Battery novices often brag about miracle batteries that offer very high energy densities, deliver 1000 charge/discharge cycles and are paper-thin. These attributes are indeed achievable but not on one and the same battery pack.
A certain battery may be designed for small size and long runtime but has a limited cycle life. Another pack may be built for durability and is big and bulky. A third may have high energy density and long durability but is made for a special application and is too expensive for the average consumer. A lithium-based battery can be designed for maximum energy density but its safety would be compromised.
Battery manufacturers are aware of customer needs and offer packs that best suit the application. The mobile phone industry is a good example of this clever adaptation. Here, small size and high energy density reign in favor of longevity. Short service life is not an issue because a device is often replaced before the battery is worn out.
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 costs and safety. Nickel-cadmium is the first rechargeable battery in small format and forms a standard against which other chemistries are commonly compared. The trend is towards lithium-based 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. High current versions are available for power tools and medical devices.
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.
2. The secrets of battery runtime
Is the runtime of a portable device directly related to the size of the battery? The answer should be 'yes' but in reality, the runtime is governed by other attributes than the specified capacity alone.
This paper examines the cause of unexpected downtime and short battery service life. We look at four renegades - declining capacity, increasing internal resistance, elevated self-discharge and premature voltage cut-off on discharge. We evaluate how these regenerative deficiencies affect nickel, lead and lithium-based batteries.
Declining capacity
The amount of charge a battery can hold gradually decreases due to usage and aging. Specified to deliver 100% capacity when new, the battery should be replaced when the capacity drops to below 80% of the nominal rating. Some organizations may use different end-capacities as a minimal acceptable performance threshold.
The energy storage of a battery can be divided into three imaginary sections consisting of: available energy, the empty zone that can be refilled, and the unusable part (rock content) that increases with aging. Figure 1 illustrates these three sections.
In nickel-based batteries, the so-called rock content is present in form of crystalline formation, also known as memory. Restoration is possible with a full discharge to one volt per cell. However, if no service is done for four months and longer, a full repair becomes increasingly more difficult the longer service is withheld. To prevent memory, nickel-based batteries should be deep-cycled once every one or two months. Nickel-cadmium and nickel-metal-hydride batteries are used for two-way radios, medical instruments and power tools.
Performance degradation of the lead-acid battery is caused by sulfation and grid corrosion. Sulfation is a thin layer that forms on the negative cell plate if the battery is being denied a fully saturated charge. Sulfation can, in part, be corrected with cycling and/or topping charge. The grid corrosion, which occurs on the positive plate, is caused by over-charge. Lead-acid batteries are used for larger portable devices and wheeled applications.
Lithium-ion batteries lose capacity through cell oxidation, a process that occurs naturally during use and aging. The typical life span of lithium-ion is 2-3 years under normal use. Cool storage a 40% charge minimizes aging. An aged lithium-ion cannot be restored with cycling. Lithium-ion is found in cell phones and mobile computing.
Increasing internal Resistance
The capacity of a battery defines the stored energy - the internal resistance governs how much energy can be delivered at any given time. While a good battery is able to provide high current on demand, the voltage of a battery with elevated resistance collapses under a heavy load. Although the battery may hold sufficient capacity, the resulting voltage drop triggers the 'low battery' indicator and the equipment stops functioning. Heating the battery will momentarily increase the output by lowering the resistance.
A battery with high internal resistance may still perform adequately on a low current appliance such as a flashlight, portable CD player or wall clock. Digital equipment, on the other hand, draw heavy current bursts. Figure 2 simulates low and high internal resistance with a free-flowing and restricted tap.
Nickel-cadmium offers very low internal resistance and delivers high current on demand. In comparison, nickel-metal-hydride starts with a slightly higher resistance and the readings increase rapidly after 300 to 400 cycles.
Lithium-ion has a slightly higher internal resistance than nickel-based batteries. The cobalt system tends to increase the internal resistance as part of aging whereas the manganese (spinel) maintains the resistance throughout its life but loses capacity through chemical reaction. Cobalt and manganese are used for the positive electrodes.
