What is a battery cell

battery

Lexicon> Letter B> Battery

Definition: a producer of electrical energy based on electrochemical processes

More specific terms: device battery, car battery, starter battery, traction battery, hybrid battery, solar battery, household battery, rechargeable battery, lithium-ion battery, lead battery, nickel-metal hydride battery, alkali-manganese battery, zinc-carbon battery, button cell, Flat battery, block battery

English: battery

Categories: electrical energy, energy storage, energy carriers, vehicles

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 04/06/2012; last change: 08/21/2020

URL: https://www.energie-lexikon.info/batterie.html

A battery is strictly speaking an interconnection (mostly series connection) of several galvanic cells. However, the term has also become common for individual galvanic cells and is therefore used here despite a certain inaccuracy - as a generic term for electrochemical producers of electrical energy.

This article covers both rechargeable and single use batteries. Rechargeable batteries are also known as accumulators and are discussed in more detail in the article on accumulators.

The names of batteries are partly based on their elementary components; for example there are lithium-ion batteries, lead batteries and nickel-metal hydride batteries. In other cases, the designation provides information about the intended application; Examples of this are traction batteries (for driving vehicles), starter batteries (for starting internal combustion engines) and hybrid batteries (for use in hybrid drives).

Basic physical principle

The basic principle of every galvanic cell is that chemical energy can be converted into electrical energy through electrochemical processes. The cell has two electrically conductive connections (Electrodes called), which are not directly connected inside the cell, but via an electrolyte (a non-metallic, but electrically conductive material, e.g. in liquid or gel form). During the discharge (energy release), material on the electrically negative electrode (e.g. zinc) is oxidized, i. H. this material gives off electrons which flow away via the electrode. At the same time, another material (e.g. manganese dioxide) is reduced on the positive electrode, i. H. it takes electrons. Since the electrode materials are different, the amount of energy required for the reduction is less than that released during the oxidation. This creates a net “drive” for the flowing electrical charges, which expresses itself as an electrical voltage. The level of this cell voltage essentially depends on the combination of electrode materials and usually ranges between approx. 1 V and a few volts.

Ideally, the electrochemical reactions only take place to the same extent that current is drawn from the battery: If the current draw is stopped, the electrochemical reactions also cease. However, batteries also show some self-discharge due to undesirable chemical reactions, especially when they are stored at elevated temperatures.

When several cells are connected in series to form a battery, the amount of electrical charge that can be drawn does not increase, but the energy per amount of charge, i.e. the electrical voltage, and thus of course the energy content as well. For example, a 9 V block battery contains six cells (round cells or rectangular button cells) with 1.5 V each. The small volume of each individual cell allows only a relatively small capacity of approx. 500 to 600 mAh (for alkaline manganese cells) - to be compared with almost 7800 mAh for an only slightly heavier 1.5 V baby cell. Because of the higher voltage, the energy content is almost half that of the baby cell.

Often, separate battery cells are also connected in series. For example, there are many devices in which the battery compartment has two or four 1.5 V round cells almost to achieve a total voltage of 3 V or 6 V. It should be noted that cells should only be connected in series with cells of the same type and previous history.

A special case is the fuel cell, in which chemical energy in the form of z. B. is supplied by hydrogen. It can be viewed as a battery in which the energy-storing material is continuously exchanged during operation.

Rechargeable batteries

Many galvanic cells (so-called Primary cells) are only intended for one-time discharge. However, there are also specially optimized cells that can be recharged. They are called rechargeable batteries, Accumulators or Secondary cells also referred to as electrochemical energy storage. When charging, an electric current flow is forced by a charger, which is opposite to the direction of current when discharging. The charger must therefore work against the cell voltage. Here, the electrochemical processes run in the opposite direction, i. H. Material on the negative electrode is reduced again and oxidized on the positive electrode.

It should be noted that some nominally non-rechargeable batteries (e.g. alkaline-manganese batteries) can be regenerated to a certain extent by charging, especially if they are recharged (refreshed) early. (This requires special chargers with a higher voltage and lower current than with nickel batteries.) However, as a rule, the full capacity is no longer achieved, and this only in a few charging cycles. In addition, the charging time is much longer than with accumulators, since only a low charging current should be used. (Ordinary chargers for accumulators are often unsuitable for this.) With some batteries (especially button cells, e.g. based on silver oxide) charging should not be attempted because they can explode.

There are also limited rechargeable batteries like that RAM cells (Engl. Rechargeable Alkaline Manganese), the z. B. can be charged about 25 times instead of hundreds of times like real batteries. The advantages are the higher cell voltage and the much lower self-discharge. However, the RAM cells are not necessarily better than ordinary alkaline manganese cells [1].

