How Batteries Work Header (6526 bytes)

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A battery is essentially a can full of chemicals that produce electrons. Chemical reactions that produce electrons are called electro-chemical reactions.

Battery Overview (1513 bytes)

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 normal cylindrical batteries the ends of the battery are the terminals. In a large lead acid battery, the kind of thing you find in full size cars, 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. 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. If you just connect a wire across the battery this is called a short circuit and will wear out the battery very quickly. Also with larger batteries, and especially rechargeable ones this also tends to be dangerous. This is because the short circuit offers no resistance to the electro chemical reaction, therefore the battery works flat out. Batteries are not designed to do this so it produces large amounts of heat, and will usually destroy your battery in a spectacular explosion.

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. This  effectively controls the maximum current that you can draw from the battery. Electrons flow from the battery into 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 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.

Battery Chemistry
If you want to learn about the electro-chemical reactions used to create batteries, it is easy to do experiments at home to try out different combinations. To do these experiments accurately, you will want to purchase an inexpensive volt-ohm meter. Make sure that the meter can read low voltages (in the 1-volt range) and low currents (in the 5- to 10-milliamp range). This way you will be able to see exactly what your battery is doing.

The first battery was created by Alessandro Volta in 1800. To create his battery, he made a stack by alternating layers of zinc, blotting paper soaked in salt water and silver. Like this:

 A pile battery example (1614 bytes)

This arrangement was known as a "voltaic pile." The top and bottom layers of the pile must be different metals, as shown. If you attach a wire to the top and bottom of the pile, you can measure a voltage and a current from the pile. The pile can be stacked as high as you like, and each layer will increase the voltage by a fixed amount. You can create your own voltaic pile using zinc plated metal sheet, a paper towel, and some 10p coins. Mix salt with water (as much salt as the water will hold) and soak the paper towel in this brine. Then create a pile by alternating the zinc plated metal and the 10p coins. See what kind of voltage and current the pile produces. Try a different number of layers and see what effect it has on voltage. Then try alternating 2p coins and 10p coins and see what happens. Other metals to try include aluminium foil and steel. Each metallic combination should produce a slightly different voltage.

Another simple experiment you can try involves a glass jar, a dilute acid, wire and nails. Fill the jar with lemon juice or vinegar (dilute acids) and place a nail and a piece of copper wire in the jar so that they are not touching. Try zinc-coated (galvanized) nails and plain iron nails. Then measure the voltage and current by attaching your volt meter to the two pieces of metal. Replace the lemon juice with salt water, and try different coins and metals as well to see the effect on voltage and current.

In the 1800s, before the invention of the electrical generator (the generator was not invented and perfected until the 1870s), the Daniell Cell (which is also known as three other names -- the "Crowfoot cell" because of the typical shape of the zinc electrode, the "gravity cell" because gravity keeps the two sulfates separated and a "wet cell" (as opposed to the modern "dry cell") because it uses liquids for the electrolytes), was extremely common for operating telegraphs and doorbells. The Daniell cell is a wet cell consisting of copper and zinc plates and copper and zinc sulphates. To make the cell, the copper plate is placed at the bottom of a glass jar. Copper sulfate solution is poured over the plate to half-fill the jar. Then a zinc plate is hung in the jar as shown and a zinc sulfate solution poured very carefully into the jar. Copper sulfate is denser than zinc sulfate, so the zinc sulfate "floats" on top of the copper sulfate. Obviously this arrangement does not work very well in a flashlight, but it works fine for stationary applications. If you have access to the sulfates, zinc and copper, you can try making your own Daniell Cell.

 A Daniell's Cell (3116 bytes)

Battery Reactions
Probably the simplest battery you can create is called a zinc/carbon battery, and by understanding the chemical reaction going on inside this battery you can understand how batteries work in general.

