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Secondary cell

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Secondary cell

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A rechargeable battery, storage battery, or accumulator is a type of electrical battery. It comprises one or more electrochemical cells, and is a type of energy accumulator. It is known as a secondary cell because its electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging from button cells to megawatt systems connected to stabilize an electrical distribution network. Several different combinations of chemicals are commonly used, including: lead–acid, nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).

Rechargeable batteries have lower total cost of use and environmental impact than disposable batteries. Some rechargeable battery types are available in the same sizes as disposable types. Rechargeable batteries have higher initial cost but can be recharged very cheaply and used many times.

Usage and applications

Rechargeable batteries are used for automobile starters, portable consumer devices, light vehicles (such as motorized wheelchairs, golf carts, electric bicycles, and electric forklifts), tools, and uninterruptible power supplies. Emerging applications in hybrid electric vehicles and electric vehicles are driving the technology to reduce cost and weight and increase lifetime.[1]

Traditional rechargeable batteries have to be charged before their first use; newer low self-discharge NiMH batteries hold their charge for many months, and are typically charged at the factory to about 70% of their rated capacity before shipping.

Grid energy storage applications use rechargeable batteries for load leveling, where they store electric energy for use during peak load periods, and for renewable energy uses, such as storing power generated from photovoltaic arrays during the day to be used at night. By charging batteries during periods of low demand and returning energy to the grid during periods of high electrical demand, load-leveling helps eliminate the need for expensive peaking power plants and helps amortize the cost of generators over more hours of operation.

The US National Electrical Manufacturers Association has estimated that U.S. demand for rechargeable batteries is growing twice as fast as demand for nonrechargeables.[2]

Charging and discharging

Further information: Battery charger

During charging, the positive active material is oxidized, producing electrons, and the negative material is reduced, consuming electrons. These electrons constitute the current flow in the external circuit. The electrolyte may serve as a simple buffer for internal ion flow between the electrodes, as in lithium-ion and nickel-cadmium cells, or it may be an active participant in the electrochemical reaction, as in lead–acid cells.



The energy used to charge rechargeable batteries usually comes from a battery charger using AC mains electricity, although some are equipped to use a vehicle's 12-volt DC power outlet. Regardless, to store energy in a secondary cell, it has to be connected to a DC voltage source. The negative terminal of the cell has to be connected to the negative terminal of the voltage source and the positive terminal of the voltage source with the positive terminal of the battery. Further, the voltage output of the source must be higher than that of the battery, but not much higher: the greater the difference between the power source and the battery's voltage capacity, the faster the charging process, but also the greater the risk of overcharging and damaging the battery.

Chargers take from a few minutes to several hours to charge a battery. Slow "dumb" chargers without voltage- or temperature-sensing capabilities will charge at a low rate, typically taking 14 hours or more to reach a full charge. Rapid chargers can typically charge cells in two to five hours, depending on the model, with the fastest taking as little as fifteen minutes. Fast chargers must have multiple ways of detecting when a cell reaches full charge (change in terminal voltage, temperature, etc.) to stop charging before harmful overcharging or overheating occurs. The fastest chargers often incorporate cooling fans to keep the cells from overheating.

Battery charging and discharging rates are often discussed by referencing a "C" rate of current. The C rate is that which would theoretically fully charge or discharge the battery in one hour. For example, trickle charging might be performed at C/20 (or a "20 hour" rate), while typical charging and discharging may occur at C/2 (two hours for full capacity). The available capacity of electrochemical cells varies depending on the discharge rate. Some energy is lost in the internal resistance of cell components (plates, electrolyte, interconnections), and the rate of discharge is limited by the speed at which chemicals in the cell can move about. For lead-acid cells, the relationship between time and discharge rate is described by Peukert's law; a lead-acid cell that can no longer sustain a usable terminal voltage at a high current may still have usable capacity, if discharged at a much lower rate. Data sheets for rechargeable cells often list the discharge capacity on 8-hour or 20-hour or other stated time; cells for uninterruptible power supply systems may be rated at 15 minute discharge.


Flow batteries, used for specialised applications, are recharged by replacing the electrolyte liquid.

Battery manufacturers' technical notes often refer to VPC; this is volts per cell, and refers to the individual secondary cells that make up the battery. (This is typically in reference to 12-volt lead-acid batteries.) For example, to charge a 12 V battery (containing 6 cells of 2 V each) at 2.3 VPC requires a voltage of 13.8 V across the battery's terminals.

Non-rechargeable alkaline and zinc–carbon cells output 1.5V when new, but this voltage drops with use. Most NiMH AA and AAA cells are rated at 1.2 V, but have a flatter discharge curve than alkalines and can usually be used in equipment designed to use alkaline batteries.

Damage from cell reversal

Subjecting a discharged cell to a current in the direction which tends to discharge it further, rather than charge it, is called reverse charging. Generally, pushing current through a discharged cell in this way causes undesirable and irreversible chemical reactions to occur, resulting in permanent damage to the cell. Reverse charging can occur under a number of circumstances, the two most common being:

  • When a battery or cell is connected to a charging circuit the wrong way around.
  • When a battery made of several cells connected in series is deeply discharged.

