Battery Basics - Lead Acid Battery

DEFINITION of  BATTERY -

Battery is an electro-chemical device, which stores energy in the chemical form, and releases it in an electrical form when call upon to do so.

BATTERY PRINCIPAL

 Galvanic Theory-  In 1780 Luigi Galvani connected  copper and zinc and then both are touched at the same time to two different parts of a muscle of a frog leg, the frog's  legs got  contracted , as frog’s body has the role of  the electrolyte.

“When two Dissimilar Metals are placed in an Acid solution and when they are connected externally, electric current gets generated.”

 The Dissimilar metals are called ELECTRODES ( Cathode or Positive Electrode & Anode or Negative electrode) and The Solution is called ELECTROLYTE.

 Battery Types –

Primary Cell Batteries -  One time use and cannot be recharged.

Secondary Cell batteries- Accumulators, can be recharged repeatedly.

Here we are focusing more on secondary cells and hence providing more information about Lead Acid Battery.

Lead Acid Battery –

Lead Acid Battery has Positive electrode  which is LEAD- PER- OXIDE,  Negative electrode is SPONGY LEAD & Electrolyte consists of a solution of SULFURIC ACID and  Demineralised water.

The chemical reaction during  Charging & Discharging of Lead Acid Battery -

Lead Acid Battery discharge facts

As the discharge continues, the active material of both plates become more alike. i.e. Lead Sulfate and the Electrolyte becomes weaker. Due to this the voltage becomes lower since it depends on the difference between two plates material and concentration of acid.  Eventually, the battery can no longer deliver electricity at a useful voltage and is said to be DISCHARGED.

Lead Acid Battery charging facts

  CHARGING - On passing electric current (Direct) through the battery in the opposite direction to the direction of discharge, its active chemicals can be restored to the charge state.The battery would then be ready to deliver its full POWER. 

Current types-

There are two types of currents. Alternate current (AC) and  Direct current (DC).

(It will be one more topic, which we will discuss some other time)

NOTE-  While Charging and Discharging of Battery , HEAT is generated which causes evaporation of water content in electrolyte and expose of plates to air will damage the plates.

Electrolyte -

The electrolyte in Lead Acid Storage Battery is dilute SULFURIC acid. Battery grade diluted Sulfuric acid to be used. The strength of the electrolyte is measured in  Specific gravity. A fully charged lead acid battery will show a specific gravity of  1.230 ~ 1.240 corrected to 27 ⁰ C.

Battery Capacity –

Voltage –Lead Acid battery consists of multiple cells which are connected in series connection . Each cell carries 2.1 Volts. Normally, now a days batteries used are of 12.6Volts, means  6 cells are connected in series connection.

 Capacity is the output in Ampere-hours (Ah) the battery will deliver which depends on the surface area of the battery plates along with electrolyte volume. Life of Battery- Depends on active material used. 

State of Charge-

 The state of charge of  Lead Acid battery can be  determined by the specific gravity of the electrolyte or by the stabilized open circuit voltage (OCV) of battery.  Specific gravity is measured by a HYDROMETER. Fully charged battery is most of sulfuric IONS in   electrolyte and as battery discharges ,  sulphatation takes place in the plates ,which converts the acid into water and specific gravity drops.   

Typical OCV ( Open circuit Voltage) &  Sp.Gr.(Specific Gravity) values for 12Volt Battery.

Charge level	Sp. GR.	Voltage (Volts)
100%	1.240	12.42
75%	1.190	12.18
50%	1.155	11.97
25%	1.120	11.76
Discharged	1.085	11.55

 What is Grid in Lead Acid Battery –

The plate of a Lead Acid storage Battery is so designed on a frame which carries and provides path to electrons. Grid framework in the meshes on which the active materials are incorporated. These grids serve to conduct current and to hold the active materials of the Positive and Negative plates.

 Plates ( Positive & Negative) –

Positive Plate – filled  / pasted with Lead Peroxide /red lead.

Negative Plate – Filled with porous mass of lead in spongy form with electrolyte along with expander of carbon black, barium sulfate etc… The plates are then partially dried , after drying , the negative plate acquire a grey color and positive has a light brown color.

Separators – It is absolute importance that the positive plate should NOT touch the negative plate or else. There will be short circuit, which can result in loss of stored energy from all the plates. 

Thin sheets of NON –CONDUCTING POROUS material  is called SEPARATORS. 

They are inserted between the plates. 

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Stay tuned for the next posts with will be even more interesting..
Also if you want to know about handling and maintenance of batteries do check this blog where I talk about the handling and maintenance of automotive batteries and the tricks about it. Link is provided here.
If you want to know about the structure of the automotive batteries and it composition please check this post.  

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