Every day, we rely on many kinds of batteries - alkaline, rechargeable, lithium, metal hydride and others. Did you know that a battery is a device consisting of one or more voltaic cells? Because the function of a battery is to convert chemical energy into electrical energy, it is sometimes referred to as an "electric cell".

1. Basic Cell Design and Properties
   
   
   • An electric cell or battery always has two electrodes - an anode and a cathode. The electrodes are usually two metals or graphite and a metal. Sometimes, the container of the cell functions as one of the electrodes.
     
   • The anode is marked negative (-) and the cathode is marked positive(+). Electrons move from the anode through the external circuit to the cathode. A battery functions only when an external conducting path is provided.
     
   • The voltage of a battery is not dependent on its size. For example, although 1.5 V batteries come in many sizes (such as AAA, AA, B, C, and D), they are all rated at 1.5 V. However, larger electric cells can produce greater current than smaller electric cells. 
     
   • An electric cell or battery also contains an electrolyte. This is usually a moist paste.
     
   
   
   
                          Fig. 4  Electric cell or battery
   
   
   
   
2. Types of Batteries

   
   Consumer cells can be classified into three types: primary cells, secondary cells, and fuel cells.
   
   

   Primary cell: a closed cell that cannot be recharged

   Secondary cell: a closed cell that can be recharged

   Fuel cell: an open cell that requires the continuous addition of fuel

   
   

   
   A. Primary Cells
   

   • Dry Cell

     The most-used type of commercial primary cell is a dry cell. A dry cell uses a paste electrolyte. Because the quantity of water in the cell has been reduced as much as possible, the dry cell can operate in any orientation without risk of spillage. The basic structure of a dry cell is shown below. Note that the paper barrier serves to separate the carbon cathode from the zinc anode (zinc cup).
     

     
     
                 Fig. 5  Dry cell         
   
   
   

   • Alkaline Dry Cell

     A modification of the dry cell is the alkaline dry cell. The modification does not affect the measured cell potential, but it does improve the performance of the cell.

     The electrolyte in a zinc-chloride cell is slightly acidic. This makes the zinc-chloride cell somewhat inefficient because both H⁺ ions and the intended oxidizing agent are reduced. In contrast, the electrolyte in an alkaline cell is basic. Recall that as the concentration of OH- increases in a solution, the concentration of H⁺ is reduced. Basic electrolytes, therefore, reduce the possibility that H⁺ ions will act as an oxidizing agent in the battery. In other words, alkaline batteries tend to last longer before "going dead". In addition, they produce a slightly higher current.
     

     Associated equations for an alkaline dry cell:
     

     Cathode:	\( \mathrm { 2MnO_2(s) + H_2O(l) + 2e^- \rightarrow Mn_2O_3(s) + 2OH^-(aq) } \)
     Anode:	\( \mathrm { Zn(s) + 2OH^-(aq) \rightarrow ZnO(s) + H_2O(l) + 2e^- } \)

     
     Alkaline cells and other cells that use zinc and manganese oxide as reactants are not rechargeable; some cells come with warnings to prevent users from attempting to recharge them.
     

   
   B. Secondary Cells
   

   
   Secondary cells are used in many applications including cellular telephones, video cameras, and personal music players. The car battery is also an example of a secondary cell.
   

   
   Secondary cells are batteries that can be recharged. During the recharging process, an input of energy reverses the chemical reaction that produced electricity in the cell.

   
   

   • Nickel-Cadmium Battery
     

     One common type of rechargeable cell is the nickel-cadmium battery (Ni-Cd). The advantage of nickel-cadmium batteries is that they have a relatively low internal resistance and, therefore, can supply high surges of current when necessary. They are relatively easy to recharge. The main disadvantage of nickel-cadmium batteries is that, typically, they have a lower cell potential (i.e. 1.2 V as compared to alkaline dry cells, which have a measured potential of 1.5 V).

   • Lead-Acid Car Battery
     

     One of the most common secondary cells in use today is the lead-acid battery, typically used in automobiles. As the battery discharges, the following redox reactions occur.
     

     Cathode:	\( \mathrm { PbO_2(s) + 4H^+(aq) + SO_4^{2-}(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l) } \)
     Anode:	\( \mathrm { Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4 (s) + 2e^- } \)

     
     
     
     
     

     Energy is required to recharge the battery. This energy comes from the alternator as the car engine runs. During the re-charging process, the half-reactions occur in reverse, converting products back to reactants as illustrated in the in the following net equation.
     

     
     
     
     

     Because they are relatively cheap and easy to recharge, lead-acid batteries are useful for automobiles.
     

     A disadvantage of the lead-acid cell is that the electrolyte may leak or spill.
     

     
     
     
     Fig 6 Lead-acid battery
     
     

   
   C. Fuel Cells
   

   
   Fuel cells are like batteries, however they do not run down or require recharging. They produce electricity as long as fuel is supplied. In theory, a fuel cell could last forever.
   

   
   By far, the most common fuel cell reaction is based on hydrogen and oxygen. In this fuel cell (shown below), hydrogen and oxygen are pumped continuously into the cell.
   

   
   
   
                        Fig. 7   Fuel cell

   
   The only products of this fuel cell are water and electricity. Hence, the fuel cell does not produce greenhouse gases or substances that contribute to acid rain.

   
   The increasing popularity of the electric car has led to renewed interest in the fuel cells. Although most electric cars run on conventional batteries, fuel cells may prove to be feasible alternatives.
   

    

   Did You Know?           NASA uses the hydrogen-oxygen fuel cell to provide electricity for manned space flights. Hydrogen and oxygen are readily available and the reaction produces water, which can be purified for drinking.

   
   

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3. Batteries and the Environment

   
   The greatest environmental concern surrounding batteries involves their disposal. Some chemicals used in batteries, such as nickel and cadmium, are extremely toxic and can cause damage to both humans and the environment. For this reason, it is important to keep batteries out of landfills.
   

   
   
   
                     Fig 8 recycle symbol on 12 V vehicle battery