This entry covers electrochemical power sources. Electricity is most often generated electromagnetically, but since these sources cannot be classified as components, they are outside the scope of the encyclopedia. Electrostatic sources are excluded for similar reasons.
A battery is sometimes referred to as a cell or power cell, but can actually contain multiple cells, as defined in this entry. It used to be called an accumulator or a pile, but those terms are now archaic.
What It Does
A battery contains one or more electrochemical cells in which chemical reactions create an electrical potential between two immersed terminals. This potential can be discharged as current passing through a load.
An electrochemical cell should not be confused with an electrolytic cell, which is powered by an external source of electricity to promote electrolysis, whereby chemical compounds are broken down to their constituent elements. An electrolytic cell thus consumes electricity, while an electrochemical cell produces electricity. Batteries range in size from button cells to large lead-acid units that store power generated by solar panels or windmills in locations that can be off the grid. Arrays of large batteries can provide bridging power for businesses or even small communities where conventional power is unreliable. Figure 2-1 shows a 60KW, 480VDC selfwatering battery array installed in a corporate data center, supplementing wind and solar sources and providing time-of-day peak shaving of energy usage. Each lead-acid battery in this array measures approximately 28” × 24” × 12” and weighs about 1,000 lb.
Schematic symbols for a battery are shown in Figure. The longer of the two lines represents
the positive side of the battery, in each case. One way to remember this is by imagining that the longer line can be snipped in half so that the two segments can combine to form a + sign. Traditionally, multiple connected battery symbols indicate multiple cells inside a battery; thus the center symbols in the figure could indicate a 3V battery, while those on the right would indicate a voltage greater than 3V. In practice, this convention is not followed conscientiously.
How It Works
In a basic battery design often used for demonstration purposes, a piece of copper serves as an electrode, partially immersed in a solution of copper sulfate, while a piece of zinc forms a second electrode, partially immersed in a solution of zinc sulfate. Each sulfate solution is known as an electrolyte, the complete battery may be referred to as a cell, and each half of it may be termed a halfcell. A simplified cross-section view is shown in Figure 2-3. Blue arrows show the movement of electrons from the zinc terminal (the anode),
through an external load, and into a copper terminal (the cathode). A membrane separator allows the electrons to circulate back through the battery, while preventing electrolyte mixing. Orange arrows represent positive copper ions. White arrows represent positive zinc ions. (An ion is an atom with an excess or deficit of electrons.) The zinc ions are attracted into the zinc sulfate electrolyte, resulting in a net loss of mass from the zinc electrode. Meanwhile, electrons passing into the copper electrode tend to attract positive copper ions, shown as orange arrows in the diagram. The copper ions are drawn out of the copper sulfate electrolyte, and result in a net accumulation of copper atoms on the copper electrode. This process is energized partially by the fact that zinc tends to lose electrons more easily than copper.
Batteries for use in consumer electronics typically use a paste instead of a liquid as an electrolyte, and have been referred to as dry cells, although this term is becoming obsolete. The two halfcells may be combined concentrically, as in a typical 1.5-volt C, D, AA, or AAA alkaline battery.
A 1.5V battery contains one cell, while a 6V or 9V battery will contain multiple cells connected in series. The total voltage of the battery is the sum of the voltages of its cells.
The electrodes of a cell are often referred to as the anode and the cathode. These terms are confusing because the electrons enter the anode inside the cell and leave it outside the cell, while electrons enter the cathode from outside the cell. while electrons enter the cathode from outside the cell and leave it inside the cell. Thus, the anode is an electron emitter if you look at it externally, but the cathode is an electron emitter if you look at it internally.
Conventional current is imagined to flow in the opposite direction to electrons, and therefore,
outside the cell, this current flows from the cathode to the anode, and from this perspective, the cathode can be thought of as being “more positive” than the anode. To remember this, think of the letter t in “cathode” as being a + sign, thus: ca+hode. In larger batteries, the cathode is often painted or tagged red, while the anode may be painted or tagged black or blue. When a reusable battery is recharged, the flow of electrons reverses and the anode and the cathode effectively trade places. Recognizing this, the manufacturers of rechargeable batteries may refer to the more-positive terminal as the anode. This creates additional confusion, exacerbated further still by electronics manufacturers using the term “cathode” to identify the end of a diode which must be “more negative”(i.e., at a lower potential) than the opposite end.
