Can Battery
Theory be fun?
Professor Overdrive can answer this question.
It can’t. Believe me...I tried.
Well...that is unless you are learning about it from
"Bob".
*Note: "Bob" is a reference to the kinda old
crusty boiler dude who "knows everything". In reality, what he knows is usually
completely wrong and can often be so wrong as to be almost funny.
How does a lead Acid Battery Work?
Bob: "Well, ya got your battery cells see.
They’re made of this special alloy made of lead and acid see. It was a byproduct of
NASA research on the space shuttle. The acid corrodes the lead and forms these little
holes in the battery, that’s where the electrons hide".
"Buzzzzz....." "Oh...no...I’m
sorry Bob. That answer is incorrect. Thank you for playing "I am an idiot!"
Batteries are a means of storing electricity by utilization
of certain chemicals and materials which react together under certain circumstances to
produce energy. Remember back in high school? Ever put "Ben Gay" in
someone’s jock strap before gym? Remember the release of energy created by that
chemical reaction? (If you did do that, you may need therapy....and how have you been
anyway, remember when we did that to Coach Peters!?)
If you the reader are Coach Peters, you are
probably already in therapy and have been looking for us for years....in that
case, my name is Bob.
But it does not matter anyway, because a battery works
nothing like that.
A typical lead acid battery is composed of cells. Each one
of these cells is made of two plates. One is Lead (Pb), the other is lead oxide (PbO2).
The Lead Plate holds a negative electrical charge, the Lead Oxide a positive charge. These
cells are separated by insulating material
When a fully charged battery is placed under load, a
reaction occurs between the Negative Lead Cell and the Positive Lead Oxide cell, through
their interaction with the electrolyte to produce electrons.
The Reaction looks like this.
Pb + PbO2 + 2H+ 2HSO4 => 2PbSO4 + 2H2O + 2e-
But I’m sure Bob knew that. He was just about to say
that.
Yeah right...as soon as he pulled his cranium from out of
his posterior.
But I digress.
A lead-acid cell develops a voltage of around 2 Volts DC.
Basically all lead acid cells produce about 2 VDC regardless of size. H2SO4 in a lead acid
battery is the catalyst behind the Electro- chemical reaction. Unlike the nickel-cadmium
cells where electrolyte merely transfers electrons and does not change chemically as the
cell discharges in a lead-acid battery the H2SO4 content of the electrolyte changes as the
cell is discharged or charged. In a fully charged battery, approx. 25% of the electrolyte
is made up of H2SO4, the remainder being H2O. Compare this to a fully discharged battery
and we find that H2SO4 makes up less than 5%.
This fun fact allows us to determine how much energy a
lead-acid cell contains by measuring the amount of acid remaining in its electrolyte. How
convenient!
A battery is actually made up of numbers of these cells. By
linking several groups of cells in series(6 for a 12VDC battery), we can increase the
voltage from 2 VDC to ...well..whatever.
If we wanted to increase the capacity, we would like them
parallel. This would give us greater amperage, but less voltage. The configuration is
dependent on the application.
Battery capacity measures the amount of energy a battery
contains Ampere-hours (A-h) at a given voltage. Watt- hours (W-h) can also be used.
As we mentioned before, though a lead acid batteries size
does not affect the amount of voltage per cell (2 VDC) the capacity of a cell, and
therefore a battery, is limited only by its size. The larger the cell, the more reactive
materials contained within it, and the larger the electrical capacity of the cell in
Ampere-hours. A battery rated at 100 Ampere-hours will deliver 100 Amperes of current for
1 hour.
If you’ve gotten this far, it means you are still
awake. I’m impressed!
This could mean one of several things.
a) Your doing a paper on Lead Acid Batteries.
b) You did not really need to see that episode of
Gilligan's Island again.
c) This article is far more entertaining than it appears.
I’m going to assume b).
In any case, the journey must continue.
As a battery is being charged, the cell voltage is at a
certain value (say 11.5 Volts) below the charger (say at 14 Volts). As the battery becomes
charged, the voltage difference decreases. This causes a corresponding decrease in current
flow. Current charge rate is an excellent way of determining where a battery is in the
charge.
The most important issue to take into consideration when
either charging or discharging a lead acid battery is the generation of Hydrogen Gas.
Hydrogen is the lightest element know. It takes the form of an odorless, colorless gas.
Why are we concerned with hydrogen gas?
The Hindenburg used hydrogen gas.....remember what happened
to her?
Hydrogen (H2) is extremely flammable and explosive. In high
enough concentrations (about 8%) it becomes very dangerous indeed. During charging or
discharge, the decomposition of water allows free hydrogen to escape the battery. It is
for this reason that you should NEVER ALLOW OPEN FLAME near a charging or discharging
battery, especially of the battery is large and/or in an enclosed space. That is unless
you want to get rid of someone...like Bob. Let him go in there with a blow torch.
Bob: "Yeah, I know that Hydrogen is dangerous,
that’s why these new batteries use helium".
Wrong again Bob.
Remember - Safety first! Always ensure that adequate
ventilation is provided at all times.
So...did we have fun?
If you said yes, you defiantly need therapy.
