explain rechargables??

I know this may sound like a crazy question however I am curious about this.I know that the main difference between standard and rechargable cells is that you can re-use the rechargables.I do not know what makes one different from the other....In other words what allows one cell to have energy put back into it when the standard cell will not work this way?
The concept sounds so easy but I can not find anything with a search here.I was actually bored tonight and my mind got to wandering.Can anyone explain what makes a rechargable cell work or how it can be renewed when others can not?

Robocop,

Not the fullest explanation but a starting point.

http://www.valence.com/BatteryEducation.asp

I am sure others will chip in.

HTH

Mark

Hello there,

In a nutshell, one chemical process is reversible
while the other is not. It's a matter of finding
combinations that work either way.

Take care,
Al

Sometimes we loose track of the fact that cells are a chemical reaction that produces electricity, and tend to think of a cell as simply a supply of electricity. It's not a matter of removing electric from the cell, or in the case of charging, putting the electricity back in. It's not the same as we might use the gasoline from a tank and then refill it.

With cells, we're actually generating the electric from a chemical process. Sometimes electrolysis, or passing a current through, can restore the chemical back to it's original status; sometimes, but not for every chemical process. In addition, sometimes the chemical process damages or destroys the "vehicle" hosting the reaction. An example could be a piece of fruit.

If you place a couple of conductors into a lemon, you can draw a current off the electrodes as the chemical process occurs. However, regardless of whether the process is reversible through electrolysis, the vehicle itself, the lemon, is destroyed in the process. There no way to pass a current back into the lemon and have the lemon become restored to its original state. The lemon is therefore, a primary cell, which means you use it, then discard it.

By building a cell in such a manner that it's construction and chemical process can be reversed, and building the "vehicle" to where it can withstand the damage significantly enough to permit repeated actions, then we've built a rechargeable cell. Electrolysis restores the chemical process to it's basic original state, and is good for an additional use.

[ QUOTE ]
MrAl said:
Hello there,

In a nutshell, one chemical process is reversible
while the other is not. It's a matter of finding
combinations that work either way.


[/ QUOTE ]

Spot on answer (the only kind Al seems to know how to deliver.....).

A minor quibble, however. Primary cells (the kind you can't recharge) generally have more than one reaction going on. The chemicals produced in discharge are themselves consumed in another reaction meaning they're not there any longer to be recombined (recharged).

Secondary cells, OTOH, preserve their chemical byproducts so they are available for the reverse reaction (recharging).

Still, it's all about chemestry. There's actually a table that you can look up the potential of the reactions at the two poles and know what the cell voltage will be.

Doug Owen

Empath and Doug thank you for the education on this.I never imagined that a battery could be so complicated yet so simple in design.Can anyone answer why all the rechargables I have seen are always rated at a little less voltage than alkalines?Is it something about the host or chemicals that limit them to a lesser voltage?

Yep - it's all about the chemical reactions. As Doug mentioned, there are two reactions happening in a cell, one at the positive terminal, and one at the negative. Some of these reactions have a positive voltage potential, some a negative. Thus, sometimes the reactions are additive, and some are subtractive.

This page gives the half reactions of common cell chemistries - so you can see what reactions contribute to the final cell voltage:

http://www.powerstream.com/BatteryFAQ.html

Spot on, Evan, as usual.

Also, not all rechargables are less than alkalines. For instance the lead acid ones in your car are typically 2.2 or more per. And the Li one in your cell phone is like 3.7.

Nickle chemesty (the basis of NiCd and NiMH) is what sets the typical voltage at a bit over 1.2 Volts.

Doug Owen