Lesson 2/Learning Event 1
Now that you are familiar with the construction of the automotive battery, let's consider the
chemical reaction that takes place in the battery.
A battery cell is said to be fully charged when the electrolyte is full strength. The active material of
the negative plates consists of spongy lead, and the active material of the positive plates consists of
lead peroxide.
When a load is connected to the battery, electrons flow from the negative plates to the positive
plates. The battery is said to be "discharging." Chemical changes take place inside the battery
during discharging:
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As the current flows, atoms of the sulfuric acid leave the electrolyte and enter the battery
plates. This decreases the amount of acid that is in the electrolyte, making the mixture
weaker.
-
As you recall, atoms from all elements are different, and the nature of all materials is
determined by the atoms they contain. Therefore, the atoms of sulfuric acid mixing with
the battery plates will change the nature of the plates. The spongy lead of the negative
plates turns into lead sulfate; the lead peroxide of the positive plates also turns into lead
sulfate.
If current flow continues, the electrolyte will get weaker and weaker until it is almost all water. At
the same time, the negative and positive plates will be gradually turning into lead sulfate so that
they are becoming alike (both lead sulfate). Finally, the battery reaches a point where it will no
longer supply enough voltage to cause the current flow, and the battery is then said to be
discharged.
This chemical reaction in the automotive battery can be reversed and the discharged battery
returned to a charged state. This is done by supplying DC from an external power source, such as
a DC generator, and running it through the battery opposite the direction it travels when the battery
is discharging. This causes the atoms from the acid to return to the electrolyte making the
electrolyte stronger. As the acid atoms leave the battery plates, the plates change back to their
original nature--spongy lead and lead peroxide.
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