Rechargeable batteries

Rechargeable battery

A rechargeable battery, also known as a storage battery, is a group of two or more secondary cells. These batteries can be restored to full charge by the application of electrical energy. In other words, they are electrochemical cells in which the electrochemical reaction that releases energy is readily reversible. Rechargeable electrochemical cells are therefore a type of accumulator. They come in many different designs using different chemicals. Commonly used secondary cell chemistries are lead and sulfuric acid, rechargeable alkaline battery (alkaline), nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).

Rechargeable batteries can offer economic and environmental benefits compared to disposable batteries. Some rechargeable battery types are available in the same sizes as disposable types. While the rechargeable cells have a higher first cost than disposable batteries, rechargeable batteries can be discharged and recharged many times. Proper selection of a rechargeable battery system can reduce toxic materials sent to landfill disposal compared to an equivalent series of disposable batteries. Some manufacturers of NiMH type rechargeable batteries claim a lifespan up to 3000 charge cycles for their batteries.

Usage and applications

Unlike nonrechargeable batteries (primary cells), secondary cells must be charged before use. Attempting to recharge nonrechargeable batteries has a small chance of causing a battery explosion.

Rechargeable batteries are susceptible to damage due to reverse charging if they are fully discharged. Fully integrated battery chargers that optimize the charging current are available.

Rechargeable batteries currently are used for applications such as automobile starters, portable consumer devices, tools, and uninterruptible power supplies. Emerging applications in Hybrid electric vehicles and electric vehicles are driving the technology to improve cost, reduce weight, and increase lifetime. Rechargeable batteries have been known since the lead acid battery was invented in 1859.

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 National Electrical Manufacturers Association has estimated that U.S. demand for rechargeables is growing twice as fast as demand for nonrechargeables.

Charging and discharging

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 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 mostly comes from AC current (mains electricity) using an adapter unit. Most battery chargers can take several hours to charge a battery. Most batteries can be charged in far less time than the most common simple battery chargers are capable of. Duracell and Rayovac now sell chargers that can charge AA- and AAA-size NiMH batteries in just 15 minutes; Energizer sells chargers that can additionally charge C/D-size and 9V NiMH batteries.

Flow batteries don't need to be charged on place, because they can be charged 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. 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.

Reverse charging

Reverse charging, which damages batteries, is when a rechargeable battery is recharged with its polarity reversed. Reverse charging can occur under a number of circumstances, the two most important being:

  • When a battery is incorrectly inserted into a charger.
  • When multiple batteries are used in series in a device. When one battery completely charges ahead of the rest, the other batteries in series may force the charged battery to discharge to below zero voltage.

DOD

DOD means "depth of discharge". It is normally stated as a percentage of the nominal amperehour capacity; 0% DOD means no discharge. Since 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 / discharge cycles. Generally a rechargeable battery system will tolerate more charge/discharge cycles if the DOD is lower on each cycle.

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 technologies

Type Voltagea Energy densityb Powerc Effi.d E/$e Disch.f Cyclesg Lifeh
(V) (MJ/kg) (Wh/kg) (Wh/L) (W/kg) (%) (%/mo) (#) (years)
Lead-acid 2.1 0.11-0.14 30-40 60-75 180 70%-92% 5-8 3%-4% 500-800 3 (car battery), 20 (stationary)
VRLAi 2.105
Alkaline 1.5 0.31 85 250 50 99.9% 7.7 <0.3 100-1000 <5
Ni-iron 1.2 0.18 50 100 65% 5-7.3 20%-40% 50+
Ni-cadmium 1.2 0.14-0.22 40-60 50-150 150 70%-90% 20% 1500
NiMH 1.2 0.11-0.29 30-80 140-300 250-1000 66% 1.37 20% 1000
Ni-zinc 1.7 0.22 60 170 900 2-3.3 100-500
Li ion 3.6 0.58 160 270 1800 99.9% 2.8-5 5%-10% 1200 2-3
Li polymer 3.7 0.47-0.72 130-200 300 3000+ 99.8% 2.8-5.0 2-3
LiFePO4 3.25 80-120 170 1400 0.7-1.6 2000+
Li sulfur 2.0 0.94-1.44 400
Nano Titanate 2.3 90 4000+ 87-95%r 0.5-1.0 9000+ 20+
Thin film Li ? 350 959 ? ?p 40000
ZnBr 75-85
V redox 1.4-1.6 25-35 96% 14,000 10(stationary)
NaS 150 89%-92%
Molten salt 70-110 150-220 4.54 3000+ 8+
Super iron
Silver zinc 130 240
Notes For brevity, entries in the table had to be abbreviated. For a full description, please refer to the individual article about each type. Battery types for which there is no article yet are listed below.

  • a Nominal cell voltage in V.
  • b Energy density = energy/weight or energy/size, given in three different units
  • c Specific power = power/weight in W/kg
  • d Charge/discharge efficiency in %
  • e Energy/consumer price in W·h/US$ (approximately)
  • j Safe Depth of Discharge to maintain lifecycles
  • 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 which was based on Thomas Edison's first alkaline battery. Using nickel oxide hydroxide and metallic cadmium as electrodes, NiCd batteries have longer life cycles and hold electrical charge longer. However, their voltage potential difference are often less than that of Nickel-metal Hydride's.Nickel-Metal Hydride Battery (NiMH): First developed around 1980's. The battery has a hydrogen-absorbing alloy for the negative electrode instead of cadmium. Even though NiMH batteries have higher voltage outputs, the batteries discharge quicker and have a limited service life compared to NiCd.Lithium-ion Battery: The technology behind 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, no memory effect, and a slow loss of charge when not in use. The popularity of Lithium-ion has spread as their technology continues to improve.

Less common types

Lithium sulfur battery: A new battery chemistry developed by Sion Power since 1994. Claims superior energy to weight than current lithium technologies on the market. Also lower material cost may help this product reach the mass market. Not to be confused with lithium sulfur dioxide (Li-SO2) batteries which explode when recharged.Thin film lithium battery: An emerging refinement of the lithium ion technology by Excellatron. 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.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, with long life and very high power density.

Recent developments

In 2007, assistant professor Yi Cui and colleagues at Stanford University's Department of Materials Science and Engineering discovered that using silicon nanowires gave rechargeable lithium ion batteries 10 times more charge. See also Nanowire battery.

Alternatives

Several alternatives to rechargeable batteries exist or are under development. For uses like portable radios and flashlights, rechargeable batteries may be replaced by clockwork mechanisms or dynamos which are cranked by the user to provide power. For transportation, uninterruptible power supply systems and laboratories, flywheel energy storage systems store energy in a spinning rotor for reconversion to electric power when needed; such systems may be used to provide large pulses of power that would otherwise be objectionable on a common electical grid.

A future development could be ultracapacitors 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.

See also

References

External links

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