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LABC1 • 4
Quick Battery Theory
To begin, we should cover a few facts about lead acid batteries in general.
Most traditional historians date the invention of batteries to the early 1800’s
when experiments by Alessandro Volta generated electrical current from
chemical reactions between dissimilar metals. Volta’s original ‘voltaic pile’
consisted of zinc and silver disks separated by a porous nonconductive
material saturated with seawater. When stacked in a particular manner, a
voltage could be measured across each silver and zinc disk.
Other more radical thinkers, however, believe that lead acid battery
technology has been around since the early days of the Egyptian Pharaohs!
Whether they discovered the electro-chemical process on their own or if the
‘Space Aliens’ using their pyramids as an intergalactic spaceport taught them
still requires a bit more clarification. We’ll leave that one for you to follow up
on!
While advances in construction and materials have come a long way over the
years, the basic principles still apply. Lead acid cells of all types (‘Wet’ or
‘VRLA’ ) undergo a specific set of chemical reactions while charging and
discharging. They are also formed from similar types of active materials. For
the most part, lead acid batteries are made up of lead plates submerged in a
sulfuric acid solution. The positive electrode plates are formed from lead
dioxide (PbO
2
) while the negative electrodes are made of sponge metallic lead
(Pb). The porous nature of the lead plates allows the electrolyte, a dilute
mixture of 35% sulfuric acid and 65% water, to efficiently contact the
maximum surface area and obtain the most charge carriers. The electrolyte
solution provides the sulfate ions formed during the discharge chemical
reaction process giving us the electrons needed for current flow into the load.
One of the byproducts created during the discharge process of freeing sulfate
ions is lead sulfate (PbSO
4
). As the battery discharges, the lead sulfate
attaches to the electrode plates raising the internal resistance of the battery
which in turn lowers its working terminal voltage.
To determine the SOC (State Of Charge) of a lead acid battery, the classic
voltmeter approach does not work well. The terminal voltage will vary widely
between batteries as a function of things like ambient temperature and the
relative age of the battery. A full set of temperature profile tables would show
big differences in the open circuit terminal voltage over a wide temperature
range. This is why a good charger must incorporate a temperature
compensation network to avoid ‘over’ or ‘under’ charging the battery at
different ambient temperatures. To test a lead acid battery’s SOC, the best
indicator is a hydrometer. When you test a battery’s SOC with a hydrometer,
you are actually measuring the amount of sulfuric acid left in the electrolyte