More details on rechargeable batteries can be found in the article on accumulators. The remainder of the present article deals essentially only with non-rechargeable batteries, although much applies to rechargeable batteries as well.

Important properties of batteries

The main properties of batteries are the following:

  • The electrical voltage is the amount of energy per unit of charge (e.g. 1.5 V = 1.5 joules per coulomb charge). In visual terms, this is the “pressure” with which the electrical charges are delivered. The voltage of a battery gradually decreases when it is discharged until the battery can no longer be used at a certain minimum voltage (the final discharge voltage). Typically, during slow discharge, the voltage hardly decreases for a long time and remains close to what is known as the nominal voltagein order to then sag significantly more quickly when it is largely discharged.
  • In addition, the voltage also decreases very briefly as long as a high current is drawn; this is due to Internal resistance the battery, which is in large part caused by the limited speed of the electrochemical reactions. The internal resistance thus limits the amount of current that can be drawn. There is no fixed limit for this, but this current strength will be kept well below the level of the short-circuit current in order to avoid an excessive drop in voltage. The internal resistance can increase with heavy load and decrease again after a recovery time of a few hours. The power that can be drawn is the product of voltage and current strength.
  • Under the Capacity of a battery one understands the total amount of charge that can be drawn - usually not given in the SI basic unit coulomb (C), but in ampere hours (Ah) or milliampere hours (mAh). For example, a battery with 5 Ah = 5000 mAh can deliver a current of 50 mA over 100 hours, or 25 mA over 200 hours. The capacity depends mainly on the volume and type of cell (e.g. zinc-carbon or alkali-manganese), hardly on the manufacturer.
  • The amount of energy that can be called up (the Energy content) is roughly the product of the nominal voltage and the capacity. When a battery contains many cells in series, it has a small capacity (for a given weight) (because the individual cells are small) but higher voltage, and effectively stores a similar amount of energy as a single cell of the same total weight.
  • The gravimetric or volumetric Energy density is the energy content per unit of mass or volume. Battery types with high energy density are of interest for devices that are small but still need to run for a long time on a battery.
  • Even if a battery is not in use, there is a certain amount of self-discharge, although this is relatively low for most non-rechargeable batteries. Some types can be stored and operated for years without any problems.

(In the case of accumulators, there are also a number of properties.)

Leak security is of practical importance. In particular, old types of zinc-carbon batteries tended to leak out of the electrolyte material after prolonged storage (especially in the discharged state), so that some of the battery boxes of a device became badly soiled; In extreme cases, a device can even be completely destroyed by corrosive substances. Today's household batteries are largely leak-proof.

Most non-rechargeable batteries can operate in any position. They hardly react to environmental influences, except that the energy density usually decreases significantly at low temperatures (e.g. below 0 ° C) as well as at high temperatures (e.g. above 50 ° C).

Some batteries can explode if they are short-circuited for a period of time. This results from the formation of gas in the interior, as a result of which a high pressure builds up.

At the end of its service life, every battery must be properly disposed of, as it can contain various toxic substances and also recyclable materials. Collection containers are available from retailers for end users [2]. Unfortunately, many used batteries still end up in the household waste. Neither waste incineration nor landfilling are suitable for defusing the great potential danger of toxic substances such as mercury, cadmium and lead.

The manufacture of batteries requires far more energy than the batteries can give off during operation. (In other words, batteries contain a lot of gray energy.) For this reason alone, it is clear that electrical energy from batteries is far more expensive than that from the power grid.

Types of non-rechargeable batteries

Used to be Zinc-carbon batteries (Zinc-manganese dioxide cells) very common. However, they were increasingly of Alkaline manganese batteries (Alkaline batteries), which are also based on zinc, but with an alkaline electrolyte and manganese dioxide instead of graphite for the positive electrode. Alkaline-manganese batteries have the same nominal voltage of approx. 1.5 V, but an energy density that is more than twice as high as that of zinc-carbon batteries and a significantly lower internal resistance. In addition, there is a much weaker self-discharge and less tendency to leak, so a much better shelf life. Alkaline-manganese batteries are a little more expensive per piece, but less expensive in terms of the amount of energy.

Battery types with increased energy density (but higher prices) are the Lithium batteries and the Nickel oxyhydroxide batteries. They also have higher cell voltages and therefore cannot generally replace conventional alkaline-manganese batteries. Lithium batteries are z. B. often used in the form of the relatively large button cell CR2032 with 3 V cell voltage, for example in pocket calculators or as a CMOS backup battery in PCs. (If a PC constantly “forgets” the time when it is switched off, this battery usually has to be replaced.) Lithium iron sulfide button cells are next to Silver oxide zinc button cells well suited for watches.

