Automotive lead acid batteries used to start car engines, are one
of the oldest design of rechargeable battery in existence. The lead
acid battery was primarily used for the storage of DC power and used in
early commercial vehicles due to the weight and size. These batteries
have literally dominated the marketplace for many years ever since their
invention back in the early 1800's and have continued to be an
important automotive component to the point were there is one in every
vehicle on the roads.
Today, automotive batteries used to start
gasoline and diesel engines, are smaller, cheaper and more cost
effective than their earlier cousins, supplying greater starting and
surge currents than ever before at a fraction of their original price.
Not
only does the lead acid battery provide the correct amount of
electricity needed to start engines, but they also supply electricity
for the ignition system, lights, indication, stereos and other such
electronic features found on today's modern vehicles.
The energy
produced by a lead acid battery occurs via an internal chemical process
which involves lead (hence their name), lead oxide and a liquid acid
solution called the electrolyte. Solid lead and lead oxide plates are
submerged within an electrolytic solution that consists of a very small
percentage of sulphuric acid (H2SO4) mixed with distilled water. When
electrical power is drawn from the battery, a chemical reaction takes
place between the plates and liquid electrolyte releasing electrons.
These free electrons in the form of an electrical current flow through
electrical conductors mounted on the battery and out via lead terminals
providing the electricity required to start the car.
As the lead
acid battery becomes discharged, the sulphuric acid forms deposits onto
the lead plates, but when the lead acid battery is recharged again, the
sulphuric acid breaks down and returns back into their separate lead and
lead oxide components. Heat is generated by this constant charging and
discharging of the battery evaporating the water inside.
This
evaporation meant that older lead acid batteries needed to be
"topped-up" with more distilled water on a monthly basis but the modern
maintenance free lead acid battery is fully sealed against leakage in
which the electrolyte in the form of a gel is contained in separate
compartments are now used. The result is that these fully sealed
batteries can be rotated upside down or positioned sideways in the event
of an accident without any risk of an acid leakage. Also these modern
batteries have safety valves fitted which allow the venting of fumes
during the charging, the discharging and changes in atmospheric pressure
when driving at altitude.
The lead acid battery is the only
batteries suitable to be used in alternative energy systems but the
continuous cycle of discharging a battery into a load (night time hours)
and then recharging the battery (sunlight hours) many times over
requires a different type of battery as not all lead acid batteries are
the same. The most important requirement of a battery is whether it is a
Deep Cycle Battery or a Shallow Cycle Battery.
Consider
automotive starting batteries. These batteries are cheap to buy but are
designed to provide high amperes of current for very short periods of
time (less than 10 seconds) to operate the starter motor and turn over
the engine. After the car has started, the battery is then trickle
charged by the cars alternator. Even on cold frosty mornings the cars
battery is only discharged to less than 10% of its rated capacity at
startup so automotive batteries are designed for this very shallow cycle
service, (100% to 90% state of charge).
As a car battery is
designed to deliver high currents for very short periods of time it is
therefore made of many thin lead plates giving a large surface area for
the chemical reaction to occur. These thin lead plates do not have the
necessary mechanical strength for repeated cycling over a period of many
years and wear out very quickly after only 200 to 400 cycles. Therefore
shallow cycle car batteries which although they work, are not designed
for a long term solar power or wind power system which requires a much
deeper cycling service.