Pique the Geek 20100425: Electricity: Cells and Batteries

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Electricity is the movement of electrons one way or another.  The electron is a very small mass particle that is classified as a lepton, meaning that is has mass and has a spin quantum number of +/- 1/2.

An electron has a mass of 9.0166 x 10^-31 kg, making it about 1/1800 the mass of a proton, which is a hadron.  Hadrons account for most of the mass in normal matter, as opposed to dark matter, the nature of which has not been elucidated nor ever proven, but that is for another series.

This series is concerned with the storage of electrical energy in the form of chemical energy, and converting the two into useful currents.  Most of the electricity that we use is quite transient in nature, but that stored chemically in batteries is much longer lasting, if not as intense.

First, we should get some nomenclature established here.  A battery is a connexion of two or more cells, and like in biology, the cell is the fundamental unit of the chemical generation of electricity, at least in modern technology.

A cell is simply two materials that, when connected properly, generate an electromotive force, otherwise known as a voltage.  I you happen to be fortunate enough to have an old silver coin, you can carefully wash it, along with a copper cent (pre-1982, see my post from last week here), and place them on opposite sides of your tongue.  Nothing happens until you touch the coins at the bottom of your tongue, but when you do that you will feel a sensation on the sides, and that is an electrical current.  It is not very powerful, but you can sort of feel it and taste it.

This is because your saliva in an electrolyte, a material (almost always liquid, usually but not always water based) that can transfer ions of differing materials from one electrode to the other.  It turns out that silver and copper have a voltage potential difference that allows the copper to corrode, losing electrons, and the silver to suck them up rapidly.  As this happens, the less noble metal is consumed, and the more noble one is not affected, except possibly on the surface.  Actually, those electrons have to go somewhere, and they act to form hydrogen from the water of your saliva.

A common school experiment is to make a lemon cell, using an ordinary lemon, and small strip of copper, and a small strip of zinc.  You make a couple of slits into the lemon, push in the metal strips, and clip a flashlight bulb to make a connexion.  You need to use a bulb designed for a single cell flashlight.  The bulb will glow.

In this series we shall examine the whole concept of chemical generation of electricity.  We shall also examine the difference between primary batteries (ones that only can be discharged with no recharging possible due to severe physical and chemical changes on the electrodes), and secondary batteries, that can be recharged if not too deeply discharged so as to ruin the geometry of the electrodes.  Once a positive electrode touches a negative one, physically, that becomes a dead cell, and weakens the output of the entire battery, usually fatally, for its intended application.

The concept of a cell, or of a battery, is ancient.  There are artifacts of Sumerian cells from many millenia BCE, but these data have been not very well deciphered.  In those days, there were no applications for current electricity, so the importance of those “jars” is obscure.  The Mythbusters have an episode where they built some.

Of course, the prehistoric folks knew all too well about lightening.  This is form of static electricity, and is not useful for running a computer or a flashlight (actually, there are components in your computer that actually use tiny bits of static electricity, but we shall talk about capacitors many moons hence).

The first real, modern battery was constructed by the Italian genius, Alessandro Volta.  He took pieces of zinc and copper and separated them with pieces of paper (more likely, parchment), and wetted the whole thing down with a dilute solution of what is now known as sulfuric acid.  When a wire was connected to the top and bottom, a very strong voltage could be discharged, depending on how may layers were used.  The zinc/copper couple is not very powerful, but if one connects enough together, it can be.

To this day, we respect Dr. Alessandro Volta by using the term volt as the SI unit of electromotive force.  Good on him!

This is just the introduction to a series about batteries.  I hope that this is interesting, because they touch us everywhere.  Just remember that a cell is just a component of a battery, and that there are very, very many different kinds of cells.  I shall use the term battery to refer to both single cells and batteries of cells henceforth unless a specific reason requires the more specific term.

Batteries can also be divided into “wet” ones and “dry” ones, but, with a few important exceptions, most dry batteries contain some water to act as the electrolyte.  They are called dry because the water has been absorbed to form a paste or gel so it does not flow.  They are also sealed.  Thus, they can be used in any position without risk of leaking.  Wet cells, on the other hand, contain liquid water along with dissolved electrolytes, and will leak if not kept in the upright position.  The most commonly encountered wet battery is the lead/acid one in your car.