Zinc-air batteries are mostly used in the form of button cells, e.g. B. in hearing aids. They replace the ones that used to be widespread Mercury Oxide-Zinc Batteries (which are rarely used because of their mercury content). They can be stored for a long time as long as their air inlet opening remains closed with a flap. After activation by removing the tab, they must be used within a few weeks because they have a high self-discharge rate. You are thus z. B. well suited for hearing aids, but not for watches.

Sizes

Device batteries (household batteries) are mainly used in the following designs:

  • The cylindrical ones are used particularly frequently AA cells (AA cells) with a length of approx. 50 mm and a diameter of 14 mm. As alkali-manganese cells, they have a nominal voltage of 1.5 V and a capacity of e.g. B. 2.8 Ah.
  • Larger devices also use cylindrical ones C cells (Baby cells) with approx. 50 mm × 26 mm, or the even larger D cells (Mono cells) with 61 mm × 34 mm, which have correspondingly higher capacities and current carrying capacities.
  • A smaller design is the AAA cell (Micro cell) with 44 mm × 10 mm.
  • A 9 volt block battery contains six primary cells, so that the voltage is 6 × 1.5 V = 9 V.
  • A variety of sizes exist for the Button cellswhich often makes finding the right cell difficult. In addition to the different sizes, there are also very different battery types and correspondingly different voltages between approx. 1.3 V and 3 V. The first letter in the type designation provides information about the cell type: C = lithium manganese dioxide, B = lithium carbon monofluoride, S = Silver, L = alkali manganese, P = zinc-air, M = mercury oxide-zinc.

Rarely come today 4.5 volt flat batteries which contain three primary elements of 1.5 V each. There are also special (often quite expensive) designs of cylindrical cells, e.g. B. Lithium batteries with approx. 34.5 mm × 17 mm for cameras.

TypeAbbreviationNominal voltageCapacity (AA cell)Self-dischargeCharging cycles
Alkali manganeseAM or RAM1.5V1.8 to 2.9 Ah0.3% per monthsome ten
Zinc-carbon1.5V0.8 to 1.2 Ah0.6% per month
Nickel metal hydrideNiMH1.2V2 to 2.7 Ah20% per monthmany hundreds
Nickel-cadmiumNiCd1.2V0.6 to 1.1 Ah20% per monthmany hundreds

Table 1: Comparison of various (partially rechargeable) battery types that are available in the form of AA cells. Note that the numerical values ​​v. a. for the discharge rates and possible charging cycles are only rough guidelines, as they depend heavily on the manufacturing details and the operating and storage conditions.

Assessment of the remaining capacity

The remaining capacity of a battery in use is not easy to determine. The electrical voltage is easy to measure, but it does not provide much information about the remaining capacity: the voltage often remains almost constant for a long time and then only quickly drops when it is largely exhausted. Often it can also be observed that the no-load voltage (i.e. the voltage in the unloaded state) is still relatively high (close to the nominal voltage), but the voltage drops sharply even with moderate load; the internal resistance has risen sharply. The easiest way to recognize this is by measuring the short-circuit current, i.e. by measuring the current strength that results when an ammeter is connected directly to the two battery terminals. (However, this should only be done for a very short time, since the battery will continue to discharge quickly.) A battery with reduced short-circuit current may hold out for a while in a consumer with low power requirements.

In summary, it can be said:

  • If the measured voltage of the battery is clearly below the nominal voltage even without a load, the battery is definitely exhausted.
  • If the voltage is still close to the nominal voltage, no reliable statement can be made on this basis. A sign of considerable exhaustion is then a sharp reduction in the short-circuit current.

The best way to estimate the remaining capacity is to measure the removed charge from the device. However, this is only possible with very few devices.

Second life of batteries

In some cases batteries that have lost too much of their capacity before the originally intended application are then used for another, in this respect less critical application; this is called a second life.This occurs above all in batteries for mobile applications (such as electric cars), in which a significantly reduced capacity is no longer tolerable due to the insufficient vehicle range; Such batteries can still be useful, for example, solar power storage or for covering peak loads in order to reduce the maximum power consumption from the grid and thus lower the power price paid. here a certain loss of capacity is less critical, and the energy efficiency of the storage hopefully will not be reduced too much.

Such secondary applications are of course to be rated positively in terms of resource efficiency. However, they are not necessarily cheaper than using new batteries, since additional work is often required to suitably integrate batteries of different types into a storage system.

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See also: accumulator, capacity of a battery, charging of electric cars, fuel cells, charger, electrical energy, energy storage, storage for electrical energy, chemical energy storage
as well as other articles in the categories of electrical energy, energy storage, energy carriers, vehicles