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How does battery works ?


By:Prayag nao

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We can't always generate electricity where and when it is needed so batteries, devices that store electrical energy in chemical form, are very important. Many different types of batteries are produced for a wide variety of applications, from storing solar power for satellites in space to powering heart pacemakers fitted inside people's chests.

You might think a battery looks just about as dull as anything you've ever seen. But the minute you hook it up to something, it starts buzzing with electricity. That dull little cylinder turns into your very own micro power plant! Let's see what's going on in there...

Anatomy of a Battery

 

Take a look at any battery, and you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In normal flashlight batteries, like AA, C or D cell, the terminals are located on the ends. On a 9-volt or car battery, however, the terminals are situated next to each other on the top of the unit. If you connect a wire between the two terminals, the electrons will flow from the negative end to the positive end as fast as they can. This will quickly wear out the battery and can also be dangerous, particularly on larger batteries. To properly harness the electric charge produced by a battery, you must connect it to a load. The load might be something like a light bulb, a motor or an electronic circuit like a radio.

How does a battery really work?

Where does the power in a battery actually come from? Let's take a closer look!




The positive and negative electrodes are separated by the chemical electrolyte. It can be a liquid, but in an ordinary battery it is more likely to be a dry powder.

When you connect the battery to a lamp and switch on, chemical reactions start happening. One of the reactions generates positive ions (shown here as big yellow blobs) and electrons (smaller brown blobs) at the negative electrode. The positive ions flow through the electrolyte to the positive electrode (from the green line to the red one). Meanwhile, the electrons (smaller brown blobs) flow around the outside circuit (blue line) to the positive electrode and make the lamp light up on the way.
The electrons and ions flow because of the chemical reactions happening inside the battery—usually two or three of them going on simultaneously.
The exact reactions depend on the materials from which the electrodes and electrolyte are made, and we won't go into them here. (If you want to know what they are, enter the type of the battery you're interested in followed by the words "anode cathode reactions" in your favorite search engine.) Whatever chemical reactions take place, the general principle of electrons going around the outer circuit and ions flowing in the opposite direction through the electrolyte happens in all batteries. As the battery generates power, the chemicals inside it are gradually converted into different chemicals. Their ability to generate power dwindles, the battery's voltage slowly falls, and the battery eventually runs flat. In other words, if the battery cannot produce positive ions because the chemicals inside it have become depleted, it can't produce electrons for the outer circuit either.

Now you may be thinking: "Hang on, this doesn't make any sense! Why don't the electrons just take a short cut and hop straight from the negative electrode through the electrolyte to the positive electrode? It turns out that, because of the chemistry of the electrolyte, electrons can't flow through it in this simple way. In fact, so far as the electrons are concerned, the electrolyte is pretty much an insulator: a barrier they cannot cross. Their easiest path to the positive electrode is actually by flowing through the outer circuit.

 

 Types of batteries

Although there are lots of different kinds of batteries, there are really only two types: disposable and rechargeable. They contain two different kinds of cells. Primary cells make the power in ordinary, disposable batteries. They produce electricity by slowly using up the chemicals from which the electrodes and electrolyte are made. Secondary cells power rechargeable batteries. You can find them in the big lead-acid batteries that start cars and the nickel-cadmium (NiCd), nickel metal hydride (NiMH) and lithium-ion batteries that power cellular phones. Unlike primary cells, secondary cells can be recharged simply by passing a current through them in the reverse direction to normal. When you charge your cellphone, you are really just running the battery (the chemical reactions inside it) in reverse.

Examples of disposable batteries (primary cells)

Zinc-chloride batteries

In a zinc-chloride long-life battery, the positive electrode is made from a carbon rod surrounded by a mixture of powdered carbon and manganese dioxide, the negative electrode is made from an alloy of zinc and the electrolyte between them is a jelly or paste of ammonium chloride. The whole battery may be sealed inside a metal or plastic case and, because there is no liquid that can be spilled, it is often referred to as a dry cell. The cheapest, ordinary, everyday batteries you get for things like flashlights are zinc carbon ones.

Alkaline batteries

Inside an alkaline battery, manganese dioxide molecules are converted into manganese oxide and hydroxyl ions. The hydroxyl ions then react with zinc to form zinc oxide and water, releasing electrons. The electrons move toward the carbon rod and flow out around the circuit, producing an electric current. The battery stops producing electricity when all the manganese dioxide is used up. Alkaline batteries look much the same as zinc carbon ones but last longer and cost more.