High internal resistance will eventually render the battery useless. The energy may still be present but can no longer be delivered. This condition is permanent and cannot be reversed with cycling. Cool storage at a partial state-of-charged (40%) retards the aging process.
The internal resistance of Lead-acid batteries is very low. The battery responds well to short current bursts but has difficulty providing a high, sustained load. Over time, the internal resistance increases through sulfation and grid corrosion.
Elevated self-discharge
All batteries suffer from self-discharge, of which nickel-based batteries are among the highest. The loss is asymptotical, meaning that the self-discharge is highest right after charge and then levels off. nickel-based batteries lose 10% to 15% of their capacity in the first 24 hours after charge, then 10% to 15% per month afterwards. One of the best batteries in terms of self-discharge is Lead-acid; it only self-discharges 5% per month. Unfortunately, this chemistry has the lowest energy density and is ill suited for portable applications.
lithium-ion self-discharges about 5% in the first 24 hours and 1-2% afterwards. Adding the protection circuit increases the discharge by another 3% per month. The protection circuit assures that the voltage and current on each cell does not exceed a safe limit. Figure 3 illustrates a battery with high self-discharge.
The self-discharge on all battery chemistries increase at higher temperatures. Typically, the rate doubles with every 10¡ãC (18¡ãF). A noticeable energy loss occurs if a battery is left in a hot vehicle.
Aging and usage also affect self-discharge. nickel-metal-hydride is good for 300-400 cycles, whereas nickel-cadmium may last over 1000 cycles before high self-discharge affects the performance. An older nickel-based battery may lose its energy during the day through self-discharge rather than actual use. Discard a battery if the self-discharge reaches 30% in 24 hours.
Nothing can be done to reverse this deficiency. Factors that accelerate self-discharge are damaged separators induced by crystalline formation, allowing the packs to cook while charging, and high cycle count, which promotes swelling in the cell. Lead and lithium-based batteries do not increase the self-discharge with use in the same manner as their nickel-based cousins do.
Premature voltage cut-off
Not all stored battery power can be fully utilized. Some equipment cuts off before the designated end-of-discharge voltage is reached and precious battery energy remains unused. Applications demanding high current bursts push the battery voltage to an early cut-off. This is especially visible on batteries with elevated internal resistance. The voltage recovers when the load is removed and the battery appears normal. Discharging such a battery on a moderate load with a battery analyzer to the respective end-of-discharge threshold will sometimes produce residual capacity readings of 30% and higher, jet the battery is inoperable in the equipment. Figure 4 illustrates high cut-off voltage.
High internal battery resistance and the equipment itself are not the only cause of premature voltage cut-off - warm temperature also plays a role by lowering the battery voltage. Other reasons are shorted cells in a multi-cell battery pack and memory on nickel-based batteries.
3. How to service laptop batteries
Most laptops batteries are 'smart', meaning that some form of communications occurs between the battery and user. The definition of 'smart' varies among manufacturers and regulatory authorities. Some manufacturers call their batteries 'smart' by simply adding a chip that sets the charger to the correct charge algorithm. The Smart Battery System (SBS) forum states that a 'smart' battery must provide state-of-charge (SoC) indications.
There are two common architectures of 'smart' batteries, consisting of the single wire system found on high-end cameras and radio communications devices, and the two-wire system typically used on laptops. The two-wire system is usually configured to the System Management Bus (SMBus). Because of its common use in laptops, we will focus on the SMBus system. Figure 1 shows the layout.
(Figure 1: Two-wire SMBus system.
The SMBus is based on a two-wire system using a standardized communications protocol. This system lends itself to standardized state-of-charge and state-of-health measurements.)
Battery connection
The SMBus battery has five or more battery connections consisting of positive and negative battery terminals, thermistor, clock and data. The connections are commonly unmarked and attempting to test this type of battery appears complicated. Figure 2 describes the functions of a battery with 6 connections.
(Figure 2: Connections of a typical laptop battery.
The positive and negative terminals are usually placed on the outside; no norm exists on the arrangement of the contacts.)
