Sub-discipline:- Electrical System( Lessons: 03 Sessions:15)
Lesson–III: Auto Electrical Session-7 : Battery: Definition of Cell & Battery,
Types, Construction, Specific Gravity, Rating
BATTERY :
The battery is the primary "source" of electrical energy on Automotives. It stores chemicals,
not electricity. Two different types of lead in an acid mixture react to produce an electrical
pressure. This electrochemical reaction changes chemical energy to electrical energy. Battery
is the Combination of cells .
CELL:
It converts chemical energy to electrical energy. In cell electrical energy is store in the form of
chemical energy. When battery is connected to any circuit then chemical energy is converted to
electrical energy current flows from +ve to –ve terminal.
Symbol Cell: + -
Battery + -
TYPES :
1. PRIMARY CELL: Non rechargeable cell is called Primary Cell.The chemical reaction
totally destroys one of the metals after a period of time. Small batteries for Torch and
radios are primary cells.
2. SECONDARY CELL: Rechargeable cell is called Secondary Cell .The metals and acid
mixture change as the battery supplies voltage. The metals become similar, the acid
strength weakens. This is called discharging. By applying current to the battery in the
opposite direction, the battery materials can be restored. This is called charging .
Automotive lead-acid batteries are secondary cells
CONSTRUCTION :
1.Case: Container which holds and protects all battery components and electrolyte, separates
cells, and provides space at the bottom for sediment (active materials washed off plates)
Translucent plastic cases allow checking .
2.Cover: Permanently sealed to the top of the case; provides outlets for terminal posts, vent
holes for venting of gases and for battery maintenance (checking electrolyte, adding water).
3.Plates: Positive and negative plates have a grid framework of antimony and lead alloy.
Active material is pasted to the grid ... brown-colored lead dioxide (Pb02) on positive plates,
gray- colored sponge lead (Pb) on negative plates. The number and size of the plates
determine current capability ... batteries with large plates or many plates produce more
current than batteries with small plates or few plates.
4. Separators: Thin, porous insulators (woven glass or plastic envelopes) are placed between
positive and negative plates. They allow passage of electrolyte, yet prevent the plates from
touching and shorting out.
5.Cells: An assembly of connected positive and negative plates with separators in between
is called a cell or element. When immersed in electrolyte, a cell produces about 2.1 volts
(regardless of the number or size of plates). Battery cells are connected in series, so the
number of cells determines the battery voltage. A"1 2 - volt" battery has six cells.
6. Cell Connectors: Heavy, cast alloy metal straps are welded to the negative terminal of
one cell and the positive terminal of the adjoining cell until all six cells are connected in
series.
7. Cell Partitions: Part of the case, the partitions separate each cell.
8. Terminal Posts: Positive and negative posts(terminals) on the case top have thick, heavy
cables connected to them. These cables connect the battery to the vehicle's electrical
system (positive) and to ground (negative).
9. Vent Caps: Types include individual filler plugs, strip-type, or box-type. They allow
controlled release of hydrogen gas during charging (vehicle operation). Removed, they
permit checking electrolyte and, if necessary, adding water.
10. Electrolyte: A mixture of sulfuric acid (H2SO4) and water (H2O). It reacts chemically with
the active materials in the plates to create an electrical pressure (voltage). And, it
conducts the electrical current produced by that pressure from plate to plate. A fully
charged battery will have about 36% acid and 64% water.
SPECIFIC GRAVITY:-
Specific gravity is a ratio comparing the weight of acid with weight of water. The state of
discharge for a lead acid cell is generally checked by measuring the specific gravity of the
electrolyte.
By weight, the electrolyte in a fully charged battery is about 36% acid and 64% water.
The specific gravity of water is 1.000. The acid is 1.835 times heavier than water, so its
specific gravity is 1.835. The electrolyte mixture of water and acid has a specific gravity
of 1.270 is usually stated as "twelve and seventy."By measuring the specific gravity of the
electrolyte, you can tell if the battery is fully charged, requires charging, or must be
replaced. It can tell you if the battery is charged enough for the capacity, or heavy- load
test. In a fully charged cell the mixture of sulphuric acid and water results in a specific
gravity of 1.280 at room temperature of 70 to 80 0F. when it down to about 1.150, the cell is
completely discharged. The open circuit voltage of a lead-acid cell is approx. equal to :
V= specific gravity +.84
STORAGE CAPACITY RATINGS:
The battery must be capable of cranking the engine and providing adequate reserve
capacity. Its capacity is the amount of electrical energy the battery can deliver when fully
charged. Capacity is determined by the size and number of plates, the number of cells, and
the strength and volume of electrolyte. Capacity of battery is expressed in amp-hours.
Two standard ratings are used to measure a battery's storage capacity.
AMP-HOURS (AH) :-
The battery must maintain active materials on its plates and adequate lasting power under
light-load conditions. This method of rating batteries is also called the 20-hour discharge
rating. Original equipment batteries are rated in amp-hours. The ratings of these batteries
are listed in the parts microfiche. The Amp-Hour Rating specifies, in amp-hours, the
current the battery can provide for 20 hours at 80˚F (26.7˚C) while maintaining a voltage
of at least 1.75 volts per cell (10.5 volts total for a 12- volt battery). For example, a battery
that can deliver 4 amps for 20 hours is rated at 80 amp-hours (4 x 20 =80).
Reserve Minutes :-
Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to
10.5 V. (10.5 volts is the fully discharged level, at which point the battery needs to be
recharged.)
Capacity of battery depends upon following factors :
1. Number and size of plates
2. Rate of discharge
3. Density and quantity of electrolyte
4. Temperature
5. Condition of battery
