Familiar Batteries

 

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Although at first they may appear to be mundane topics for discussion, batteries are contributing in ever increasingly important ways to the modern lifestyle.   Items from spacecraft to even some failing human hearts depend upon batteries.    Although a myriad of battery types and chemistries exist, none are ideal or are very long lasting.   In almost every category the quest for a better battery solution, is an urgent one.   Here in this post an attempt is made to illuminate the construction, advantages or limitations, recycle-ability and where applicable the proper recharging of some of the most common consumer grade batteries.

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The “dry cell” battery made its first appearance during the Paris 1900 World’s Fair.    From this evolved the zinc-carbon cells which were so ubiquitous during the 1960’s.   Such cells are not dry, but actually contain an electrolyte of moist paste.   So-called “heavy duty” (zinc-chlorine) cells began to appear which offered an improvement in performance by featuring purer chemicals and an electrolyte of zinc chloride.   In today’s marketplace zinc-carbon and zinc-chloride cells have been largely displaced by the more expensive alkaline cell.   So far, all of these cell types are progressive variations of the original Leclancé cell invented around 1866.  In some circles zinc-carbon cells are simply referred to as Leclancé cells.

Leclancé cell image modified from public domain source

The term “primary cell” denotes a battery which is intended to be thrown away and not recharged after use, while the term “secondary cell” denotes a rechargeable type.   Battery chargers for zinc-carbon and zinc-chloride batteries have been built in the past but their effectiveness was minimal and the process fairly pointless.  The components of these old style cells contain materials however which might be useful to an experimenter.   For example the cases of Leclancé type primary cells are made of useful zinc.  The carbon graphite rod at the cell’s center can be filed to a point at one end and when attached to a 12v automotive battery, can be used to expeditiously solder electrical connections in an emergency.

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Furthermore both zinc-carbon and alkaline cells contain manganese oxide (specifically manganese (IV) oxide) ; a common inorganic pigment used in dyes, paints, ceramics and in glassmaking.   As long as 19,000 years ago in Europe prehistoric cavemen were painting cave walls black and dark brown with manganese oxide and were achieving umber, sienna & burnt sienna hues by mixing or cooking in with this, varying amounts of iron oxide.    Ammonium chloride (NH4Cl) composing the electrolyte of the zinc-carbon cell is made by the reaction of hydrochloric acid and ammonia.  This chemical has a wide range of applications.   Also known as Sal ammoniac, ammonium chloride can be found in food additives, baked bread where it acted as a yeast nutrient, salty licorice candy, cattle feed and in some cough medicines where it acts as an expectorant.   Ammonium chloride acts as a nitrogen source in some fertilizers, it can be found in the glue that bonds plywood and as a thickening agent in certain hair shampoos.   Ammonium chloride can clean a soldering iron.   It is used in some soldering fluxes and once upon a time in the past it was even used (along with the help of a little copper) to produce green and blue colors in fireworks.   Zinc chloride (in Heavy Duty cells) can also be found in non electrical, corrosive soldering fluxes.  Sometimes used as a disinfectant, in antiseptic mouthwashes and dental fillings, zinc chloride of higher concentrations can also dissolve cellulose, starch and silk.  Zinc chloride is also a frequent ingredient in military smoke grenades.

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In a battery, energy density is the amount of energy stored in a given space per unit volume or mass.  Alkaline battery cells have 3-4 times the energy density and a much improved shelf life compared to a zinc-carbon cell.  Appearing on the market in the late 1960’s, these usually have an outer shell made of steel.  Although generally thought of as primary batteries and contrary to what might be stated on a label, alkaline cells can be recharged.  A small current at about 65mA, interrupted periodically will do the trick.  Commercial pulse chargers for alkaline cells are available but rare.   Alkaline cells get their name from the strong base of potassium hydroxide (caustic potash) used in the electrolyte.   Potassium hydroxide (KOH) is hygroscopic (with a high affinity for water) and is sometimes used as a desiccant.   Some shaving creams, cuticle removers and leather tanning solutions to remove hair from animal hides – employ potassium hydroxide.

The 9 volt battery is properly termed a ‘battery’ because it is composed of a bank of individual 1.5 volt cells.  The construction of the 9 volt battery has varied over the years but nowadays the most common assembly is of 6, rarely seen AAAA type cells.