The positive and negative battery terminals are commonly located at the outer edges of the connector. The inner contacts accommodate the clock and data. (On a one-wire system, clock and date are combined.) For safety reasons, a separate thermistor wire is brought to the outside. This allows temperature protection if the digital communication is disabled.
Some batteries are equipped with a solid-state switch that is normally in the off position. In such a case, no voltage is present. Connecting the switch terminal to ground will turn the battery on. If this does not work, a proprietary code may be needed to activate the battery.
How can I find the correct terminals? To begin with, use a voltmeter to locate the positive and negative battery terminals. Establish the polarity. If no voltage is available, a solid-state switch may need to be activated. With the voltmeter connected on the outer terminals, take a 100-Ohm resistor (other values may also work), connect one end of the resistor to ground, and with the other end touch each terminal while observing the voltmeter. If no voltage appears, the battery may be dead or the pack requires a digital code to activate. The resistor protects the battery against a possible electrical short.
Once the connection to the battery terminals is established, charging should be possible. If the charge current stops after 30 seconds, an activation code may be required. This code is often difficult, if not impossible to obtain.
Some battery manufacturers even add an end-of-battery-life switch. At a preset age, cycle count or capacity level, the battery stops functioning. Manufacturers explain that customer satisfaction and safety can only be guaranteed if the battery is regularly replaced. Such policy tends to satisfy the manufacturer more than the user. Newer batteries generally do not have this feature.
It is recommended to utilize the thermistor during charge and discharge to protect the battery against over heating. The thermistor can be measured with the Ohmmeter. The most common thermistors are 10 Kilo Ohm NTC or 10kOhm at 20¡ãC (68¡ãF). NTC stands for negative temperature coefficient, meaning that the resistance decreases with rising temperature. A positive temperature coefficient (PTC) will increase the resistance. Warming the battery with your hand may be sufficient to detect a small change in resistor value.
An SMBus battery contains permanent and temporary data. The permanent data is programmed into the battery at time of manufacturing and includes battery ID number, battery type, serial number, manufacturer and date of manufacture. The temporary data is acquired during use and consists of cycle count, user pattern and maintenance requirements. Some of this information is renewed during the life of the battery.
Repairing a 'smart' battery
Laptop batteries can be repaired but the work is often time consuming. The success rate varies with battery type. One must remember that the 'smart' battery consists of two parts, the chemical cells and the digital circuit. In some cases, the chemical battery can be fully restored but the fuel gauge may be inaccurate or its data is corrupt.
Anyone attempting to repair SMBus battery must be aware of some non-compliance. Unlike other tightly regulated standards, the SMBus allows some variations. This may cause problems with existing chargers and the SMBus battery should be checked for compatibility before use. More information on SMBus is available on www.sbs-forum.org and www.acpi.info.
If the cells are weak, cell replacement makes economic sense. While nickel-based cells are readily available, lithium-ion cells are not sold on the open market. This precaution is understandable when considering the danger of explosion and fire if the cells are assembled in a careless way. Always replace the pack with the same chemistry cells.
During cell replacement, the circuit of the 'smart' battery may need to be kept alive with a supply voltage. Disconnecting the circuit, if only for a fraction of a second, can erase vital data and render the circuit unusable. To assure continued operation when changing the cells, connect a secondary voltage through a 100-Ohm resistor before disconnecting the cells. Remove the secondary supply only after the circuit is fed from the new cells.
The open terminal voltages of the replacement cells should be within 10% of each other. Welding the cells is the only reliable way to get dependable service. Attention must be paid to limiting the amount of heat transferred to the cells during welding. Excess heat can damage the cells.
During storage, each cell may have self-discharged to a different charge level. This is especially evident on nickel-based batteries. To assure proper charge of all cells without overcharging some, trickle charge the newly repaired pack for about 14 hours, then discharge and recharge normally. Such a cycle is also needed to reset the battery's fuel gauge circuit. Lithium-ion can accept a normal charge in about 3 hours. The service should also include calibrating the battery.