Imagine that you have a jar of sulfuric acid (H2SO4). Stick a zinc rod in it, and the acid will immediately start to eat away at the zinc. You will see hydrogen gas bubbles forming on the zinc, and the rod and acid will start to heat up. Here's what is happening:

If you now stick a carbon rod in the acid, the acid does nothing to it. But if you connect a wire between the zinc rod and the carbon rod, two things change:

The electrons go to the trouble to move to the carbon rod because they find it easier to combine with hydrogen there. There is a characteristic voltage in the cell of 0.76 volts. Eventually the zinc rod dissolves completely or the hydrogen ions in the acid get used up and the battery "dies."

In any battery, the same sort of electrochemical reaction occurs so that electrons move from one pole to the other. The actual metals and electrolytes used control the voltage of the battery -- each different reaction has a characteristic voltage. For example, here's what happens in one cell of a lead-acid battery:

A lead-acid battery has a nice feature -- the reaction is completely reversible. If you apply current to the battery at the right voltage, lead and lead dioxide reform on the plates so you can reuse the battery over and over. In a zinc-carbon battery, there is no easy way to reverse the reaction because there is no easy way to get hydrogen gas back into the electrolyte.

From all this experimenting you will be noticing that different combinations of metals produce different voltages. This relationship between different metals is called the Galvanic Series. It indicates the relative nobility of different metals and alloys in seawater. In a galvanic cell, the more noble material in this series will become the cathode (+ve) (A cathode is an electrode at which point electrons enter a chemical reaction), while the less noble material will become the anode (-ve) (An anode is an electrode at which point electrons leave a chemical reaction). A greater separation of the materials in the galvanic series indicates a bigger potential difference between the materials. Note that this galvanic series was derived for one specific electrolyte (seawater) only. The materials can have a different nobility ranking in different environments and at different temperatures series.

The Galvanic Series (7020 bytes)

Chemistry Comparison

Let's examine the advantages and limitations of today’s popular battery systems. Batteries are scrutinized not only in terms of energy density but service life, load characteristics, maintenance requirements, self-discharge and operational costs. Since NiCd remains a standard against which other batteries are compared, let’s evaluate alternative chemistries against this classic battery type.

Nickel Cadmium (NiCd) — mature and well understood but relatively low in energy density. The NiCd is used where long life, high discharge rate and economical price are important. Main applications are cars, boats, helicopter, aircraft, radio sets, just about every aspect of modelling. The NiCd contains toxic metals and is not environmentally friendly.

Nickel-Metal Hydride (NiMH) — has a higher energy density compared to the NiCd at the expense of reduced cycle life. NiMH contains no toxic metals. Applications include cars, boats, aircraft, some helis, and radio sets.

Lead Acid (Pb) — most economical for larger power applications where weight is of little concern. The lead acid battery is the preferred choice for boats, and flight boxes.

Lithium Ion (Li-ion) — fastest growing battery system. Li-ion is used where high-energy density and light weight is of prime importance. The Li-ion is more expensive than other systems and must follow strict guidelines to assure safety. Applications are starting to include basic electric flight and radio sets.

Lithium Ion Polymer (Li-ion polymer) — a potentially lower cost version of the Li-ion. This chemistry is similar to the Li-ion in terms of energy density. It enables very slim geometry and allows simplified packaging. Main applications as Lithium Ion.

Reusable Alkaline — replaces disposable household batteries; suitable for low-power applications. Its limited cycle life is compensated by low self-discharge, making this battery ideal for radio sets.

The below table compares the characteristics of the six most commonly used rechargeable battery systems in terms of energy density, cycle life, exercise requirements and cost. The figures are based on average ratings of commercially available batteries. Exotic batteries with above average ratings are not included.

