Science Questions

Why do some batteries last longer than others?

Sun, 11th Apr 2010

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Question

David Waltham asked:

Why do some batteries last much longer than other less expensive brands? And are different types more suitable for certain applications than others where the savings would not be as great?

Answer

Fundamentally, a battery is a chemical reaction going on, and part of that chemical reaction is it pushes electrons from one side of the battery to the other side.  So, actually, you have two halves of a chemical reaction, one which absorbs electrons and another that gives them out.  And the only way that the chemical reaction can carry on is when these electrons are going around and getting back to the other side through your circuit and they can do work when it's doing that.  Now, there are lots of different chemical reactions you can use and so, basically, the number of atoms – the number of molecules which can be active or which can move electrons across is important.  The more you can get in the battery, the more current, more charge you can store in the battery.

So, you've got to have lots of complex things that hold it together, which take up space and take up weight – and, normally, it's only on the surface where these things can react.  So, any kind of centre of an electrode which isn't able to react with things is useless and doesn't work very well.  So, basically, the way they last longer is by having more of the battery, which is active, you can do different chemical reactions which can store more energy, take up less space.  You can also use different chemical reactions if to produce different voltages.  So, a lithium ion battery is at 3V whereas a standard one and a half volt alkaline cell is 1.5V, and a rechargeable battery is 1.2 volts. 

Different batteries have different properties.  Alkaline cell will last for a very, very long time.  It doesn't lose its charge.  It could just sit back and it will last for several years.  It has a shelf life of several years.  Whereas a rechargeable battery will just discharge itself in maybe a month or so.  Also, a rechargeable battery can give out much more current.  So, if you've got a high current application, a rechargeable battery works a lot better.  So, yes, there are different circumstances where some are better than others.

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Derek asked the Naked Scientists: Why do some batteries last much longer than other, cheaper ones and by what method is this achieved?   Derek, Wellingborough What do you think? Derek , Thu, 25th Mar 2010

Presumably you mean primary (non rechargeable) batteries.  All such batteries produce their electricity by using a chemical reaction which only happens when an electric current is allowed to pass between the positive and negative electrodes.  This is in effect the reverse of electrolysis when chemicals  (notably metals) can be separated from a conducting (ionic) solution by passing an electrical current through it.  This chemical reaction can only continue until the reagents run out.  There are several different chemical reactions that are used.  the original one (the Le Clanche cell)  uses a reaction using zinc metal and an acidic gel with a carbon positive electrode.  These have relatively short lives and are standard batteries.  Alkaline batteries use manganese ions and have much longer lives better storage properties and greater current capacity.  There are several other primary cell reactions that are used mostly in button cells for low power applications and very long shelf lives.

This wikipedia article gives lots more detailed information


http://en.wikipedia.org/wiki/Battery_(electricity)

Sorry I can't get this link to paste properly the last bracket always drops out of the html <a>  it will offer you the correct page if you follow it though.



It is interesting to note that all these reactions are inherently reversible  that is if you pass force electric current backwards through a partially discharged battery you can recharge them although batteries will often swell and may explode if this is done too violently.  To be successful you should also recharge a battery with precisely the correct amount of current.  if this is done carefully it is possible to increase the life of a cheap battery by five to ten times in undemanding applications. Soul Surfer, Sat, 27th Mar 2010

The chemistry of the cell determines the basic energy-capacity of the cell.

However, an important thing to consider with batteries is that the 'available' energy decreases with high-loads - which is related to the concept of 'internal resistance' in the battery. The physical contruction of the battery, including the surface area of the elctrodes (look for words like 'sintered') have an important role in decreasing the internal resistance (good) and maximising the available energy at high loads. Not all batteries are equal, and you may find, for example, that on a modest load (perhaps an LED cycle light) which might run for a few 10's of hours works comparably on both premium and supermarket own-brand alkalines, yet if you used the same batteries in a high-drain appliance (like a digital camera) where you might only get 1-2 hours use you might find the cheaper batteries last less than half as long.

As I understand it, the Duracell 'Ultra' range of batteries are optimised for high-drain use (the capacity doesn't fall so much with high current-drain) and will outlast normal Duracell for digital cameras and the like. If you used them in something like a modest LED cycle-light or radio, you'd probably find they last pretty much the same as regular Duracell.


Another complication is not just the energy-capacity, but how the voltage on the cell falls with use... for a simple bulb-torch, the brightness of the light just fades as the batteries get used - but in a modern electronic widget (like an mp3 player or digital camera) there's usually a distinct cut-off voltage below which the gadget declares 'low battery' and refuses to function. It is therefore generally preferable to have a battery which maintains it's original voltage for as long as possible, then 'falls off a cliff' - rather than a voltage which gradually just droops...
Again different manufacturers batteries may have relatively different energy capacities depending on where you define the end-of-life, eg "to 1.2V" or "to 0.9V".

I suspect that some of the minor chemical additives have a bearing on the voltage-drop with use - but I'm not a chemist (and certainly not a battery chemist), so I'll leave that for others to comment on.
techmind, Sat, 27th Mar 2010

My lantern has a five watt fluorescent U-tube. Battery life tends to be short with it. Has anyone got any recommendations as to what batteries I should be using? The makers don't recommend rechargeables. rhade, Tue, 30th Mar 2010

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