4. The high-power lithium-ion
Most lithium-ion batteries for portable applications are cobalt-based. The system consists of a cobalt oxide positive electrode (cathode) and a graphite carbon in the negative electrode (anode). One of the main advantages of the cobalt-based battery is its high energy density. Long run-time makes this chemistry attractive for cell phones, laptops and cameras.
The widely used cobalt-based lithium-ion has drawbacks; it offers a relatively low discharge current. A high load would overheat the pack and its safety would be jeopardized. The safety circuit of the cobalt-based battery is typically limited to a charge and discharge rate of about 1C. This means that a 2400mAh 18650 cell can only be charged and discharged with a maximum current of 2.4A. Another downside is the increase of the internal resistance that occurs with cycling and aging. After 2-3 years of use, the pack often becomes unserviceable due to a large voltage drop under load that is caused by high internal resistance. Figure 1 illustrates the crystalline structure of cobalt oxide.
(Figure 1: Cathode crystalline of lithium cobalt oxide has 'layered' structures. The lithium ions are shown bound to the cobalt oxide. During discharge, the lithium ions move from the cathode to the anode. The flow reverses on charge.)
In 1996, scientists succeeded in using lithium manganese oxide as a cathode material. This substance forms a three-dimensional spinel structure that improves the ion flow between the electrodes. High ion flow lowers the internal resistance and increases loading capability. The resistance stays low with cycling, however, the battery does age and the overall service life is similar to that of cobalt. Spinel has an inherently high thermal stability and needs less safety circuitry than a cobalt system.Low internal cell resistance is the key to high rate capability. This characteristic benefits fast-charging and high-current discharging. A spinel-based lithium-ion in an 18650 cell can be discharged at 20-30A with marginal heat build-up. Short one-second load pulses of twice the specified current are permissible. Some heat build-up cannot be prevented and the cell temperature should not exceed 80¡ãC.
(Figure 2: Cathode crystalline of
lithium manganese oxide has a
'three-dimensional framework structure'.
This spinel structure, which is usually composed of diamond shapes connected into a lattice, appears after initial formation. This system provides high conductivity but lower energy density)
The spinel battery also has weaknesses. One of the most significant drawbacks is the lower capacity compared to the cobalt-based system. Spinel provides roughly 1200mAh in an 18650 package, about half that of the cobalt equivalent. In spite of this, spinel still provides an energy density that is about 50% higher than that of a nickel-based equivalent.
(Figure 3: Format of 18650 cell.
The dimensionsof this commonly used cell are: 18mm in diameter and 650mm in length.)
Types of lithium-ion batteries
Lithium-ion has not yet reached full maturity and the technology is continually improving. The anode in today's cells is made up of a graphite mixture and the cathode is a combination of lithium and other choice metals. It should be noted that all materials in a battery have a theoretical energy density. With lithium-ion, the anode is well optimized and little improvements can be gained in terms of design changes. The cathode, however, shows promise for further enhancements. Battery research is therefore focusing on the cathode material. Another part that has potential is the electrolyte. The electrolyte serves as a reaction medium between the anode and the cathode.
The battery industry is making incremental capacity gains of 8-10% per year. This trend is expected to continue. This, however, is a far cry from Moore's Law that specifies a doubling of transistors on a chip every 18 to 24 months. Translating this increase to a battery would mean a doubling of capacity every two years. Instead of two years, lithium-ion has doubled its energy capacity in 10 years.
Today's lithium-ion comes in many "flavours" and the differences in the composition are mostly related to the cathode material. Table 1 below summarizes the most commonly used lithium-ion on the market today. For simplicity, we summarize the chemistries into four groupings, which are Cobalt, Manganese, NCM and Phosphate.