Some other less common primary dry cells that won’t be discussed here include: the aluminum battery, chromic acid cell, nickel oxyhydroxide battery, silver-oxide battery, and zinc air batteries.  Again, the main difference between a primary battery and a secondary battery is the ease with which the chemical reaction within the cell can be reversed.   “A battery charger functions by passing a current through the cell in a direction opposite to that of the flow of electricity during discharge.”

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It may be useful at this point to realize that the spiral wound (Jelly-roll or Swiss-roll) construction of some of the next battery cells to be mentioned and the construction of some capacitors can appear to be similar.  Over time in spiral wound batteries and capacitors alike, crystalline structure in the plate material or electrolyte eventually changes and causes complications.   Also the separators that isolate plates can deteriorate with age, eventually allowing opposing plates to make contact and short out.  When old devices like radios, stereos and TV’s stop working it is often discovered that bad capacitors caused the problem.   Simpler than a battery cell, a capacitor doesn’t produce electrons – it only stores them.  A capacitor can dump its entire charge in a split second whereas a battery cell discharges much more slowly.

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Nickel–cadmium batteries (NiCd or NiCad) got their name from the chemical symbols of their electrodes.   The first NiCD battery was a wet cell created in Sweden around 1899.   Beneficial attributes of this type of rechargeable battery include its tolerance to being deeply discharged for long periods, the fact that it can withstand very high discharge rates with virtually no loss of capacity, its lower self-discharge rate, and its performance in cold weather.  Outdoor solar patio lamps are one application where NiCds work admirably.   Negative attributes of the NiCd type would include a phenomenon known as voltage depression (voltage depletion, “lazy battery” or “memory” effect).   Voltage depression in this case is attributable to increased internal resistance caused by metallic crystal growth in the cadmium.   Improper or unsophisticated recharging of NiCads is probably the main reason for the continuing decline of their popularity.  The surface of the cadmium plate in a good NiCD cell has a small crystalline structure.  When these crystals begin to grow then the surface area is reduced so voltage depression and loss of capacity result.  The crystals can grow large enough and sharp enough to penetrate the separator between electrodes.

* It is sometimes possible to temporarily reclaim “spent” NiCD cells or battery packs with a trick.  By zapping NiCd cells or battery packs with a strong DC current like that from a welder or automotive battery charger (where positive to positive and negative to negative terminals meet) the size and sharpness of the crystalline dendrites within the cadmium hydroxide electrode can be reduced and performance partially restored.   Even battery packs that seem to have been dead for several years can be recovered this way.  If not used constantly however these seem to return to their original dormant state much sooner than they should.  Nickel based cells have venting mechanisms to allow gasses to escape in the event of heat from overcharging.  Therefore the electrolyte might dry up.  As with other “dry cells” the electrolyte can migrate away from the terminals over time also.

There is not much that can be reclaimed from either a NiCd or NiMH cell.   Cadmium hydroxide is more basic than zinc hydroxide.   Cadmium (Cd / atomic # 48) is a rare, soft, ductile and toxic transition metal that is found in trace amounts in most zinc ores and is often collected as a byproduct of zinc production.  Sometimes replacing zinc for corrosion resistant coatings, cadmium electroplating of steel is common with aircraft parts.  Cadmium can be found in nuclear reactors where it controls neutrons in nuclear fission.   Red, orange and yellow paint and plastic pigments are often made with cadmium.

NiMH cells (Nickel-Metal Hydride) are similar to NiCd cells, having replaced the negative cadmium electrode with one of a hydrogen absorbing alloy.   Superior in some ways but not in others to NiCads, NiMH cells only arrived in the marketplace in 1989.   Having 2-3 times the capacity of a NiCd cell they are useful in high drain applications like the demands from digital cameras as an example.   NiMH cells however have a very high self-discharge rate (perhaps 30% a month).  That means that they lose their charge just by doing nothing.   NiMH cells exhibit much less apparent voltage depression or recharge “memory” than NiCd types but it can still occur.  Unlike NiCd cells, NiMH cells should not be deeply discharged (except upon occasion before recharging) and they should be kept “toped up” or recharged frequently.

 * The amount of energy expended by a typical “AA” alkaline battery is about 5,000 C (Coulombs -> 1C = about 6,241,000,000,000,000,000 electrons).  Rechargeable “AA’s” and some alkalines display the relative capacitance of a cell with a “mAh” (Milliamp hour) rating.  One mAh = 3.6 C and 1,000 mAh’s = 1 Amp hour = 3,600 C.  Often compared to the gas tank of a car, the voltage represents how much gas is being used while the mAh represents the size of the gas tank.  A car with a bigger gas tank will go farther but the bigger gas tank will also take longer to refill.    