  NiCd NiMH Lead Acid Li-ion Li-ion polymer Reusable
Alkaline
Gravimetric Energy Density (Wh/kg) 45-80 60-120 30-50 110-160 100-130 80 (initial)
Internal Resistance
(includes peripheral circuits) in mW
100 to 2001
6V pack
200 to 3001
6V pack
<1001
12V pack
150 to 2501
7.2V pack
200 to 3001
7.2V pack
200 to 20001
6V pack
Cycle Life (to 80% of initial capacity) 15002 300 to 5002,3 200 to
3002
500 to 10003 300 to
500
503
(to 50%)
Fast Charge Time 1h typical 2-4h 8-16h 2-4h 2-4h 2-3h
Overcharge Tolerance moderate low high very low low moderate
Self-discharge / Month (room temperature) 20%4 30%4 5% 10%5 ~10%5 0.3%
Cell Voltage (nominal) 1.25V6 1.25V6 2V 3.6V 3.6V 1.5V
Load Current
-    peak
-    best result

20C7
1C

5C
0.5C or lower

5C7
0.2C

>2C
1C or lower

>2C
1C or lower

0.5C
0.2C or lower
Operating Temperature (discharge only) -40 to
60oC
-20 to
60oC
-20 to
60oC
-20 to
60oC
0 to
60oC
0 to
65oC
Maintenance Requirement 30 to 60 days 60 to 90 days 3 to 6 months9 not req. not req. not req.
Typical Battery Cost £25
(7.2V)
£30
(7.2V)
£12
(6V)
£50
(7.2V)
£50
(7.2V)
£2
(9V)
Cost per Cycle11 £0.02 £0.10 £0.15 £0.30 £0.60 £0.10-0.50
Commercial use since 1950 1990 1970 1991 1999 1992

Foot note reference.

  1. Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of Li-ion and Li-polymer adds about 100mW.
  2. Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
  3. Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
  4. The discharge is highest immediately after charge, then tapers off. The NiCd capacity decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
  5. Internal protection circuits typically consume 3% of the stored energy per month.
  6. 1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference between the cells; it is simply a method of rating.
  7. Capable of high current pulses.
  8. Applies to discharge only; charge temperature range is more confined.
  9. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
  10. Cost of battery for commercially available portable devices.
  11. Derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.

Observation: It is interesting to note that NiCd has the shortest charge time, delivers the highest load current and offers the lowest overall cost-per-cycle, but has the most demanding maintenance requirements.

Battery Arrangements
In almost any device that uses batteries, you do not use just one cell at a time. You normally group them together serially to form higher voltages, or sometimes in parallel to a) form the ability to draw higher currents, and b) to give a greater capacity. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents, and capacities add up. The following diagram shows these two arrangements:

Examples of Series and Parallel battery arrangements (5997 bytes)

The upper arrangement is called a parallel arrangement. If you assume each cell produces 1.5 volts, then four batteries in parallel will also produce 1.5 volts, but the potential current that can supplied will be four times that of a single cell. Also if you assume that one battery would go flat in one our then when wired in parallel, four batteries would take four hours to go flat.

The lower arrangement is called a serial arrangement. The four voltages add together to produce 6 volts. Their ability to supply current remains the same as one battery, and given any current draw the duration they will remain charged remains the same.

Normally, when you buy a pack of batteries, the package will tell you the voltage and current rating for the battery. For example, most recevier packs use four nickel-cadmium batteries that are rated at 1.2 volts and 700 milliamp-hours (mAh) for each cell. The milliamp-hour rating means that the cell can produce 700 milliamps for one hour. In general, you can scale milliamp-hours linearly -- this battery could produce 350 milliamps for two hours or 1,400 milliamps for half an hour. It is not completely linear -- all batteries have a maximum current they can produce, and many battery chemistries have longer or shorter than the expected life at very low currents -- but it is generally linear over a normal range. Using the amp-hour rating, you can estimate how long the battery will last under a given load.

If you arrange four of these batteries in a serial arrangement, you get 4.8 volts (1.2 x 4) at 700 milliamp-hours. If you arrange them in parallel, you get 1.2 volts at 2,800 (700 x 4) milliamp-hours.

Have you ever looked inside a normal 9-volt battery? It contains six, very small batteries producing 1.5 volts each in a serial arrangement.

 A 9v battery with the cover on (10040 bytes)

 A 9v battery with the cover off (10172 bytes)

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