In the latter case, the problem occurs due to the different cells in a battery having slightly different capacities. When one cell reaches discharge level ahead of the rest, the remaining cells will force the current through the discharged cell. This is known as "cell reversal". Many battery-operated devices have a low-voltage cutoff that prevents deep discharges from occurring that might cause cell reversal.

Cell reversal can occur to a weakly charged cell even before it is fully discharged. If the battery drain current is high enough, the cell's internal resistance can create a resistive voltage drop that is greater than the cell's forward emf. This results in the reversal of the cell's polarity while the current is flowing.[3][4] The higher the required discharge rate of a battery, the better matched the cells should be, both in kind of cell and state of charge, in order to reduce the chances of cell reversal.

In some situations (such as when correcting Ni-Cad batteries that have been previously overcharged[5]), it may be desirable to fully discharge a battery. To avoid damage from the cell reversal effect, it is necessary to access each cell separately: each cell is individually discharged by connecting a load clip across the terminals of each cell, thereby avoiding cell reversal.

Damage during storage in fully discharged state

If a multi-cell battery is fully discharged, it will often be damaged due to the cell reversal effect mentioned above. It is possible however to fully discharge a battery without causing cell reversal--either by discharging each cell separately, or by allowing each cell's internal leakage to dissipate its charge over time.

Even if a cell is brought to a fully discharged state without reversal, however, damage may occur over time simply due to remaining in the discharged state. An example of this is the sulfation that occurs in lead-acid batteries that are left sitting on a shelf for long periods. For this reason it is often recommended to charge a battery that is intended to remain in storage, and to maintain its charge level by periodically recharging it. Since damage may also occur if the battery is overcharged, the optimal level of charge during storage is typically around 30% to 70%.

Depth of discharge

Main article: Depth of discharge

Depth of discharge (DOD) is normally stated as a percentage of the nominal ampere-hour capacity; 0% DOD means no discharge. Seeing as the usable capacity of a battery system depends on the rate of discharge and the allowable voltage at the end of discharge, the depth of discharge must be qualified to show the way it is to be measured. Due to variations during manufacture and aging, the DOD for complete discharge can change over time or number of charge cycles. Generally a rechargeable battery system will tolerate more charge/discharge cycles if the DOD is lower on each cycle.[6]

Active components

The active components in a secondary cell are the chemicals that make up the positive and negative active materials, and the electrolyte. The positive and negative are made up of different materials, with the positive exhibiting a reduction potential and the negative having an oxidation potential. The sum of these potentials is the standard cell potential or voltage.

In primary cells the positive and negative electrodes are known as the cathode and anode, respectively. Although this convention is sometimes carried through to rechargeable systems — especially with lithium-ion cells, because of their origins in primary lithium cells — this practice can lead to confusion. In rechargeable cells the positive electrode is the cathode on discharge and the anode on charge, and vice versa for the negative electrode.

Table of rechargeable battery types

Type Voltagea Energy densityb Powerc E/$e Disch.f Cyclesg Lifeh
(V) (MJ/kg) (Wh/kg) (Wh/L) (W/kg) (Wh/$) (%/month) (#) (years)
Lead–acid 2.1 0.11-0.14 30-40 60-75 180 5-8 3%-4% 500-800 5-8 (automotive battery), 20 (stationary)
Alkaline 1.5 0.31 85 250 50 7.7 <0.3 100-1000 <5
Nickel–iron 1.2 0.18 50 100 5-7.3[7] 20%-40% 50+
Nickel–cadmium 1.2 0.14-0.22 40-60 50-150 150 1.25-2.5[7] 20% 1500
Nickel–hydrogen 1.5 0.27 75 60 220 20,000+ 15+ (satellite application with frequent charge-discharge cycles)
Nickel–metal hydride 1.2 0.11-0.29 30-80 140-300 250-1000 2.75 30% 500-1000
Nickel–zinc 1.7 0.22 60 170 900 2-3.3 100-500
Lithium-air (organic)[8] 2.7 7.2 2000 2000 400 ~100
Lithium-ion 3.6 0.58 150-250 250-360 1800 2.8-5[9] 5%-10% 400–1200[10] 2-6
Lithium-ion polymer 3.7 0.47-0.72 130-200 300 3000+ 2.8-5.0 5% 500~1000 2-3
Lithium iron phosphate 3.25 0.32-0.4 80-120 170 1400 0.7-3.0 2000+[11] >10
Lithium sulfur[12] 2.0 0.94-1.44[13] 400[14] 350 ~1400 [15]
Lithium–titanate 2.3 0.32 90 4000+ 0.5-1.0 9000+ 20+
Sodium-ion[16] 1.7 30 3.3 5000+ Still testing
Thin film lithium  ? 300[17] 959[18] 6000[19]  ?p[20] 40000[17]
Zinc bromide 0.27-0.31 75-85
Vanadium redox 1.15-1.55 0.09-0.13 25-35[21] 20%[22] 14,000[23] 10(stationary)[22]
Sodium-sulfur 0.54 150
Molten salt 2.58 0.25-1.04 70-290 [24] 160[7] 150-220 4.54[25] 3000+ <=20
Silver-oxide 1.86 0.47 130 240
Notes
  • b Energy density = energy/weight or energy/size, given in three different units
  • c Specific power = power/weight in W/kg
  • e Energy/consumer price in W·h/US$ (approximately)
  • f Self-discharge rate in %/month
  • g Cycle durability in number of cycles
  • h Time durability in years
  • i VRLA or recombinant includes gel batteries and absorbed glass mats
  • p Pilot production
  • r Depending upon charge rate

