Re: Redox reactions and rechargable batteries.

Thank you for your question, Miriam.  Probably others in the MadSci Network, 
like Australia's own John Christie, are better qualified to answer you, but let 
me give it a go.

Strictly defined, a "cell" produces a voltage difference and produces electric 
power through chemistry.  Strictly defined, a "battery" comprises cells in 
series to produce voltages higher than a single cell can.  For example, a 
Nickel-Cadmium (NiCad) cell produces 1.2 volts and a 6.0 volt NiCad battery 
comprises 5 NiCad cells in series.  But common use allows calling both of them 
"batteries."

Cells - rechargeable or not - all use Redox (Reduction-Oxidation) reactions to 
produce their voltage difference.

http://www.duracellusa.com/Education/index.html is a site explaining 
batteries.  Click on Chemistry for (guess what) a description of the 
chemistry.  You will see  the redox reactions taking place in a Zinc-Carbon 
cell.  The zinc serves as the anode, where oxidation produces electrons that 
want to get to the cathode, but they need a path.  Provide that path, and they 
will go, doing work (in the physics sense) as they go.  At the cathode, 
reduction reaction with the cathode material, manganese dioxide, takes place.  
Note that the carbon doesn't enter into the chemistry.  It just provides a 
place for reduction.

http://www.chemistry.ucsc.edu/stanley/teaching/batteries/recharge.html is a 
site from University of California, Santa Cruz, USA, describing the anode 
oxidation and cathode reduction for a NiCad cell and a Lead-Acid cell.

Professor Michael Lerner, and scientist, Frank McLarnon, in answering a 
question not too different from yours, at:
 
http://www.scientificamerican.com/askexpert/chemistry/chemistry5.html

explain what makes some cells rechargeable and others not.  It boils down to 
chemical and physical reversibility.  If, after discharging a cell through a 
load, you can restore the original chemical and physical configuration to the 
cell by passing current backwards, (requires voltage higher than the cell 
voltage), then the cell will be reversible.  Another name for an irreversible 
cell is primary cell. Another name for a reversible cell is secondary cell.

Hope this helps. 

Larry Skarin