Think for a minute about how many uses for batteries there are.  From portable radios, to automobiles, to cellular telephones, to computers (even desktops have a battery in them to keep the clock running), to hearing aids, and thousands of other applications, batteries are everywhere.  Without getting out of my seat, I see my cordless telephone on its charger, my cellular telephone on my belt, my torch (the British expression for flashlight, and a more apt term today), my laptop, the remote control for my TeeVee, my portable radio, and my laptop.  This is without getting up from my chair.  If I were to move about a bit, I would see my cordless drill, another torch, another remote control, my digital camera, and my smoke detectors.  Certainly I have left out some.

There are three critical parts to a cell.  Those are the cathode (whither the electrons go), the anode (whence the electrons come), and the electrolyte (the medium that conducts the electrons).  Note that the convention used for electricity is exactly backwards to the actual direction of flow of electrons.  The basic laws of electricity were developed long before the electron was discovered, and the current carrying species were assumed to be positive.  It does not really matter, but is confusing.  In electrical work, the current is really the reverse of the electron flow, which mathematically is equivalent.  In other words, a flow of positive charges in a given direction is the same as a flow of negative charges in the opposite direction.  This convention, by the way, goes all the way back to Benjamin Franklin who incorrectly guessed that the carrier of current were positive.

The cathode and anode MUST be made of different materials.  This is because the anode material has to undergo a chemical reaction to generate electrons, and the cathode material has to be able to accept them before they finally undergo a terminal chemical reaction.  The electrolyte is necessary to provide a pathway between the cathode and the anode without them physically touching.

There are lots of candidates for both electrodes and for electrolytes.  The Volta cell used zinc as the cathode and copper as the anode, with sulfuric acid as the electrolyte.  Dry cells use zinc as the cathode and carbon as the anode, with either ammonium chloride in a paste of water and manganese dioxide as the electrolyte (“heavy duty” dry cells), or in the case of alkaline cells, potassium hydroxide replacing the ammonium chloride.

Batteries, since they depend on a chemical reaction to produce electrical current, are temperature sensitive.  As a very rough general rule, the rate of many chemical reactions double for every 10 degrees C of temperature increase.  Likewise, the rate is halved for every 10 degrees C of temperature decrease.  Those of you who camp know that your torch is dimmer on cold winter nights than warm summer ones, and most drivers are familiar with the fact that cars are harder to start in the winter.  This is a direct result of the slowing of the chemical reactions that produce current in cold conditions.  It can get so cold that a battery will no longer produce usable amounts of current.

This sort of anticipates the next point.  All batteries, except for extremely specialized ones specifically designed to avoid it, undergo self discharge.  This is a slow process of the cathode reacting even without a current drain, and eventually the cathode is degraded to the point that the battery will not produce a usable current.  Alkaline dry cells self discharge more slowly than heavy duty ones, so have a longer shelf life.  All dry cells can have essentially an infinite shelf life if kept in the freezer (the same is true of photographic film), because the very low temperatures greatly retard the rate of self discharge.

Even though batteries perform better at higher temperatures, high temperatures cause them to degrade quickly.  Self discharge rates increase, and in car batteries high temperatures cause the lead plates to distort, finally shorting out and ruining the battery.  Although it is often noticed that a car’s battery is bad when the weather gets cold, the real damage was done in the summer when temperatures under the hood are extremely high.  It is the natural decrease in current output with the low temperatures that make the damage that weakened the battery apparent.

In the next few installments in this series, we shall discuss dry cells, automobile batteries, and the common rechargeable ones, nickel cadmium, nickel metal hydride, and of course, the lithium ion battery.  The latter is becoming more and more important since it is the only current battery technology that can provide the current density necessary for electric cars without extremely high weight and size.

You might be interested in knowing that electric automobiles were quite competitive with internal combustion engine ones during the early days of the automobile.  They were more reliable, simpler, and obviously quieter.  However, improvements in the internal combustion engine rapidly outpaced battery technology, so electric cars lost out in the late 1910’s.  It is notable that we are coming full circle.

Well, you have done it again.  You have wasted many more einsteins of perfectly good photons reading this non/-electrifying material.  And even though Brit Hume develops a personality when he reads me say it, I always learn much more than I possibly could hope to teach writing this series, so keep those comments, recommendations, questions, corrections, and other remarks coming.  Remember, no science or technology subject is off topic here.

Warmest regards,

Doc

Crossposted at Dailykos.com