Explaining Why Dry Cell Battery Resistance Increases & Power Pack Resistance

[Canuck156]

Hi,

I just need a proper explanation as to why the internal resistance of a dry cell battery increases as the battery is used?

Also, what would cause the internal resistance, if there is any, in a power pack? (A power pack is a device that allows electricity from a powerpoint to be lowered to 2-12 Volts, suitable for experimental purposes.)

Thanks in advance for any help.

Canuck156

 

[arcnets]

Canuck156,
1) I guess the reason is the acid inside getting weaker as it dissolves the electrodes.
2) Due to hysteresis, you lose some energy in the transformer's core, producing heat, which probably is the cause of output resistance.

 

[Chi Meson]

When a battery is part of a flowing circuit, it too must be part of the conducting loop. The electric flow in the battery is due to the flow of ions through a solution (in a wet cell). As the electrolytic solution (the acid or alkaline) is "used up" in the chemical reaction, there are fewer and fewer ions to carry the charge, therefore it's harder for the battery's own emf to push charges through itself. Hence: larger internal resistance.

Side note: if you are ever given the choice of having a "fully charged" car battery, or a "dead" car battery dumped on you, choose the latter.

Second question: A little nifty phenomenon called "self-induction" introduces "impedence" which has the same effect as "resistance" and is also measured in ohms. On top of that, all wires have resistance, and a transformer will have a lot of wire inside it. wound around and around. Also, as the current in a wire increases, the temperature of the wire increases (check your toaster for validity). As the temperature of a wire increases, the resistance of that wire increases. So the internal resistance of a transformer will also increase as the current through it increases.

 

[Canuck156]

Thanks for the help, I now understand the second question that I posted. However, is the explanation regarding the acid still relevant for a 'dry cell' battery? (As specified in the original post) ?

 

[Chi Meson]

The typical dry cell uses the chemical reaction between zinc and manganese dioxide. I don't remember the specifics, but the reaction itself is not so straightforward, so a chemistry type person would give you a better answer if you need detail

As the zinc is turned into zinc oxide, it stops being a metal (now it's a compound) and thus is not a good conductor anymore. The layer of zinc oxide gets thicker and thicker and so there is more and more non-conductive stuff for the electrons to flow through.

 

[Canuck156]

Thankyou, that has now answered both of my original question.

Thanks for the help!