Button cells

Button cells are used inside calculators and watches (and you find really tiny ones in hearing aids). The top of the cell is the negative electrode, made from powdered zinc trapped between two metal layers. The bottom of the cell and the case make up the positive electrode, made from mercury oxide and graphite. In between the electrodes is an alkaline electrolyte of potassium hydroxide. During operation, the zinc loses electrons to become zinc oxide and the mercury oxide changes to mercury metal.

Examples of rechargeable batteries (secondary cells)

This a quick overview of rechargeables.


Nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries

Until recently, virtually all rechargeable batteries were nickel-cadmium (NiCd, usually pronounced "nicad"). Although very dependable, it's often said that they need to be discharged fully before you charge them up or the amount of charge they will store (and their effective lifespan) can be greatly reduced. Nickel metal hydride work in a similar way, but suffer less from this so-called "memory effect." Another problem with NiCd batteries is the toxic cadmium metal they contain. If they are buried in a landfill, instead of properly recycled the cadmium can escape into the soil and could potentially pollute watercourses nearby.


Lithium-ion batteries

Lithium is a lightweight metal that easily forms ions, so it is excellent for making batteries. The latest lithium-ion batteries can store about twice as much energy as traditional NiCd rechargeables, work at higher voltages, and are more environmentally friendly, but do not last as long. There are probably lithium-ion batteries in your cellphone, MP3 player, and laptop computer.

How do they work? When you plug a cellphone or laptop into the power supply, the lithium-ion battery inside starts buzzing with chemical activity. The battery's job is to store as much electricity as possible, as fast as possible. It does this through a chemical reaction that shunts lithium ions (lithium atoms that have lost an electron to become positively charged) from one part of the battery to another. When you unplug the power and use your laptop or phone, the battery switches into reverse: the ions move the opposite way and the battery gradually loses its charge. Lithium-ion batteries also have special electronic circuits that can interrupt charging and discharging. These switch off the power to prevent overcharging and overheating and to prevent too much discharging, which makes the battery unstable and harder to charge up again.

Accumulators

Accumulators are most familiar to us as large, powerful car batteries. A lead-acid accumulator contains three or six separate cells inside a tough plastic casing. Each cell contains lead electrodes and an electrolyte of sulfuric acid and water. During operation, the sulfuric acid is gradually turned into water, the lead electrodes are converted into lead sulfate, and the battery becomes unable to supply more charge. But unlike a dry cell, it can be recharged simply by passing a current through it in the opposite direction.

Battery Arrangement and Power

In many devices that use batteries -- such as portable radios and flashlights -- you don't use just one cell at a time. You normally group them together in a serial arrangement to increase the voltage or in a parallel arrangement to increase current. The diagram shows these two arrangements.



The upper diagram shows a parallel arrangement. The four batteries in parallel will together produce the voltage of one cell, but the current they supply will be four times that of a single cell. Current is the rate at which electric charge passes through a circuit, and is measured in amperes. Batteries are rated in amp-hours, or, in the case of smaller household batteries, milliamp-hours (mAH). A typical household cell rated at 500 milliamp-hours should be able to supply 500 milliamps of current to the load for one hour. You can slice and dice the milliamp-hour rating in lots of different ways. A 500 milliamp-hour battery could also produce 5 milliamps for 100 hours, 10 milliamps for 50 hours, or, theoretically, 1,000 milliamps for 30 minutes. Generally speaking, batteries with higher amp-hour ratings have greater capacities.

The lower diagram depicts a serial arrangement. The four batteries in series will together produce the current of one cell, but the voltage they supply will be four times that of a single cell. Voltage is a measure of energy per unit charge and is measured in volts. In a battery, voltage determines how strongly electrons are pushed through a circuit, much like pressure determines how strongly water is pushed through a hose. Most AAA, AA, C and D batteries are around 1.5 volts.

Imagine the batteries shown in the diagram are rated at 1.5 volts and 500 milliamp-hours. The four batteries in parallel arrangement will produce 1.5 volts at 2,000 milliamp-hours. The four batteries arranged in a series will produce 6 volts at 500 milliamp-hours.

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