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Zinc-carbon batteries of different sizes.
A Zinc-carbon dry cell or battery is packaged in a zinc can that serves as both a container and anode. It was developed from the wet Leclanché (IPA pronunciation: [lɛklɑːnˈʃei]) cell. "Super" or "Heavy Duty" batteries, technically called zinc chloride cells, are an improved version from the cheaper "General Purpose" variety. The cathode is a mixture of manganese dioxide and carbon powder. The electrolyte is a paste of zinc chloride and ammonium chloride dissolved in water. Carbon-zinc batteries are the least expensive primary batteries and thus a popular choice by manufacturers when devices are sold with batteries included. They can be used in remote controls, flashlights, toys, or transistor radios, where the power drain is not too heavy.
A primary cell
A Zinc-carbon dry cell is described as a primary cell because as the cell is discharged, it is not intended to be recharged and must be discarded. "Battery Rejuvenators" were once marketed to restore partially discharged Zinc-carbon cells by applying a reverse current to them. However the effects of such devices were only temporary and prone to cause the cell to leak or burst.  zinc-carbon cells are more likely to leak
The container of the zinc-carbon dry cell is a zinc can. This contains a layer of NH4Cl with ZnCl2 aqueous paste separated by a paper layer from a mixture of powdered carbon & manganese (IV) oxide (MnO2) which is packed around a carbon rod.
Cross-section of a zinc-carbon battery.
In a dry cell, the outer zinc container is the anode (-). The zinc is oxidised according to the following half-equation.
- Zn(s) → Zn2+(aq) + 2 e-
A graphite rod surrounded by a powder containing manganese(IV) oxide is the cathode(+). The manganese dioxide is mixed with carbon powder to increase the conductivity of the cathode mixture. The cathode reaction is as follows:
- MnO2(s) + 2 H+(aq) + 2 e- → Mn2O3(s) + H2O(l)
The H+ comes from the NH4+(aq):
- NH4+(aq) + H2O(aq) → H+(aq) + NH3(aq)
and the NH3 combines with the Zn2+.
In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3).
There are other possible side-reactions, but the overall reaction in a zinc-carbon cell can be represented as:
- Zn(s) + 2 MnO2(s) + 2 NH4+ → Mn2O3(s) + Zn(NH3)22+(aq)
The battery has an e.m.f. of about 1.5 V. Side reactions and depletion of the active chemicals increases the internal resistance of the battery, and this causes the e.m.f. to drop.
When the dry cell has been used for a certain time, the zinc container becomes thinner because zinc metal is oxidised to zinc ions. Therefore Zinc Chloride Solution leaks out the battery. The old dry cell is not leakproof. It becomes very sticky as the paste leaks through the holes in the zinc case. The service life of the battery is short, with a shelf life of around 1.5 years.
Furthermore, the zinc casing in the dry cell gets thinner slowly, even when the cell is not being used. It is because the ammonium chloride inside the battery is acidic, reacting with the zinc.
The Zinc Chloride Cell
The (heavy duty) zinc chloride cell is an improvement on the original zinc-carbon cell, using purer chemicals and giving a longer life and steadier voltage output as it is used. Instead of an electrolyte mixture containing much NH4Cl, it is largely only ZnCl2 paste. The anode reaction is thus a little different:
- MnO2(s) + H2O(l) + e- → MnO(OH)(s) + OH-(aq)
as is the overall reaction:
- Zn(s) + 2 MnO2(s) + ZnCl2(aq) + 2 H2O(l) → 2 MnO(OH)(s) + 2 Zn(OH)Cl(aq)
Eveready: Carbon Zinc Application Notes
Rayovac: Alkaline and Heavy Duty Application Notes
Power Stream Battery Chemistry FAQs
Categories: Cleanup from December 2006 | Wikipedia articles needing clarification | Electric batteries