*  The mAh rating stamped on an “AA” battery can be misleading if comparing different types of batteries.  New alkaline “AA’s” might have a 2,500 mAh rating, while rechargeable  NiCd’s or NiMH’s might only be rated at 1,200, 1,900, etc.  In high-drain applications (like digital cameras) however these rechargeable cells will far outlast the alkaline types even before being in need of a recharge.  Alkaline batteries are not designed for high discharge demands, and only deliver full capacity if the power is used slowly.    

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* Some of the first “button” type battery cells were mercury or mercuric oxide batteries.  Used in hearing aids, watches, calculators and other small portable electronic devices mercury cells were popular and common between 1942 and the early 1990’s.  In the 1990’s the European Union and the United States began to legislate this type of chemistry out of existence.  Mercury cells had a 1.35 nominal voltage and high capacity, achieved from using an alkali electrolyte with zinc and mercuric oxide electrodes.

Cells in the Lithium battery family use lithium metal or lithium compounds in the anode but vary widely in choice of cathode and electrolyte.  Lithium cells offer higher voltage and larger energy density than most other battery types but they are also far more expensive.    Depending on its chemistry a lithium cell can provide 3.3 – 3.7v of nominal cell voltage (compared to: 1.5v for zinc carbon, zinc chloride and alkaline cells, or 1.2v for NiCd and NiMH cells).

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Lithium Prismatic cells of monopolar or stacked configuration are similar to the voltaic pile in concept – with the positive and negative plates are sandwiched together in layers with separators between them.   A new way to construct multiple electrode cells is to arrange them in what is called a “bipolar configuration”.   This looks like a stacked sandwich or prismatic configuration but here the negative plate of one cell becomes the positive plate of the next cell.   Almost a play upon words is the term “bipolar” because of the historic use of this unusual metal in treating manic depression (more commonly referred to today as “bipolar disorder”).   More on this bipolar topic momentarily.

Because they are pressurized and may use a flammable electrolyte “Lithium-ion” batteries can be dangerous.   A standard lithium cell is not rechargeable but a lithium-ion cell is.  While lithium primary cells have electrodes (generally anodes) of metallic lithium, rechargeable lithium-ion battery (LIB or Li-ion battery) cells use electrodes composed of various materials impregnated with lithium ions.   [Some examples are: lithium iron phosphate (LFP), lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt oxide (NMC)and lithium manganese oxide (LMO)].   Some of the newest battery designs being contemplated by researchers are impregnating carbon nanotube cathodes with lithium on a nanoscopic scale (particles usually measuring between 1 and 100 nanometers).  In the near future we may witness the commercialization of the so-called “nanobattery”.

The capacity of Li-ion type rechargeable batteries will diminish substantially after a few years.   Li-ion cells don’t have a “memory” and don’t get confused by shallow discharges.   It is not wise to strain such a battery by frequently discharging it completely, nor is it beneficial to keep it fully charged all the time.   Quick discharges also place strain on this battery type.  Over time a regularly used Li-ion battery will suffer less capacity loss than one that is used infrequently.   These cells don’t like extreme cold but they hate hot temperatures.

Popular lithium-ion polymer batteries (LiPo, LIP) should connote cells built with a non liquid polymer electrolyte that does not leak.   Confusing the issue however, manufacturers soon expanded this meaning to include lithium cells with pouch type flexible polymer casings.

* Lithium is a very curious material.  It does not occur naturally in a pure state because it is a highly reactive alkali metal with one of the highest reduction potentials of any element.  With an atomic number of only 3, refined lithium metal would be so soft in could be cut with a knife and so light that it would float on water.  A comparatively rare element and strategically important material, it is hard to acquire and therefore costly.   The price of the metal has sky-rocked since WWII.  During that war lithium was mainly used as Hi-temp grease for aircraft engines.  Soon later it was used to stage man’s 1st nuclear fusion reaction (1952 / lithium transmutation to tritium).  In 1954 when mixed with hydrogen (as lithium deuteride) it composed the fuel of the Bikini Atoll (Marshal Islands / Castle Bravo) thermonuclear “H” bomb.  This particular test surprised its designers by being a far more powerful blast than expected (@ 15MT the greatest yield of any U.S. nuclear test) and also created international repercussions concerning atmospheric thermonuclear testing.  Low concentration but stable lithium hydroxide was stockpiled for many years due to its strategic value in the manufacture of hydrogen bombs.  In nuclear power plants or ship/submarine reactors lithium might be employed as a coolant by moderating boric acid in the absorption of neutrons.   In some underwater torpedoes a block of lithium might be sprayed with sulfur hexafluoride to produce the steam which cranks the propeller.  Lithium is used in heat-resistant glass and the manufacture of telescope lenses because lithium fluoride crystals have a very low refractive index.  Chosen for their resonance, lithium niobate crystals are used in mobile phones.  Lithium is used as an oxidant in red flares & red fireworks, as a flux for welding and soldering and as a fusing flux for enamels and ceramic glazes because it lowers their melting points. 