(Table 1: Most common types of lithium-ion batteries. )
The cobalt-based lithium-ion appeared first in 1991, introduced by Sony. This battery chemistry gained quick acceptance because of its high energy density. Possibly due to lower energy density, spinel-based lithium-ion had a slower start. When introduced in 1996, the world demanded longer runtime above anything else. With the need for high current rate on many portable devices, spinel has now moved to the frontline and is in hot demand. The requirements are so great that manufacturers producing these batteries are unable to meet the demand. This is one of the reasons why so little advertising is done to promote this product. E-One Moli Energy (Canada) is a leading manufacturer of the spinel lithium-ion in cylindrical form. They are specializing in the 18650 and 26700 cell formats. Other major players of spinel-based lithium-ion are Sanyo, Panasonic and Sony.
Sony is focusing on the nickel-cobalt manganese (NCM) version. The cathode incorporates cobalt, nickel and manganese in the crystal structure that forms a multi-metal oxide material to which lithium is added. The manufacturer offers a range of different products within this battery family, catering to users that either needs high energy density or high load capability. It should be noted that these two attributes could not be combined in one and the same package; there is a compromise between the two. Note that the NCM charges to 4.10V/cell, 100mV lower than cobalt and spinel. Charging this battery chemistry to 4.20V/cell would provide higher capacities but the cycle life would be cut short. Instead of the customary 800 cycles achieved in a laboratory environment, the cycle count would be reduced to about 300.
The newest addition to the lithium-ion family is the A123 System in which nano-phosphate materials are added in the cathode. It claims to have the highest power density in W/kg of a commercially available lithium-ion battery. The cell can be continuously discharged to 100% depth-of-discharge at 35C and can endure discharge pulses as high as 100C. The phosphate-based system has a nominal voltage of about 3.3V/cell and peak charge voltage is 3.60V. This is lower than the cobalt-based lithium-ion and the battery will require a designated charger. Valance Technology was the first to commercialize the phosphate-based lithium-ion and their cells are sold under the Saphiona name.
In Figure 4 we compare the energy density (Wh/kg) of the three lithium-ion chemistries and place them against the traditional lead acid, nickel-cadmium, nickel-metal-hydride. One can see the incremental improvement of Manganese and Phosphate over older technologies. Cobalt offers the highest energy density but is thermally less stable and cannot deliver high load currents.
Figure 4: Energy densities of common battery chemistries.
(Lithium-cobalt enjoys the highest energy density. Manganese and phosphate systems are terminally more stable and deliver high load currents than cobalt.)
Definition of Energy Density and Power Density
Energy Density (Wh/kg) is a measure of how much energy a battery can hold. The higher the energy density, the longer the runtime will be. Lithium-ion with cobalt cathodes offer the highest energy densities. Typical applications are cell phones, laptops and digital cameras.
Power Density (W/kg) indicates how much power a battery can deliver on demand. The focus is on power bursts, such as drilling through heavy steel, rather than runtime. Manganese and phosphate-based lithium-ion, as well as nickel-based chemistries, are among the best performers. Batteries with high power density are used for power tools, medical devices and transportation systems.
An analogy between energy and power densities can be made with a water bottle. The size of the bottle is the energy density, while the opening denotes the power density. A large bottle can carry a lot of water, while a large opening can pore it quickly. The large container with a wide mouth is the best combination.
Confusion with voltages
For the last 10 years or so, the nominal voltage of lithium-ion was known to be 3.60V/cell. This was a rather handy figure because it made up for three nickel-based batteries (1.2V/cell) connected in series. Using the higher cell voltages for lithium-ion reflects in better watt/hours readings on paper and poses a marketing advantage, however, the equipment manufacturer will continue assuming the cell to be 3.60V.
The nominal voltage of a lithium-ion battery is calculated by taking a fully charged battery of about 4.20V, fully discharging it to about 3.00V at a rate of 0.5C while measuring the average voltage.
Because of the lower internal resistance, the average voltage of a spinel system will be higher than that of the cobalt-based equivalent. Pure spinel has the lowest internal resistance and the nominal cell voltage is 3.80V. The exception again is the phosphate-based lithium-ion. This system deviates the furthest from the conventional lithium-ion system
Prolonged battery life through moderation
Batteries live longer if treated in a gentle manner. High charge voltages, excessive charge rate and extreme load conditions have a negative effect on battery life. The longevity is often a direct result of the environmental stresses applied. The following guidelines suggest ways to prolong battery life.
