Introduction to Batteries
This article will give you a basic concept of how battery works and in later articles, dive further into the different components and chemistry of a battery.
Batteries are all over the place -- in our cars, our PCs, laptops, portable MP3 players and cell phones. A battery is essentially a can full of chemicals that produce electrons. Chemical reactions that produce electrons are called electrochemical reactions.
If you look at any battery, you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In an AA, C or D cell (normal flashlight batteries), the ends of the battery are the terminals. In a large car battery, there are two heavy lead posts that act as the terminals.
Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous, especially with large batteries, so it is not something you want to be doing). Normally, you connect some type of load to the battery using the wire. The load might be something like a light bulb, a motor or an electronic circuit like a radio.
Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts.
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Modern Battery Chemistry
Modern batteries use a variety of chemicals to power their reactions. Typical battery chemistries include:
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Zinc Chloride/Zinc Carbon - Commonly known as Extra Heavy Duty battery, zinc-chloride/zinc-carbon chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte.
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Alkaline - Alkaline chemistry is used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte.
- Lithium-iodide battery - Lithium-iodide chemistry is used in pacemakers and hearing aides because of their long life.
- Lead-acid battery - Lead-acid chemistry is used in automobiles, the electrodes are made of lead and lead-oxide with a strong acidic electrolyte (rechargeable).
- Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with potassium-hydroxide as the electrolyte (rechargeable).
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Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable).
- Lithium-ion battery - With a very good power-to-weight ratio, this is often found in high-end laptop computers and cell phones (rechargeable).
- Zinc-air battery - This battery is lightweight and rechargeable.
- Zinc-mercury oxide battery - This is often used in hearing-aids. But because of the mercury content, this battery is no longer available on the market.
- Silver-zinc battery - This is used in aeronautical applications because the power-to-weight ratio is good.
As you can see, several of these batteries are rechargeable. What makes a battery rechargeable? We will answer this question in a future knowledge articl
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Battery Arrangement and Power
In many devices that use batteries - such as portable radios and flashlights, you do not use just one cell at a time. You normally group them together serially to form higher voltages, or in parallel to form higher currents. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents add up. The following diagram shows these two arrangements:
The upper arrangement is called a parallel arrangement. If you assume that each cell produces 1.5 volts, then four batteries in parallel will also produce 1.5 volts, but the current supplied will be four times that of a single cell. The lower arrangement is called a serial arrangement. The four voltages add together to produce 6 volts.
Normally, when you buy a pack of batteries, the package will tell you the voltage and current rating. For example, a digital camera might use four nickel-cadmium batteries that are rated at 1.25 volts and 500 milliamp-hours for each cell. The milliamp-hour rating means, theoretically, that the cell can produce 500 milliamps for one hour. You can slice and dice the milliamp-hour rating in lots of different ways. A 500 milliamp-hour battery could produce 5 milliamps for 100 hours, or 10 milliamps for 50 hours, or 25 milliamps for 20 hours, or (theoretically) 500 milliamps for 1 hour, or even 1,000 milliamps for 30 minutes.
However, batteries are not quite that linear. For one thing, all batteries have a maximum current they can produce -- a 500 milliamp-hour battery cannot produce 30,000 milliamps for 1 second, because there is no way for the battery's chemical reactions to happen that quickly. And at higher current levels, batteries can produce a lot of heat, which wastes some of their power. Also, many battery chemistries have longer- or shorter-than-expected lives at very low current levels. But milliamp-hour ratings are somewhat linear over a normal range of use. Using the amp-hour rating, you can roughly estimate how long the battery will last under a given load.
If you arrange four of these 1.25-volt, 500 milliamp-hour batteries in a serial arrangement, you get 5 volts (1.25 x 4) at 500 milliamp-hours. If you arrange them in parallel, you get 1.25 volts at 2,000 (500 x 4) milliamp-hours.
Have you ever looked inside a normal 9-volt battery? The familiar 9-Volt battery goes into smoke detectors and garage door openers.
Manufacturers caution against disassembling batteries, to avoid personal injury. However, a partially disassembled 9-volt battery would look like this.
It contains six, very small batteries producing 1.5 volts each in a serial arrangement!
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