Battery
The battery is the primary "source" of electrical energy on vehicles. 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 Functions:
1. ENGINE OFF: Battery energy is used to operate the lighting and accessory
systems.
2. ENGINE STARTING: Battery energy is used to operate the starter motor and
to provide current for the ignition system during cranking.
3. ENGINE RUNNING: Battery energy may be needed when the vehicle's
electrical load requirements exceed the supply from the charging system. In
addition, the battery also serves as a voltage stabilizer, or large filter,
by absorbing abnormal, transient voltages in the vehicle's electrical system.
Without this protection, certain electrical or electronic components could
be damaged by these high voltages.
Battery Types
1. PRIMARY CELL: The chemical reaction totally destroys one of the metals
after a period of time. Small batteries for flashlights and radios are
primary cells.
2. SECONDARY CELLS: 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.
3. WET-CHARGED: The lead-acid battery is filled with electrolyte and charged
when it is built. During storage, a slow chemical reaction will cause
self-discharge. Periodic charging is required.
4. DRY-CHARGED: The battery is built, charged, washed and dried, sealed, and
shipped without electrolyte. It can be stored for 12 to .18 months. When put
into use, it requires adding electrolyte and charging.
5. LOW-MAINTENANCE: Such batteries are built to reduce internal heat and
water loss. The addition of water should only be required every 15,000 miles
or so.
The battery is one of the most important components on a vehicle today. It
provides the amps needed to crank and start the engine, and it stores the
voltage that runs everything from the ignition system and fuel injectors to
the vehicles lights and all of its electrical accessories. Lead-acid
batteries have been around since the earliest days of the automobile and
have steadily improved over the years. Todays batteries are more durable,
have high power-to-weight ratios and lighter cases. But the basic chemistry
is unchanged. A chemical reaction between two dissimilar metals in an acid
solution creates voltage. The two dissimilar metals are lead and lead
peroxide. The active material on the positive plates are lead peroxide (a
soft, dark brown material), while that on the negative plates is finely
ground sponge lead (which is gray in color). The positive and negative
plates are sandwiched together and separated by a nonconductive insulating
layer of paper, plastic or mirco-woven glass.
The acid is a mixture of 25 percent water and 75 percent sulfuric acid
(H2SO4). Battery acid is called the electrolyte because it allows charged
particles (called ions) to move between the plates when current is drawn
from the battery. A 12-volt battery has six cells, each of which contains
9 to 20 positive and negative plates. The greater the number of plates, the
higher the power output (measured in amps) of the battery. Each cell
produces 2.11 volts, so when all six cells are connected together in series,
the batterys total output is actually 12.66 volts. As a battery discharges,
sulfate combines chemically with the positive and negative plates. This
lowers the concentration of acid in the solution, which can be measured by
checking the specific gravity (density) of the liquid with a hydrometer or
built-in charge indicator. As more and more current is pulled out of the
battery, sulfate continues to build up on the plates, and the concentration
of the acid drops until the battery becomes discharged and no longer
produces enough voltage to crank the engine or power the lights or other
accessories.
To reverse the chemical reaction and recharge the battery, the charging
system senses any drop in voltage and increases its voltage output to push
amps back into the battery. This forces the sulfate away from the plates and
puts it back into the solution. The concentration of acid goes back up, and
the battery returns to full charge.
To produce maximum cranking power and remain healthy, an automotive
lead-acid battery must be kept at or near full charge. If the battery is
allowed to run down (leaving the lights on, low charging output, slipping
drive belt, frequent short-trip driving, long periods of inactivity, etc.),
sulfate can form a barrier on the surface of the plates that makes it
difficult for the battery to accept a charge. Over time, this can ruin the
plates and cause the battery to fail.