How a Lead Acid Cell Works

Basic Lead Acid Cell Construction

While battery manufacturers will employ different construction techniques and technologies, all lead acid batteries have the same three fundamental components.

These three fundamental components rely on the physical laws associated with their chemistry to provide a behavior that is relatively consistent from manufacturer to manufacturer.

The three fundamental components are:

Anode – This is the also known as the negative electrode or terminal. It is made of lead.

Cathode – This is known as the positive electrode or terminal. It is made of lead dioxide.

Electrolyte – This is a mixture of sulfuric acid and water. If you have the type of lead acid battery known as a flooded cell, the electrolyte is the liquid that is sloshing around on the inside.

Definition and Discussion of Open Circuit Output Voltage

There are two ways to measure the output voltage on a battery. The first is to take the measurement with the battery disconnected.  The second is to take the measurement with the battery powering various devices.

Taking a voltage measurement with the battery disconnected from devices will provide what is known as an open circuit output voltage. Sometimes this will be referred to as a “no load’ condition. That is to say, there are no devices placing a demand on the battery.

Taking the same voltage measurement with the battery providing power to devices is taking the measurement of the battery under load.  This can be a more difficult measurement to interpret as the voltage of a battery under load will vary depending on the amount of current being provided.

In some cases, it is far simpler to analyze the state of a battery by taking an open circuit voltage measurement.

Lead Acid Cell versus Lead Acid Battery

A single cell is a single combination of a lead, lead dioxide and and electrolyte composed of sulfuric acid and water.  It’s output and behavior are governed by this chemistry.  A single cell will provide an open circuit output voltage of about 2.13 volts when fully charged and will be considered discharged when its open circuit output voltage drops to 1.95 volts.

A battery may be a single lead acid cell or it may be several connected lead acid cells.

For example, a 12 volt car battery is made up of six lead acid cells that are connected in series. The total open circuit output voltage from these cells when full charged is approximately 12.78 volts.   It is generally considered to be fully discharged when its open circuit output voltage drops to 11.7 volts.

Two Cycles of a Lead Acid Battery

The two cycles of a lead acid battery are referred to as a discharge cycle and a charge cycle.

During the discharge cycle, the battery is connected to a device and it is supplying power to that device. It accomplishes this by converting chemical energy into electrical energy. While in this cycle, the battery is considered to be acting as a Galvanic Cell.  When the useable electrical energy within the battery has been completed, the battery is considered to be fully discharged.

During the charge cycle, a power source (like a charger or alternator) is connected to battery and supplies the battery with an electrical current.  During this process, electrical energy is being converted into stored chemical energy.  While being charged, the battery is often considered to be acting as an electrolytic cell.   When the battery has reached its chemical energy storage capacity, it is considered to be fully charged.

Functional Description of Lead Acid Battery Discharge Cycle

Electrical power depends on a difference of electrons between two points. The fully charged battery has more electrons at the negative terminal and fewer at the positive terminal.

When a fully charged battery is connected to a device (or load), the battery begins its discharged cycle.  During this cycle, electrons are leaving the negative terminal, powering the device and re-entering the battery at the positive terminal.  With the battery, the electrolyte is working to keep maintain the difference of electrons between the negative terminal and positive terminal.

During this process, the electrolyte and the electrodes begin to change chemically begin exhausting their ability to supply electrical power.  When that ability to supply electrical power has been depleted, the battery is said to be discharged.

When the battery is completely discharged, the electrolyte will be  water and the electrodes will have been transformed into lead sulfate.

What’s important to know here is that a dead battery will still have voltage when measured by a meter. If devices are connected to the battery when the measurement is taken, the battery will read about 10.5 volts. If devices are not connected, the voltage will be about 11.7 Volts.

Functional Description of Lead Acid Battery Charge Cycle

The second cycle is known as the charge cycle. Its essentially the reverse of the discharge cycle.

During this cycle a a charger is connected to the battery. The charger output voltage is set to a voltage that is higher than that of the battery. Current is essentially forced through the battery in the opposite direction.   Like the discharge cycle,  a chemical change is occurring within the battery.

When the battery is completely charged,  the negative electrode will be lead, the positive electrode will be lead dioxide and the electrolyte will return to a mixture of sulfuric acid and water.