﻿How a lead acid battery works EngineerGuy Series #4
The increasing amount of technology we rely on in
everyday life makes portable energy sources essential
to our world today.
We can power our cellphones and laptops with
lithium ion batteries, run flashlights from alkaline ones,
and many of our watches depend on batteries constructed with silver oxide.
This one, though, may be the most important of them all.
Now I know it seems a clunky, old-fashioned thing
it was invented in the 19th century
but it still makes our world move by starting our
cars, motorcycles, and trucks.
No wonder it’s the best selling battery.
If you understand how this battery operates
you can grasp the principles underlying any of the newer ones
and even see why no single type of battery can be used in all applications.
Let’s take a look inside.
The first thing to note about this motorcycle battery is that it's heavy
That’s because it’s tightly packed with lead and lead oxide sheets
both of which are very dense.
This is a cell from an identical battery
and you can see the lead and underneath this the lead oxide.
They alternate throughout the interior.
So, let me show you how they store electricity using just two plates.
So, I have here a lead plate and a lead oxide plate from the battery
and the sulfuric acid.
And now watch what happens when I connect the leads
The LED lights up.
A current flows from the lead oxide cathode to the lead anode.
The lead gives up electrons, which the lead oxide accepts.
This exchange turns both plates into solid lead sulfate.
Now let me measure the voltage difference between the two leads.
And you can see that it’s about two volts
which means that to make this 12-volt battery
six cells are linked in series to make up the 12 volts.
So, that’s the basic electrochemical reaction
now let’s look at how you engineer a battery.
We want a battery to have a high density of either energy or power.
That difference being that batteries with a high energy density
can store large amounts of energy,
and release it reliably over long periods of time,
whereas batteries with a high power density release
large amounts of energy quickly.
This battery was designed with power in mind
because we need a burst of nearly four hundred amps to start a motorcycle.
To save space we need to pack the plates in here tightly
but in order to ensure that the electron transfer
takes place through the terminals
the plates are mechanically separated by these permeable layers.
Now, let me show you two plates from a discharged battery
so you can see the effect of a deep discharge of a battery.
If we look at these two plates we can see
the effect of a long term discharge.
This light coating here is lead sulfate.
That explains why if you run a car battery to zero charge
a few times you’re likely to kill it.
As the battery discharges, lead sulfate cakes the space between the plates.
If too much builds up, you’ll never be able to recharge the battery.
This highlights how every different application
needs a different type of battery.
You see with most engineered objects
there are going to be trade-offs
giving away some characteristic you want
to gain others that you must have.
Indeed, the toughest part of engineering often ends up
being balancing the trade-offs in a design.
For example, a car battery does a great job starting a vehicle
but not running one, like an electric car.
Nor is it a good candidate to store energy
from solar panels in a house.
There we harvest energy from the sun
charge up the batteries
and then use the stored energy
in a variety of ways until the batteries die and
then recharge them with the sun’s energy.
For this use you would use what is called a deep cycle battery.
As the name implies this battery's capacity can be largely used
and then recharged easily.
To convert an SLI battery
like used to start a motorcycle or a car
into a deep cycle battery we do three things.
We use thicker electrodes to increase energy density
space them further apart so lead sulfate debris can fall off of them
and add room below where that debris can accumulate.
The trade off is that it’s larger, heavier
and gives off a lower current than a car battery.
It would seem that we’d want to get rid of the lead-acid battery
It poses a significant environmental risk if disposed of improperly.
We still have the 19th century lead-acid battery
because of what nature has handed us
plentiful electrode materials, lead and lead oxide
both very high in conductivity.
Once we arrange these materials correctly we have a
cheap battery with a high power density.
No other materials found in nature meet these criteria so well
which means that with current technology it is extremely difficult
to overcome this barrier.
That’s why 19th century technology is still
the go to for starting our cars.
I’m Bill Hammack, the Engineer guy.
This video is based on a chapter in the book
Eight Amazing Engineering Stories.
The chapter features more information about this subject.
Learn more about the book at the address below.