Common rechargeable battery types

Nickel–cadmium battery (NiCd)

Created by Waldemar Jungner of Sweden in 1899, it used nickel oxide hydroxide and metallic cadmium as electrodes. Cadmium is a toxic element, and was banned for most uses by the European Union in 2004. Nickel–cadmium batteries have been almost completely superseded by nickel–metal hydride (NiMH) batteries.

Nickel–metal hydride battery (NiMH)

First commercial types were available in 1989.[26] These are now a common consumer and industrial type. The battery has a hydrogen-absorbing alloy for the negative electrode instead of cadmium.

Lithium-ion battery

The technology behind the lithium-ion battery has not yet fully reached maturity. However, the batteries are the type of choice in many consumer electronics and have one of the best energy-to-mass ratios and a very slow loss of charge when not in use.

Lithium-ion polymer battery

These batteries are light in weight and can be made in any shape desired.

Less common types

Lithium sulfur battery
A new battery chemistry developed by Sion Power since 1994.[27] Claims superior energy to weight than current lithium technologies on the market. Also lower material cost may help this product reach the mass market.[28]
Thin film battery (TFB)
An emerging refinement of the lithium ion technology by Excellatron.[29] The developers claim a very large increase in recharge cycles, around 40,000 cycles. Higher charge and discharge rates. At least 5C charge rate. Sustained 60C discharge, and 1000C peak discharge rate. And also a significant increase in specific energy, and energy density.[30]
Also Infinite Power Solutions makes thin film batteries (TFB) for micro-electronic applications, that are flexible, rechargeable, solid-state lithium batteries.[31]
Smart battery
A smart battery has the voltage monitoring circuit built inside. See also: Smart Battery System
Carbon foam-based lead acid battery
Firefly Energy has developed a carbon foam-based lead acid battery with a reported energy density of 30-40% more than their original 38 W·h/kg,[32] with long life and very high power density.
Potassium-ion battery
This type of rechargeable battery can deliver the best known cycleability, in order of a million cycles, due to the extraordinary electrochemical stability of potassium insertion/extraction materials such as Prussian blue.
Sodium-ion battery
This type is meant for stationary storage and competes with lead–acid batteries. It aims at a very low total cost ownership per kWh of storage. This is achieved by a long and stable lifetime. The number of cycles is above 5000 and the battery does not get damage by deep discharge. The energy density is rather low, somewhat lower than lead–acid.

Developments since 2005

In 2007, Yi Cui and colleagues at Stanford University's Department of Materials Science and Engineering discovered that using silicon nanowires as the anode of a lithium-ion battery increases the volumetric charge density of the anode by up to a factor of 10, leading to the development of the nanowire battery.[33][34]

Another development is the paper-thin flexible self-rechargeable battery combining a thin-film organic solar cell with an extremely thin and highly flexible lithium-polymer battery, which recharges itself when exposed to light.[35]

Ceramatec, a research and development subcompany of CoorsTek, as of 2009 was testing a battery comprising a chunk of solid sodium metal mated to a sulfur compound by a paper-thin ceramic membrane which conducts ions back and forth to generate a current. The company claimed that it could fit about 40 kilowatt hours of energy into a package about the size of a refrigerator, and operate below 90 °C; and that their battery would allow about 3,650 discharge/recharge cycles (or roughly 1 per day for one decade).[36]

Alternatives

Several alternatives to rechargeable batteries exist or are under development. For uses such as portable

Ultracapacitors—capacitors of extremely high value—are being developed for transportation, using a large capacitor to store energy instead of the rechargeable battery banks used in hybrid vehicles. One drawback to capacitors compared with batteries is that the terminal voltage drops rapidly; a capacitor that has 25% of its initial energy left in it will have one-half of its initial voltage. Battery systems tend to have a terminal voltage that does not decline rapidly until nearly exhausted. This characteristic complicates the design of power electronics for use with ultracapacitors. However, there are potential benefits in cycle efficiency, lifetime, and weight compared with rechargeable systems. China started using ultracapacitors on two commercial bus routes in 2006; one of them is route 11 in Shanghai.[39]

See Battery (electricity) for comparisons between battery types.

See also

energy portal

References

Further reading

  • Vlasic, Bill. The New York Times, published online December 9, 2012, p. B1.
  • Witkin, Jim. lithium ion battery.
  • Wald, Matthew L. The New York Times, January 7, 2011. Discusses AES Energy Storage.
  • Woody, Todd. The New York Times, September 6, 2010. Discusses lithium-air batteries.

External links

  • High-performance lithium battery anodes using silicon nanowires
  • Scientific American - How Rechargeable Batteries Work

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