Sodium affects excitation or mania in the human brain so doctors and psychiatrist might often issue lithium as a mood stabilizer.  For treatment of bipolar disorder / manic depressive disorder, lithium affects the flow of sodium through nerve and muscle cells in the body.   The terms for this disorder denote uncontrolled mood swings from up to down or high to low and back.   Lithium treats the aggressive, hyperactive and manic symptoms of the disorder.   In humans amphetamines produce effects similar to the symptoms of mania and herein lay another interesting quality of lithium.   Apparently lithium battery cells (a cheap source of the metal) are frequently used as a reducing agent in the illicit manufacture of methamphetamine.  One recipe called the “Nazi method” requires anhydrous ammonia, ether, lithium and pseudoephedrine.  A more complicated recipe also uses lithium but substitutes anhydrous ammonia with ammonium nitrate, lye, salt and a caustic drain opener composed of sulfuric acid and a cationic acid inhibitor.  Double methylated phenylethylamine (Meth) and its precursor amphetamine are both built upon the plant derived alkaloids ephedrine and pseudoephedrine.   Ephedrine and pseudoephedrine (from the Ephedra distachya plant) are active ingredients found in several brands of effective antihistamine.

The largest producers of lithium are Chile and Argentina.  Large deposits of lithium have been discovered on the Bolivian side of the Andes and there is a lot of the metal dissolved in the oceans.  Acquired primarily from brine lakes, clays and salt pans where it is refined electrolytically, production of the metal is slow.  There is no standard spot price for the metal in a futures market or stock exchange.  China has become the world’s largest producer and consumer of lithium ion batteries.   Presently ever-growing in utility and popularity and expecting huge requirements of this metal in future electrical automobiles, market analysts predict that production of lithium will soon fall short of fulfilling its demand.

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The construction of lead-acid automobile batteries has changed very little in the last 50-60 years.   A standard car battery has a conventional voltage of 12.6 volts achieved with only 6 cells, because the nominal voltage of each cell is 2.1 volts.   Typically in each cell alternating plates of different polarity {+ containing lead dioxide (PbO2) and (-) of plain lead (Pb)} are separated by nonconductive paper or synthetic dividers and surrounded by an electrolyte of about 35% sulfuric acid (H2SO4) and 65% water.  The electrolyte of a healthy cell should have a specific gravity of 1.265 @ 80°F.

*   During the discharge cycle of a lead-acid battery; the negative plates (lead) combine with the SO4 (of the sulfuric acid- H2SO4) to produce lead sulfate (PbSO4) and the electrolyte’s specific gravity goes down.   The electrolyte becomes weaker and the potential between ± plates diminishes.   Conversely, during the charge cycle electricity is passed through the plates forcing SO4 back into the electrolyte.   The lead sulfate is broken up as lead oxide and plain lead are re-deposited upon their respective plates.   The specific gravity and voltage (potential between plates) are re-elevated to the proper levels.

Industry nomenclature- lead acid batteries

Aside from common automotive batteries, buyers also have access to “maintenance free” batteries, “deep cycle” batteries, “hybrid” or “marine” batteries, “gelled” deep cycle batteries and “AGM”(Absorbed Glass Mat) batteries.   The chemistry of these differing lead-acid batteries remains the same but the quality or quantity of the components change.  “Maintenance free” batteries are usually just heavy duty versions of the same basic design.   Generally the construction is better, components are thicker and the materials are more durable.   Commonly the plate grids contain cadmium, strontium or calcium to help reduce water loss by reducing gas.   Such batteries are often closed systems (can’t add water or check specific gravity) and they are often referred to as “lead-calcium” batteries.