To minimize the risk of errors, it is easiest to avoid the terms “anode” and “cathode” when referring to batteries, and speak instead of the negative and positive terminals. This encyclopedia uses the common convention of reserving the term “cathode” to identify the “more negative” end of any type of diode.
Three types of batteries exist.
This type of batteies properly (but infrequently) referred to as primary cells. They are
not reliably rechargeable because their chemical reactions are not easily reversible.
This type of battery properly (but infrequently) known as secondary cells. They can
be recharged by applying a voltage between the terminals from an external source such
as a battery charger. The materials used in the battery, and the care with which the battery is maintained, will affect the rate at which chemical degradation of the electrodes
gradually occurs as it is recharged repeatedly. Either way, the number of charge/
discharge cycles is limited.
It require an inflow of a reactive gas such as hydrogen to maintain an electrochemical reaction over a long period.
Capacitor as a Battery
A large capacitor may be substituted for a battery for some applications, although it has a lower energy density and will be more expensive to manufacture than a battery of equivalent power storage. A capacitor charges and discharges much more rapidly than a battery because no chemical reactions are involved, but a battery sustains its voltage much more successfully during the discharge cycle. Capacitors that can store a very large amount of
energy are often referred to as supercapacitors.
The energy density of any disposable battery is higher than that of any type of rechargeable battery, and it will have a much longer shelf life because it loses its charge more slowly during storage (this is known as the self-discharge rate). Disposable batteries may have a useful life of five years or more, making them ideal for applications such as smoke detectors, handheld remotes for consumer electronics, or emergency flashlights.
Disposable batteries are not well suited to delivering high currents through loads below 75Ω.
Rechargeable batteries are preferable for higher current applications. The bar chart
shows the rated and actual capabilities of an alkaline battery relative to the three most commonly used rechargeable types, when the battery is connected with a resistance that is low enough to assure complete discharge in 1 hour.
The manufacturer’s rating of watt hours per kilo is typically established by testing a battery with a relatively high-resistance load and slow rate of discharge. This rating will not apply in practice if a battery is discharged with a C-rate of 1, meaning complete discharge during 1 hour. Common types of disposable batteries are zinccarbon cells and alkaline cells. In a zinc-carbon cell, the negative electrode is made of zinc while the positive electrode is made of carbon. The limited power capacity of this type of battery has reduced its popularity, but because it is the cheapest to manufacture, it may still be found where a company sells a product with “batteries included.” The electrolyte is usually ammonium chloride or zinc chloride. The 9V battery in Figure 2-7 is actually a zinc-carbon battery according to its supplier, while the smaller one beside it is a 12V alkaline battery designed for use
in burglar alarms. These examples show that batteries cannot always be identified correctly by a casual assessment of their appearance.
In an alkaline cell, the negative electrode is made of zinc powder, the positive electrode is manganese dioxide, and the electrolyte is potassium hydroxide. An alkaline cell may provide between three to five times the power capacity of an equal size of zinc-carbon cell and is less susceptible to voltage drop during the discharge cycle.
Extremely long shelf life is necessary in some military applications. This may be achieved by
using a reserve battery, in which the internal chemical compounds are separated from each
other but can be recombined prior to use.
Commonly used types are lead-acid, nickel cadmium (abbreviated NiCad or NiCd), nickel-metal hydride (abbreviated NiMH), lithium-ion (abbreviated Li-ion), and lithium-ion polymer.
Lead-acid batteries have existed for more than a century and are still widely used in vehicles, burglar alarms, emergency lighting, and large power backup systems. The early design was described as flooded; it used a solution of sulfuric acid (generically referred to as battery acid) as its electrolyte, required the addition of distilled water periodically, and was vented to allow gas to escape.