-The time at which the battery stays at 4.20/cell should be as short as possible. Prolonged high voltage promotes corrosion, especially at elevated temperatures. Spinel is less sensitive to high voltage.
-3.92V/cell is the best upper voltage threshold for cobalt-based lithium-ion. Charging batteries to this voltage level has been shown to double cycle life. Lithium-ion systems for defense applications make use of the lower voltage threshold. The negative is a much lower capacity.
-The charge current of Li-ion should be moderate (0.5C for cobalt-based lithium-ion). The lower charge current reduces the time in which the cell resides at 4.20V. A 0.5C charge only adds marginally to the charge time over 1C because the topping charge will be shorter. A high current charge tends to push the voltage into voltage limit prematurely.
-Do not discharge lithium-ion too deeply. Instead, charge it frequently. Lithium-ion does not have memory problems like nickel-cadmium batteries. No deep discharges are needed for conditioning.
-Do not charge lithium-ion at or below freezing temperature. Although accepting charge, an irreversible plating of metallic lithium will occur that compromises the safety of the pack.
Not only does a lithium-ion battery live longer with a slower charge rate; moderate discharge rates also help. Figure 5 shows the cycle life as a function of charge and discharge rates. Observe the improved laboratory performance on a charge and discharge rate of 1C compared to 2 and 3C.
(Figure 5: Longevity of lithium-ion as a function of charge and discharge rates.
A moderate charge and discharge puts less stress on the battery, resulting in a longer cycle life.
Battery experts agree that the longevity of lithium-ion is shortened by other factors than charge and discharge rates. Even though incremental improvements can be achieved with careful use, our environment and the services required are not always conducive for optimal battery life. In this respect, the battery behaves much like us humans - we cannot always live a life that caters to achieve maximum life span.
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(From batteryuniversity.com)
5. Glossary of Common Battery Terms:
Battery - a device that converts energy, by chemical reaction or physical reaction, into electric current.
Primary Battery- energy is exhausted when active materials are consumed (carbon-zinc dry cell, lithium battery, silver oxide battery, alkaline battery)
Secondary Battery - active materials are regenerated by charging (nickel cadmium (NiCd), nickel metal hydride (NiMH), Lithium Ion, Lithium Polymer, Sealed Lead Acid.
Series Connection Connection of a group of battery cells by sequentially interconnecting the terminals of opposite polarity thereby increasing the voltage of the battery group but not increasing capacity (i.e. positive to negative connections).
Parallel Connection - Connection of a group of batter cells by interconnecting all terminals of the same polarity, thereby increasing the capacity of the battery group but not increasing the voltage (i.e. positive to positive and negative to negative).
Cadmium- Chemical symbol Cd. This metallic element is the chemically active material of a nickel cadmium battery's negative electrode. When the battery is charged, the negative electrode surface consists of cadmium. As the battery discharges, the cadmium progressively changes into cadmium hydroxide (Cd (OH2)).
Cadmium Hydroxide - Active material used at the negative electrode of the Nickel-Cadmium Cell.
Metal Hydride - A general name for chemical compounds consisting of metal elements and hydrogen.
Nickel Hydroxide - The active material in the positive electrode of NiMH and NiCd batteries.
Nickel Oxyhydroxide - The chemical name of NiOOH. Indicates that oxidation of Ni (OH)2 has progressed, and that the active material of the positive electrode of an NiCd or NiMH battery is charged.
Capacity - The quantity of electricity that can be obtained from a battery in one cycle from full charge to full discharge when the battery is discharged under conditions of rated current level and ambient temperature within the predetermined range. Generally, capacity is expressed in units of mAh (milliampere-hour).
Nominal Capacity - The standard capacity designated by a battery manufacturer to identify a particular cell model.
Nominal Voltage - The standard voltage used to express the capacity of a particular battery model. It is generally equal to its electromotive force or its approximate voltage during normal operation. Typical Values:
