devices. Priming is also recommended after storing a battery for six months and longer. Battery analyzers provide the priming service automatically.

It is believed that a partial or full discharge applied once every six months or so enhances the performance of lead-acid batteries. Avoid too many full discharges, as this would wear down the battery unnecessarily.

While capacity loss during a battery's life cannot be eliminated, simple guidelines minimize the effect:

• Keep batteries in a cool and dry storage area. Refrigeration is recommended but freezers should be avoided. When refrigerated, the battery should be placed in a plastic bag to protect against condensation
   
• Do not fully charge lithium and nickel-based batteries before storage. Keep them partially charged and apply a full charge before use. Store lithium-ion at about 40% state-of-charge (3.75-3.80V/cell open terminal). Lead-acid batteries must be stored fully charged.
   
• Do not store lithium-ion fully depleted. If empty, charge for about 30 minutes before storage. Self-discharge on a depleted battery may cause the protection circuit to trip, preventing a recharge.
   
• Do not stockpile lithium-ion batteries; avoid buying dated stock, even if offered at a reduced price. Observe the manufacturing date, if available.
   
• Never leave a nickel-based battery sitting on a charger for more than a few days. Prolonged trickle charge causes crystalline formation (memory).
   
• Always store a lead acid battery in full-charge condition. Observe the open terminal voltage and recharge the battery every 6 months or as recommended by the manufacturer.

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Created: May 2003, Last edited: March 2004

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.

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• Over 3 billion batteries are sold each year in the U.S.
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Recycling batteries

Modern batteries are often promoted on their environmental qualities. lithium-based batteries fall into this category. While nickel-cadmium presents an environmental problem on careless disposal, this chemistry continues to hold an important position among rechargeable batteries. Power tools are almost exclusively powered by nickel-cadmium. Lead-acid batteries continue to service designated market niches and these batteries also need to be disposed of in a proper manner. lithium-ion would simply be too fragile to replace many of these older, but environmentally unfriendly, battery chemistries.

Our quest for portability and mobility is steadily growing, so is the demand for batteries. Where will the mountains of batteries go when spent? The answer is recycling.

The lead-acid battery has led the way in recycling. The automotive industry should be given credit in organizing ways to dispose of spent car batteries. In the USA, 98% of all lead-acid batteries are recycled. In comparison, only one in six households in North America recycle batteries.

Careless disposal of nickel-cadmium is hazardous to the environment. If used in landfills, the cadmium will eventually dissolve itself and the toxic substance can seep into the water supply, causing serious health problems. Our oceans are already beginning to show traces of cadmium (along with aspirin, penicillin and antidepressants) but the source of the contamination is unknown.

Although nickel-metal-hydride is considered environmentally friendly, this chemistry is also being recycled. The main derivative is nickel, which is considered semi-toxic. nickel-metal-hydride also contains electrolyte that, in large amounts, is hazardous. If no disposal service is available in an area, individual nickel-metal-hydride batteries can be discarded with other household wastes. If ten or more batteries are accumulated, the user should consider disposing of these packs in a secure waste landfill.

Lithium (metal) batteries contain no toxic metals, however, there is the possibility of fire if the metallic lithium is exposed to moisture while the cells are corroding. Most lithium batteries are non-rechargeable and are used in cameras, hearing aids and defense applications. For proper disposal, the batteries must first be fully discharged to consume the metallic lithium content.
Lithium-ion batteries used for cell phones and laptops do not contain metallic lithium and the disposal problem does not exist. Most lithium systems contain toxic and flammable electrolyte.

In 1994, the Rechargeable Battery Recycling Corporation (RBRC) was founded to promote recycling of rechargeable batteries in North America. RBRC is a non-profit organization that collects batteries from consumers and businesses and sends them to recycling organizations. Inmetco and Toxco are among the best-known recycling companies in North America Europe and Asia have had programs to recycle spent batteries for many years. Sony and Sumitomo Metal in Japan have developed a technology to recycle cobalt and other precious metals from spent lithium-ion batteries.

Battery recycling plants require that the batteries be sorted according to chemistries. Some sorting must be done prior to the battery arriving at the recycling plants. nickel-cadmium, nickel-metal-hydride, lithium-ion and lead acid are placed in designated boxes at the collection point. Battery recyclers claim that if a steady stream of batteries, sorted by chemistry, were available at no charge, recycling would be profitable. But preparation and transportation add to the cost.

The recycling process starts by removing the combustible material, such as plastics and insulation, with a gas fired thermal oxidizer. Gases from the thermal oxidizer are sent to the plant's scrubber where they are neutralized to remove pollutants. The process leaves the clean, naked cells, which contain valuable metal content.

The cells are then chopped into small pieces, which are heated until the metal liquefies. Non-metallic substances are burned off; leaving a black slag on top that is removed with a slag arm. The different alloys settle according to their weights and are skimmed off like cream from raw milk.

Cadmium is relatively light and vaporizes at high temperatures. In a process that appears like a pan boiling over, a fan blows the cadmium vapor into a large tube, which is cooled with water mist. This causes the vapors to condense and produces cadmium that is 99.95 percent pure.

Some recyclers do not separate the metals on site but pour the liquid metals directly into what the industry refers to as 'pigs' (65 pounds) or 'hogs' (2000 pounds). The pigs and hogs are then shipped to metal recovery plants. Here, the material is used to produce nickel, chromium and iron re-melt alloy for the manufacturing of stainless steel and other high-end products.

Current battery recycling methods requires a high amount of energy. It takes six to ten times the amount of energy to reclaim metals from recycled batteries than it would through other means.

Who pays for the recycling of batteries? Participating countries impose their own rules in making recycling feasible. In North America, some recycling plants bill on weight. The rates vary according to chemistry. Systems that yield high metal retrieval rates are priced lower than those, which produce less valuable metals.

Nickel-metal-hydride yields the best return. It produces enough nickel to pay for the process. The highest recycling fees apply to nickel-cadmium and lithium-ion because the demand for cadmium is low and lithium-ion contains little retrievable metal.

Not all countries base the cost of recycling on the battery chemistry; some put it on tonnage alone. The flat cost to recycle batteries is about $1,000 to $2,000US per ton. Europe hopes to achieve a cost per ton of $300US. Ideally, this would include transportation, however, moving the goods is expected to double the overall cost. For this reason, Europe sets up several smaller processing locations in strategic geographic locations.

Significant subsidies are sill required from manufacturers, agencies and governments to support the battery recycling programs. This funding is in the form of a tax added to each manufactured cell. RBRC is financed by such a scheme.

Important: Under no circumstances should batteries be incinerated as this can cause explosion. If skin is exposed to electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.

_________________________
Created: May 2003, Last edited: July 2003

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.