BATTERIES

 

 

Many of the electronic devices are operated on mains power only; in early days. With the rapid increase in semiconductor technology; voltage (power) required to the electronic equipment is on decreasing. So devices like; calculator, Walkman stereo, mobile phone, toys, etc uses batteries rather than mains power. There for portability such devices increased. The Volta’s Pile in 1980 was invented the dry cell. A cell is a device constructed to do some chemical reactions to generate electric current. If a set a set of voltaic cells designed (connected in series or parallel) to provide greater voltage and/or current than is possible with one cell alone, it is called battery. Some commercially available (dry) batteries and their typical ratings are shown below. 

 

 

 

 

Size	Typical current rating	Short circuit current	Out put voltage	Internal resistance
AA	10mA	4.5A	1.5 V	0.33 W
C	20mA	5.5A	1.5 V	0.27 W
D	35mA	6.5A	1.5 V	0.23 W
Rectangular 9V battery		0.5A	9 V	19 W

 

 

Rectangular 9 V battery

The table above shows that when the size of the battery increases, internal resistance decreases and hence current handling capacity also increases. Note that the physical size of a cell has no impact on its voltage; but it has importance in its resistance, which in turn affects the maximum amount of current that a cell can provide. Every cell contains some amount of internal resistance due to the electrodes and the electrolyte. If a cell larger, the greater the electrode contact area with the electrolyte, and thus the less internal resistance it will have.

The current handling capacity can be increased also by no: of batteries are connected in parallel. Although we generally consider a cell or battery in a circuit to be a perfect source of voltage (constant), and the current is flowing only through the external resistance (load) of the circuit to which it is attached, this is not exactly true in practical. Since every cell or battery contains some internal resistance, that resistance must affect the current in any given circuit.

The current handling capacity of a battery can be increased by no: of batteries are connected in parallel. i.e. If two batteries are connected in parallel, as shown fig (a) below, and its internal resistances are as in fig (b).

 

 

 

fig (1 a) Two batteries connected in parallel

fig (1 b) Batteries with their equivalent resistances Note: equivalent resistance 0.33W is selected for AA batteries

fig (1 c) Equivalent circuit when batteries replaced with short circuit

fig (1 d) Thevenins equivalent circuit

 

 

 

 

 

 

 

 

 

 

 

 

 

 

From the fig above we can conclude that when two batteries are connected in parallel; their equivalent internal resistances also act as a parallel resistance combination. Hence the internal resistance decreases so more current can supply with less drop in voltage.

 

 

         

fig (2a) Two batteries connected in series

fig(2b) Batteries with theirs with their internal resistances

fig (2b) Thevenins equivalent circuit

 

In the same way when two batteries are connected in series; their internal resistances also act in series. There for the Thevenins voltage (V_TH ) is the sum of two voltage sources and Thevenins resistance (R_TH) is the sum of two series resistances as shown in fig(2b).

An ideal battery (which has only 0Ω internal resistance) would be able to supply a short circuit current of infinite amount. But the real battery, on the other hand, can supply only a few amps of currant to a short circuit due to its internal resistance. For example consider a battery with electrode voltage of 12V and an internal resistance of 0.3 Ω, it can supply a maximum current (short circuit current) of 12/0.3= 40Amps only. Typically, batteries are selected for applications where their internal resistance is negligible compared to that of the circuit load (i.e. load resistance is >> internal resistance of battery), and so the performance is very close to that of an ideal voltage source.

Development of rechargeable Ni-Cd batteries brought a new dimension to battery usage. Ni-Cd batteries provide a steady voltage and moderate power. They could be rechargeable several times before becoming unserviceable. Today they are used in cameras, computers, torchlights, etc. Lead acid cell is a heavy-duty rechargeable cell, mainly used for automobiles, invertors, and high capacity CPU’s.

 

 

TESTING OF A BATTERY

fig (2 a)

The right method of checking a battery is by connecting a load across the battery and measuring the out put voltage across the load. If the measured voltage is not in acceptable limit, eliminate the battery.

A popular method of checking a battery is the short circuit *SPARK*!! (due to infinite current) observation. NEVER USE THIS METHOD because; it is very harmful and may damage the battery

Remember that as a battery discharges, not only its internal energy decays, but also its internal resistance increases. The best check for a battery's condition is a voltage measurement under load as mentioned as above. A simple voltmeter check across the terminals may falsely indicate the condition of as battery healthy even though the internal resistance has increased considerably.

 

 

 

 

 

Method for measuring the internal resistance of a battery     Find the no load voltage across the terminals of the battery using a digital multi meter     Connect a suitable load across the battery and note the value of voltage and resistance [selection of load: fist select a high value load such as 10KΩ; if there is no noticeable decrees in o/p voltage compared to no load voltage decrees the load to 1KΩ, 470Ω, 100Ω, 47Ω, 10Ω, 4.7Ω,etc. Plz note the load resistance must have a sufficient power rating]      Find out the internal resistance using the formula                    For example, if the voltage across the battery under no load is 12V we can say that its Thevenins equivalent voltage (VTH) is 12V. When the battery is connected with a load of 100Ω, noted that its terminal voltage drops to 10V. the we can say that the voltage drop across its internal resistance is 12-10=2V, at the same time the current flowing through the circuit will be 10/100=0.1A. So the internal resistance is V/Ið2/0.1=0.2 Ω

 

The factors that depends internal resistance of a cell is

 Increases with age of the cells

 Decreases with concentration of electrolyte,

 Decreases with increasing the size of electrodes

 Decreases with area of the plates inside the electrolyte

 Increases with increase in distance b/w the two electrodes

 

 

SOLAR CELL [PHOTO VOLTAIC CELL]

Solar cell is semiconductor devices that convert solar energy into electrical energy.  A silicon solar cell consist thin wafer of silicon with its back surface completely silvered. Light waves entering the surface force electron to move towards silvered surface. An individual solar cell with width of 1 cm can supply open circuit voltage of 0.8V and 0.55V with load. Connecting many solar cells in series can increase the voltage out put of a solar system. . Conversion efficiencies of silicon solar cells are still very low, but their benefits as power sources are legion. It has no moving parts, no waste products, any noise or pollution; it has an indefinite life also.

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