Automotive batteries are optimized to start car engines.  The hardest work they are expected to do is to start a cold engine on a cold day.  Hence they are constructed internally with many thin plates within each cell, to maximize surface area and therefore current output.  An automotive battery is designed to produce a large current for a short time.  Unless abused, a car battery is seldom drained to less than 20% of its total capacity.  Allowing this type of battery to drain beyond that point (or allowing it to self-discharge by not using it for long periods) can be very detrimental to the battery’s longevity.   By contrast “deep cycle” batteries as used in golf carts, electric fork-lifts and for boat trolling motors are optimized to provide a steady amount of current for a protracted period of time.  These can be deeply discharged (80% of capacity –although doing so strains the battery), repeatedly whereas an automotive battery cannot be.   Deep cycle batteries have fewer plates but thicker ones within each galvanic cell.  The plates have higher density plate material.  Less electrolyte and better separators are also used.  Alloys used for the deep cycle cell plates may incorporate more antimony than car batteries do.

* Lead acid batteries generally have two common ratings stamped upon them; CCA & RC.   Cold Cranking Amps (CCA) is the number of amps a battery can produce –for 30 seconds @ freezing temperature (32°F or 0°C).    Reserve Capacity (RC) is the number of minutes that a battery can deliver 25 amps at or above a 10.5 volt threshold.

Generally, a deep cycle battery will possess only one half to three quarters the cold cranking amps but twice to three times the reserve capacity that an automobile battery will contain.  A deep cycle battery can endure several hundred total (complete) discharge/recharge cycles whereas a car battery is simply not designed to be totally discharged.  This reserve capacity and discharge tolerance makes deep cycle batteries preferable to automotive types for the purpose of off-grid electrical storage purposes.  Any lead acid battery however will last longer if it is not allowed to discharge to a great degree.  A battery discharged to 50% every day will last about twice as long as one cycled to 80% of capacity daily.  For less strain and increased longevity, deep cycle batteries should probably be drained no more than 10% on a daily basis.

Hybrid” batteries or “Marine” batteries may be labeled deep cycle, but are something of an undesirable compromise.  “Gelled” deep cycle batteries offer a safer, less hazardous electrolyte in gel form but at a heftily increased price.  AGM (Absorbed Glass Mat) batteries incorporate a Boron-Silicate glass mat between plates.   Also called “starved electrolyte” batteries, the mats are partially soaked in acid.  These are less hazardous because they won’t spill or leak acid if damaged.  These sealed batteries also recombine oxygen and hydrogen back into water during charging.  The lifecycle of an AGM deep cycle battery typically ranges between 4 to 7 years.  Deep cycle Gelled and AGM type batteries can get pretty big and might cost well over $1,000 each when new.

All lead-acid automotive and deep cycle type batteries will eventually age or fail, but for a wide variety of reasons.  A normal automotive battery might age because lead dioxide flakes off the positive plate due to natural contraction and expansion during everyday discharge and charge cycles.  Shorts between plates, buckling of plates, loss of water, negative grid shrinkage, positive grid growth and positive grid metal corrosion can cause a battery to fail.  Battery aging can be accelerated by fast charging, overcharging, deep discharging, high heat and excessive vibration.  Acid stratification is a situation where weak acid is at the top and concentrated acid at the bottom of an automotive battery, and is a condition caused perhaps by a power hungry car that is not driven enough to fully charge its battery.  Sulfation is also caused by undercharging or by allowing a lead-acid battery to self discharge by sitting for a long period in an undercharged condition.   In a sulfated battery hard lead sulfate crystals will fill the pours and coat the plates.  In a few instances it may be possible to rectify sulfation in a battery but beware of false claims and salesmen selling snake oil.

* It is interesting to note that a lead-acid battery does not require sulfuric acid as an electrolyte, to work.   Alum (hydrated potassium aluminium sulfate) solutions work and alkali or base solutions may work as well.  An evident superiority of sulfuric acid is that it works as antifreeze by causing a significant freezing-point depression of water.   Alum solutions tend to crystallize as well as freeze.

Methanol fuel cell / NASA image

Methanol fuel cell / NASA image

Fuel cells

Fuel cells are similar to batteries in that they convert chemical energy into electricity.  Like battery cells, fuel cells have anodes, cathodes and electrolytes.  The main difference between the two is that the chemicals are self contained within a battery’s cell(s) but must be imported or fed to a fuel cell.  A continual supply of fuel and of oxygen or another oxidizing agent must be fed or input to the fuel cell to perpetuate its chemical reaction and electrical output.   Methanol, natural gas or hydrogen perhaps from these are the most commonly used fuel cell fuels.

