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Author Topic: Why do batteries go "flat" ?  (Read 15036 times)

paul.fr

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Why do batteries go "flat" ?
« on: 18/05/2007 19:55:52 »
when a battery goes dead, what is not happening and why?
« Last Edit: 13/02/2009 18:31:22 by chris »


 

Offline kdlynn

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Re: Why do batteries go "flat" ?
« Reply #1 on: 18/05/2007 19:59:26 »
...and does the brand of battery that you buy really matter? does one brand last longer than another? (sorry to add to your question paul. i was going to start a new topic with my question, but i thought that too many battery questions all at one time was silly)
 

paul.fr

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Re: Why do batteries go "flat" ?
« Reply #2 on: 18/05/2007 20:01:52 »
...and does the brand of battery that you buy really matter? does one brand last longer than another? (sorry to add to your question paul. i was going to start a new topic with my question, but i thought that too many battery questions all at one time was silly)

No problem, Kadie. That is a very good question in it's own right.
 

another_someone

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Re: Why do batteries go "flat" ?
« Reply #3 on: 18/05/2007 22:17:03 »
In very broad terms, a battery is a chemical reaction that is in some way constrained to create an electrical current.

Any chemical reaction is caused by electrons moving between atoms.  If you can create a situation where you can divert those electrons to run around an electrical circuit before completing the chemical reaction, you have a battery.

Clearly, when all the chemical reagents that you have within the battery have been used up (i.e. they have all completed the chemical reaction, so you have no more unreacted chemicals left waiting to form the reaction), then you have no more electrons that want to move from one reagent to another, and thus the battery is dead.  With rechargeable batteries, the reaction is reversible, so that if you feed an electrical current through the reaction product, you can reverse that reaction to form the original reagents, and then you can use those reagents to again run through the chemical process that creates the reaction product, and in the process you can again sideline those electrons to run around your electrical circuit.

The problems arise in that if you are trying to stop the chemical reaction from taking place when the battery is disconnected, it is extremely difficult to stop the reaction overall, and so the reaction will continue at a very low pace (with the electrons finding ways to pass between the reagents that does not involve going via an outside circuit), and this causes the battery to go flat even when it is not connected.

The degree to which the reaction can take place even when the battery is not connected to a circuit will depend upon the design of the battery (both the chemical reaction used, but also how the chemical reagents are separated, what additives might be added to alter the nature of the reaction, and many other design factors).

Also, different designs will contain different amounts of reagent (thus allowing a longer life in circuit), will have different designs of electrodes (thus allowing different maximum current drain, and leakage current, from the battery), and many other design factors can play a part in the shelf life of the battery, in the life of the battery when in circuit, and even in what applications the battery can be used in.

So, yes, brand can make a substantial difference.

 

paul.fr

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Re: Why do batteries go "flat" ?
« Reply #4 on: 18/05/2007 22:22:18 »
years back, i would get a bit of life from my batteries by putting them on a warm radiator or heater. Why did warming them up give "some" life to the old batteries?
 

another_someone

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Re: Why do batteries go "flat" ?
« Reply #5 on: 18/05/2007 22:53:32 »
Most chemical reaction run slower in the cold, so if the reaction is almost completely finished, there may be too little reagents left to generate a significant current; but warming the battery could speed up the residual chemical reaction just that little bit more to get a measurable current.
 

Offline kdlynn

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Re: Why do batteries go "flat" ?
« Reply #6 on: 18/05/2007 22:55:08 »
my grandma used to keep them in the refrigerator before she used them
 

another_someone

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Re: Why do batteries go "flat" ?
« Reply #7 on: 18/05/2007 23:01:52 »
my grandma used to keep them in the refrigerator before she used them

But this is exactly the contrary situation.

You remember that I said that even when the battery was not part of the circuit, the reaction would find ways of progressing at a low level, thus running the battery flat even without it being used.

Keeping the battery in the fridge can slow down the unwanted reaction that is going on while the battery is not being used.  Ofcourse, you do have to make sure it warms up properly before you put it into the circuit.

That having been said, while these tricks will often work, it may not necessarily work for all designs of battery, and some batteries may be permanently damaged by pushing them to extremes of temperature (the fridge may not be a problem, but a freezer might well represent more of a problem).
 

Offline syhprum

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Re: Why do batteries go "flat" ?
« Reply #8 on: 19/05/2007 17:25:38 »
When I was a boy (many years ago) I used to reactivate Carbon-Zinc cells by connecting them to a 4 volt transformer and heating them up till the pitch with which they were sealed in those days melted.
Do not try this at home!
 

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Re: Why do batteries go "flat" ?
« Reply #8 on: 19/05/2007 17:25:38 »

 

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