The venting also allowed acid to spill if the battery was tipped over. The valve-regulated lead-acid battery (VRLA) has become widely used, requiring no addition of water to the cells. A pressure relief valve is included, but will not leak electrolyte, regardless of the position of the battery. VRLA batteries are preferred for uninterruptible power supplies for
data-processing equipment, and are found in automobiles and in electric wheelchairs, as their
low gas output and security from spillage increases their safety factor.
VRLA batteries can be divided into two types: absorbed glass mat (AGM) and gel batteries. The electrolyte in an AGM is absorbed in a fiber-glass mat separator. In a gel cell, the electrolyte is mixed with silica dust to form an immobilized gel. The term deep cycle battery may be applied to a lead-acid battery and indicates that it should be more tolerant of discharge to a low level—perhaps 20 percent of its full charge (although manufacturers may claim a lower number). The plates in a standard lead-acid battery are composed of
a lead sponge, which maximizes the surface area available to acid in the battery but can be physically abraded by deep discharge. In a deep cycle battery, the plates are solid. This means they are more robust, but are less able to supply high amperage. If a deep-discharge battery is used to start an internal combustion engine, the battery should be larger than a regular lead-acid battery used for this purpose.
A sealed lead-acid battery intended to power an external light activated by a motion detector is shown in Figure. This unit weighs several pounds and is trickle-charged during the daytime by a 6” × 6” solar panel. Nickel-cadmium (NiCad) batteries can withstand extremely high currents, but have been banned in Europe because of the toxicity of metallic cadmium. They are being replaced in the United States by nickel-metal hydride (NiMH) types,
which are free from the memory effect that can prevent a NiCad cell from fully recharging if it has been left for weeks or months in a partially discharged state.
Lithium-ion and lithium-ion polymer batteries have a better energy-to-mass ratio than NiMH
batteries, and are widely used with electronic devices such as laptop computers, media players, digital cameras, and cellular phones. Large arrays of lithium batteries have also been used in some electric vehicles.
Various small rechargeable batteries are shown in Figure. The NiCad pack at top-left was
manufactured for a cordless phone and is rapidly becoming obsolete. The 3V lithium battery at top-right was intended for a digital camera. The three batteries in the lower half of the photograph are all rechargeable NiMH substitutes for 9V, AA, and AAA batteries. The NiMH chemistry results in the AA and AAA single-cell batteries being rated for 1.2V rather than 1.5V, but the manufacturer claims they can be substituted for 1.5V alkaline cells because NiMH units sustain their rated voltage more consistently over time. Thus, the output from a fresh NiMH battery may be comparable to that of an alkaline battery that is part-way through its discharge cycle.
The electrical capacity of a battery is measured in amp-hours, abbreviated Ah, AH, or (rarely) A/H. Smaller values are measured in milliamp-hours, usually abbreviated mAh. If I is the current being drawn from a battery (in amps) and T is the time for which the battery can deliver that current (in hours), the amp-hour capacity is given by theformula:
Ah = I * T
By turning the formula around, if we know the amp-hour rating that a manufacturer has determined for a battery, we can calculate the time in hours for which a battery can deliver a particular current:
T = Ah / I
Theoretically, Ah is a constant value for any given battery. Thus a battery rated for 4Ah should provide 1 amp for 4 hours, 4 amps for 1 hour, 5 amps for 0.8 hours (48 minutes), and so on.
How to Use it
When choosing a battery to power a circuit, considerations will include the intended shelf life,
maximum and typical current drain, and battery weight. The amp-hour rating of a battery can be used as a very approximate guide to determine its suitability. For 5V circuits that impose a drain of 100mA or less, it is common to use a 9V battery, or six 1.5V batteries in series, passing current through a voltage regulator such as the LM7805. Note that the voltage regulator requires energy to function, and thus it imposes a voltage drop that will be dissipated as heat. The minimum drop will vary depending on the type of regulator used.