Difference Between Primary and Secondary Cells

Main Difference – Primary vs Secondary Cell

Batteries are cells which can generate electricity. A battery can have one or more electrochemical cells and it is composed of terminals that can be connected with external devices which work with a power supply. Primary cells and secondary cells are two types of batteries. The main difference between primary and secondary cells is that primary cells can be used only once whereas secondary cells can be used more than once.

Key Areas Covered

1. What are Primary Cells
      – Definition, Structure, Chemcial Reactions
2. What are Secondary Cells
      – Definition, Structure, Chemcial Reactions
3. What is the difference between Primary and Secondary Cells
      – Comparison of Key Differences

Key Terms: Battery, Primary Cell, Secondary Cell, Anode, Cathode, Electricity

What is a Primary Cell?

Primary cells or primary batteries are also called non-rechargeable batteries. This is mainly because these batteries cannot be charged again when they are exhausted and stop working. Although they are not reusable, primary cells are very useful to store power for long-term utilization because their self-discharge rate is comparatively low. Therefore, their use can be observed in service pacemakers in heart patients, smart meters, and military campaigns where charging is impractical and impossible

Primary cells are non-rechargeable due to the chemical reactions that occur inside the battery, which are irreversible. The chemical reactions use all the chemicals present in the cell and when all the chemical species are used, power generation is terminated.

A primary battery or a primary cell is composed of an anode (positively charged end) and cathode (negatively charged end). Graphite is usually used as the cathode and Zinc is used as the anode. Oxidation reactions occur in the anode where electrons are donated to the circuit and reduction reactions occur in cathode where electrons are accepted from outside. There is also an electrolyte which is helpful in making a passage for electric current. The electrolyte is composed of electrically charged ions which can carry their charge between cathode and anode.  

Figure 1: A Primary Battery having a Zinc Anode and Cu Cathode

A common example of primary cell is Leclanche cell. It is composed of a Zinc anode and porous graphite cathode. The electrolyte present inside the battery is a moist mixture of NH₄Cl (ammonium chloride), Zinc chloride (ZnCl₂) and Manganese dioxide (MnO₂). The chemical reactions that occur inside the cell can be given as below.

Anode:                Zn_(s) →    Zn²⁺_(aq)       +       2e

Cathode:   2NH₄⁺_(aq)    +    2e       → 2NH_3(g)   +    H_2(g)

Two gases NH₃ and H₂ are produced in the cathode. But these gases again will participate in reactions as shown below.

2NH_3(g) + Zn²⁺_(aq) → [Zn(NH₃)₂]²⁺_(aq)

2MnO_2(s) + H_2(g) →Mn₂O_3(s) +H₂O_(l)

What is a Secondary Cell?

Secondary cells are also known as secondary batteries or rechargeable batteries. These batteries can be used multiple times since they can be charged when they stop working. The chemical reactions that occur in these batteries are reversible. This means that those reactions can be reversed by applying an electrical charge.  But unlike in primary batteries, secondary batteries must be charged before use. Chargers are used for the recharging process.

Different secondary batteries provide different functions. Therefore, the user should know which type of battery should be used for a particular need.

These batteries also have a cathode and anode same like primary batteries. Reduction reactions occur in the cathode whereas oxidation reactions occur in the anode. A good example for secondary cells is the Lead storage/acid battery. The reactions that occur inside that cell are shown below.

Cathode:   PbO_2(s)   +   HSO₄^–_(aq)   +   3H⁺_(aq) +   2e   →   PbSO_4(s)   +   H₂O_(l)

Anode:       Pb(s)      +   HSO₄^–_(aq)   →   PbSO_4(s)     +   H⁺_(aq)    +   2e  

Figure 2: A schematic diagram of Lead acid battery.

Difference Between Primary and Secondary Cells

Definition

Primary Cells: Primary cells are batteries that cannot be recharged or reused. 

Secondary Cells: Secondary cells are batteries that can be recharged and reused.

Chemical Reactions

Primary Cells: In primary cells, irreversible reactions occur.

Secondary Cells: In secondary cells, reversible reactions occur.

Usage

Primary Cells: Primary cells can be used only once.

Secondary Cells: Secondary cells can be used more than once.

Importance

Primary Cells: Primary cells can produce current immediately; thus, they are used in portable devices.

Secondary Cells: Secondary cells should be charged before use. So they can be used in automobiles.

Self-Discharge Rate

Primary Cells: Primary cells have lower self-discharge rates and can be used for long term storage of power.

Secondary Cells: Secondary cells have a higher self-discharge rate compared to primary cells. 

Conclusion

Batteries are very important in technological applications. All batteries are made out of electrochemical cells. Primary and secondary cells are two types of batteries which are useful in day to day life. The main difference between primary and secondary cells is that primary cells can be used only once whereas secondary cells can be used more than once.

References:

1. “Primary and secondary batteries.” Lead Storage Battery I. N.p., n.d. Web. Available here. 12 June 2017. 
Libretexts. “Rechargeable Batteries.” Chemistry LibreTexts. Libretexts, 22 Jan. 2017. Web. Available here. 12 June 2017. 

Image Courtesy:

1. “Hand-painted primary battery” By 脂肪酸钠 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Discharged” (CC BY 3.0) via Commons Wikimedia