Although “fuel cell technology” may seem like new buzzwords in the automotive industry an Allis-Chambers tractor was driving around under fuel cell power more than half a century ago.   A Welshman invented the first fuel cell in 1839 and below is one of his sketches.

1839_William_Grove_Fuel_Cell

Of the several types of fuel cells designed thus far the electrolyte (whether it be liquid or solid) chosen determines the composition of the anode, cathode and usually a catalyst as well.   Alkali fuel cells use an electrolyte of potassium hydroxide, operate around 350° F and require an expensive platinum catalyst to improve the ion exchange.   A  Proton Exchange Membrane fuel cell uses a permeable sheet of polymer as its electrolyte, works around 175° F and also requires a platinum catalyst.   Platinum catalysts are also required for Phosphoric Acid fuel cells, which use corrosive phosphoric acid as the electrolyte and work at about 350° F.   High temperature salts or carbonates of sodium or magnesium are generally the electrolyte of choice in Molten Carbonate fuel cells.  These fuel cells work at a hot 1,200° F perhaps and employ a non-precious metal like nickel as the catalyst at both electrodes.   Hotter yet, Solid Oxide fuel cells require an operating temperature of about 1,800° F before the chemical reactions begin to work.  The electrolyte in one of these cells is frequently a hard ceramic compound of zirconium oxide and the catalytic activity is enhanced by the complicated composition of its electrodes.

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Homemade Batteries

In a previous post Luigi Galvani, Alessandro Volta, the voltaic pile and Benjamin Franklin’s coinage of the term “battery” were discussed.   The image above shows several ways to construct simplistic batteries.  Each of these examples exploit dissimilar metals and an electrolyte that can be either acid or alkali based.

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In the image above a potential and usable current should be created once an electrolyte is poured into the can, except for one problem.  Beer and soda cans are spayed with a plastic polymer coating to prevent interaction of the beverage with the metal, and this coating would interfere with ion exchange.  In a battery cell the current carrying capacity (or power) is governed by the area of the electrodes, the capacity is governed by the weight of the active chemicals and the cell voltage is controlled by the cell chemistry.  While a strong electrolyte might produce more voltage it would also eat through the very thin wall of the aluminum can sooner.

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The notion intended by the image above is that a PVC pipe holds an electrolyte (preferably a mild mixture of bleach and water) and electrodes of copper pipe and tin solder are used.  Obviously the anode and cathode must not make contact but the closer they are suspended together- the better the ion exchange will be.   House wiring (just called “Romex” by some American electricians) comes in both copper and aluminum versions and could also be applied in this fashion.

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To prevent sacrificial damage of the electrodes when this type of primary is not being used, it would be beneficial to be able to remove the electrolyte.  This image above suggests a way to connect PVC pipe together so that electrolyte could be added or removed when necessary.   Eight cells would produce 12v if their nominal voltage was 1.5 volts each.

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The antique Daniell cell (above) probably could have gone without mention here except that some artistic types might find it interesting.  Looking to eliminate hydrogen bubbles, Daniell (1836) came up with a battery cell that used two electrolytes rather than one.   Originally, solutions of copper sulfate (deep blue in color) and of zinc sulfate (or sulfuric acid) were separated by a porous barrier of unglazed ceramic (or plaster of Paris, later used by Bird).  Operation of a single cell (2 half cells) worked fine until the porous barrier became clogged up with deposited copper.   Later a ceramic pot inserted inside a copper jar separated the two solutions.  Because of the flow of current the ceramic eventually became coated with copper.  Even later variations of the Daniell cell included the Bird’s cell and the gravity or crow’s foot cell.  In the gravity cell the difference in specific gravity of the two solutions is all that is necessary to keep them separated.  The containers of such cells should not be jostled.  Gravity cells were the favored source of power for telegraph stations especially in remote areas, for about 90 years.  Their zinc electrode resembled a crow’s foot, the batteries were easily maintained by replacing simple components – as needed.   Modern incarnations of the Daniell cell incorporate a “salt bridge” (either a glass tube filled with a fairly inert jellified solution of potassium or sodium chloride, or filter paper soaked in the same two chlorides).  When breaching two separate containers, the salt bridge completes the circuit – allowing only ions to flow back to the anode.

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