Batteries

 Types of batteries

All the electrochemical cells and batteries are classified into two types:

• Primary batteries (non-rechargeable)
• Secondary batteries (rechargeable)

A Primary Battery is one of the simple and convenient sources of power for several portable electronic and electrical devices like lights, cameras, watches, toys etc. As they cannot be recharged electrically, they are of “use and throw” type.

Usually, primary batteries are

1. Inexpensive 
2. light weight
3. small and very convenient to use with relatively no or less maintenance.

Majority of the primary batteries that are used in domestic applications are single cell type and usually come in cylindrical configuration (although, it is very easy to produce them in different shapes and sizes).

A Secondary Battery is also called as Rechargeable Battery as they can be electrically recharged after discharge. The chemical status of the electrochemical cells can be “recharged” to their original status by passing a current through the cells in the opposite direction of their discharge.

Basically, secondary batteries can be used in two ways:

• In the first category of applications, the secondary batteries are essentially used as energy storage devices where they are electrically connected to a main energy source and also charged by it and also supplying energy when required. Examples of such applications are Hybrid Electric Vehicles (HEV), Uninterrupted Power Supplies (UPS), etc.
• The second category of applications of secondary batteries are those applications where the battery is used and discharged as a primary battery. Once it is completely discharged (or almost completely discharged), instead of discarding it, the battery is recharged with an appropriate charging mechanism. Examples of such applications are all the modern portable electronics like mobiles, laptops, electric vehicles, etc.

Energy Density of secondary batteries are relatively lower than that of primary batteries but have other good characteristics like high power density, flat discharge curves, high discharge rate, low temperature performance.

There are some other types of Secondary Batteries but the four major types are:

• Lead – Acid Batteries
• Nickel – Cadmium Batteries
• Nickel – Metal Hydride Batteries
• Lithium – Ion Batteries

Characteristics of batteries

1) Voltage

The theoretical standard cell voltage can be determined from the electrochemical series using Eo values:

Eo (cathodic) – Eo (anodic) = Eo (cell)

This is the standard theoretical voltage.

2) Discharge Curve

The discharge curve is a plot of voltage against percentage of capacity discharged. A flat discharge curve is desirable as this means that the voltage remains constant as the battery is used up.

3) Capacity

The theoretical capacity of a battery is the quantity of electricity involved in the electro-chemical reaction. It is denoted Q and is given by:

where x = number of moles of reaction, n = number of electrons transferred per mole of reaction and F = Faraday's constant

The capacity is usually given in terms of mass, not the number of moles:

where Mr = Molecular Mass. This gives the capacity in units of Ampere-hours per gram (Ah/g).

4) Energy density

The energy density is the energy that can be derived peer unit volume of the weight of the cell.

5) Power density

The power density is the power that can be derived per unit weight of the cell (W/kg).

6) Specific energy density

The specific energy density is the energy that can be derived per unit weight of the cell (or sometimes per unit weight of the active electrode material). It is the product of the specific capacity and the operating voltage in one full discharge cycle. Both the current and the voltage may vary within a discharge cycle and thus the specific energy derived is calculated by integrating the product of current and voltage over time. The discharge time is related to the maximum and minimum voltage threshold and is dependent upon the state of availability of the active materials and/or the avoidance of an irreversible state for a rechargeable battery.

7) Temperature dependence

The rate of the reaction in the cell will be temperature dependent according to theories of kinetics. The internal resistance also varies with temperature; low temperatures give higher internal resistance. At very low temperatures the electrolyte may freeze giving a lower voltage as ion movement is impeded. At very high temperatures the chemicals may decompose, or there may be enough energy available to activate unwanted, reversible reactions, reducing the capacity.
The rate of decrease of voltage with increasing discharge will also be higher at lower temperatures, as will the capacity- this is illustrated by the following graph:

Battery Backup

A battery backup is a system in some power supplies that switches between a main power source and a battery. Battery backup provides emergency lighting in the event of power failure. This device is called UPS (Uninterrupted Power Supplies).

There are two methods for battery backup 

• Online battery backup

• Offline battery backup

 

Offline Uninterruptible Power Supply (UPS)