Lead–Acid Battery, Nickel–Cadmium Battery (NiCd), Nickel–Metal Hydride Battery (NiMH) and Lithium-Ion Battery

Posted by PITHOCRATES - January 9th, 2013

Technology 101

The Chemical Reactions in a Zinc-Carbon Battery are One Way

A battery uses chemistry to make electricity.  An electric current is a flow of electrons that can do useful work.  The chemical reaction inside a battery creates that flow of electrons to produce an electric current.  In a common zinc-carbon battery, for example, a zinc electrode dissolves in an electrolyte.  As it does atoms release free electrons and become positive ions (cations) in the electrolyte.  Giving this solution a positive charge.  At the same time a carbon electrode is in a different electrolyte solution.  One filled with negative ions (anions).  Giving this solution a negative charge.

With no electrical load attached to the battery these electrodes and electrolytes are in equilibrium.  When we attach an external circuit across the battery terminals they provide a pathway for those free electrons.  As the free electrons travel through the external circuit the cations and anions travel through a porous membrane from one electrolyte to the other.  The positive cations (atoms with room for an additional electron) flow towards the carbon electrode.  And combine with the free electrons on the surface of the carbon electrode and become electrically neutral.

We can stop this chemical reaction.  Say by turning a flashlight or a portable radio off.  But we can’t reverse it.  This is a one-way chemical reaction that eventually dissolves away the anode.  A Zinc-carbon battery is inexpensive.  The amount of battery life we get out of it more than offsets the price.  And they’re easy to change.  But sometimes an application calls for a battery that isn’t easy to change.  Like a car battery.  Imagine having to change that a few times a year when it ran down.  No, that would be far too inconvenient.  Difficult.  And costly.  So we don’t.  Instead, we recharge car batteries.

The Chemical Reactions in a Lead-Acid Battery are Reversible allowing these batteries to be Recharged

A car battery is a lead-acid battery.  Each cell of a lead-acid battery has a positive electrode (i.e., plate) of lead dioxide.  A negative electrode of lead.  And an electrolyte of a sulfuric acid-water solution containing sulfate ions.  The lead chemically reacts with the sulfate ions to produce lead sulfate on the negative electrode while producing positive ions.  The lead dioxide chemically reacts with the sulfuric acid to produce lead sulfate on the positive electrode while giving up free electrons.

When we attach an external circuit to the battery (such as starting a car) the free electrons leave the positive electrode, travel through the external circuit and return to the battery.  Where they combine with those positive ions.  Lead sulfate forms on both electrodes.  These reactions consume the sulfuric acid in the electrolyte and leave mostly water behind.  Reducing the available charge in the battery.  But unlike zinc-carbon batteries these chemical reactions are reversible.  After a car starts, for example, the alternator provides the electric power needs of the car.  While applying a charging voltage to the battery.  This voltage will ionize the water in the battery which will break down the lead sulfate.  Deposit lead oxide back onto the positive electrode.  And deposit lead back onto the negative electrode.  Giving you a charged battery for the next time you need to start your engine.

A lead acid battery can provide a strong current to spin an internal combustion engine.  Which takes a lot of energy to fight the compression of the pistons.  And it can work in some very cold temperatures.  But it’s big and heavy.  And works best in things bigger and heavier.  Like cars.  Trucks.  Trains.  And ships.  But they don’t work well in things that are smaller and lighter.  Like cordless power tools.  Cell phones.  And laptop computers.  Things where battery weight is an important issue.  Requiring an alternative to the lead-acid battery.  One of the earliest rechargeable battery alternatives was the nickel–cadmium battery.  Or NiCad battery.

The Chemical Reactions produce Heat in a Lithium Ion Battery and can Catch Fire or Explode

The nickel–cadmium battery works like every other battery.  With chemical reactions that produce electrons.  And chemical reactions that consumes electrons.  The NiCad battery uses nickel (III) oxide-hydroxide for the positive electrode.  Cadmium for the negative electrode.  And potassium hydroxide as the electrolyte.  A NiCad battery may look like a zinc-carbon battery.  But the electrodes are different.  Instead of the zinc canister and a carbon rod the electrodes in a NiCad battery are long strips.  One is placed onto the other with a separator in between.  Then rolled up like a jelly-roll.

NiCad batteries have a memory effect.  If they were recharged without being fully discharged the battery ‘remembers’ the amount of charge it took to recharge the partially discharged battery.  So even if you fully discharged the battery it would only recharge it as if you partially discharged it.  Reducing the battery capacity over time.  The nickel–metal hydride battery (NiMH) eliminated this problem.  And improved on the NiCad.  Giving it 2-3 times the capacity of a NiCad battery.  NiCad and NiMH batteries are very similar.  They use the same positive electrode.  But instead of the highly toxic cadmium NiMH batteries use a mixture of a rare earth metal mixed with another metal.

Today battery technology has evolved into the lithium-ion battery.  Where the positive electrode is a compound containing lithium.  The negative electrode is typically graphite.  The electrolyte is a lithium salt.  Lithium ions travel between the electrodes through the electrolyte.  And electrons flow between the electrodes via the external circuit.  They have a greater capacity, no memory effect and hold their charge for a long time when not being used.  Making the lithium ion battery ideal for cell phones and other consumer electronics.  These chemical reactions produce heat, though.  And can catch fire or explode.  Trying to prevent this from happening increases their manufacturing costs, making them expensive batteries.  So expensive that people will buy cheaper generic brands.  Cheaper because they are not built to the same quality standards of the more expensive ones.  And are more prone to catching fire or exploding.

Something to think about when you feel the heat of your cell phone after a long conversation.  Only use a battery recommended by the manufacturer.  Even if it costs a small fortune.  It may be expensive.  But probably not as expensive as your monthly airtime charges.  So don’t skimp when it comes to lithium ion batteries.  For those cheap ones do have a tendency to catch fire.  Or explode.

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