A lead acid battery is a rechargeable battery that uses lead and sulphuric acid to function. The lead is submerged into the sulphuric acid to allow a controlled chemical reaction.

This chemical reaction is what causes the battery to produce electricity. Then, this reaction is reversed to recharge the battery.

The technology of lead acid battery

Lead-acid batteries usually consist of an acid-resistant outer skin and two lead plates that are used as electrodes. A sulfuric acid serves as electrolyte.

Lead acid battery structure

The first lead-acid battery was developed as early as 1854 by the German physician and physicist Wilhelm Josef Sinsteden. He used two lead plates arranged side by side in a vessel containing diluted sulfuric acid and placed it under voltage. After a few charging and discharging processes, he determined a measurable capacity. However, accumulators were hardly used at that time.

It was not until twenty years later – in 1866 – that the technology achieved its breakthrough. Werner von Siemens developed the electric generator, and from then on the demand for ways to store electrical energy increased. From that point on, it was impossible to imagine industry without the lead battery. Even more than 150 years later, the lead battery is still one of the most important and widely used battery technologies.

Different battery types

The lead acid battery types are mainly categorized into five types and they are explained in detail in the below section.

Flooded Type – This is the conventional engine ignition type and has a traction kind of battery. The electrolyte has free movement in the cell section. People who are using this type can have accessibility for each cell and they can add water to the cells when the battery gets dried up.

Sealed Type – this kind of lead-acid battery is just a minor change to the flooded type of battery. Even though people hold no access to each cell in the battery, the internal design is almost similar to that flooded type one. The main variation in this type is that there exist enough amount of acid which withstands for the happening of smooth flow of chemical reactions throughout the battery life.

VRLA Type – These are called Valve Regulated Lead Acid batteries which are also termed as a sealed type of battery. The value controlling procedure permits for the safe evolution of O2 and H2 gases at the time of charging.

AGM Type – This is the Absorbed Glass Matte type of battery that permits the electrolyte to get stopped near to the plate’s material. This kind of battery augments the performance of the discharge and charging processes. These are especially utilized in the power sports and engine initiation applications.

Gel Type – This is the wet kind of lead-acid battery where the electrolyte in this cell is with silica-related which makes stiffening of the material. The recharge voltage values of the cell ate minimal when compared with other types and it has more sensitivity too.

How does a lead acid battery work?

AGM lead acid battery in series or in parallel connection

To put it simply, the battery’s electrical charge is generated when the sulphate in the sulphuric acid becomes bonded to the lead. The electrical charge is replenished by reversing this reaction. That is, the sulphate goes back into the sulphuric acid and, thus, the battery is recharged.

Now, obviously, there’s a finite amount of sulphate ions in the acid. And the available surface area of the lead it bonds to is limited, too. So, as the sulphate is depleted, the charge becomes weaker.

For this reason, lead-acid batteries are not ideal for powering devices for a long period of time. Instead, they’re best for applications that need a short, powerful burst of energy.

Chemical Reaction for Discharging

When you use a 12V battery with a DC stabilizer, the output is always stable at 12V. The DC stabilizer is designed for devices that do not accept an input voltage that is too high or too low. Suppose you have a device (for example a depth sounder) with an operating voltage of 10.5V-12.9V then you need to use the DC stabilizer with a lithium battery. With a “normal” lithium battery because the maximum voltage of 13.6V is above the maximum voltage of 12.9V of the depth sounder. With a battery from the AV line you need the DC stabilizer because the minimum input voltage of 10V is above the minimum voltage of 9V of the AV battery. In this case the device would switch off while there is still 25% of capacity in the battery remaining.

Lead acid battery discharging

Negative plate reaction

Pb(s) + HSO−4(aq) → PbSO4(s) + H+(aq) + 2e−

The release of two conducting electrons gives the lead electrode a negative charge.

Positive plate reaction

PbO2(s) + HSO−4(aq) + 3H+(aq) + 2e− → PbSO4(s) + 2H2O(l)

taking advantage of the metallic conductivity of PbO2.

The total reaction as

Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l) 

The net energy released per mol (207 g) of Pb(s) converted to PbSO4(s), is ca. 400 kJ, corresponding to the formation of 36 g of water.

Lead Acid Battery Charging

The sulphuric acid existing in the lead discharge battery decomposes and needs to be replaced. Sometimes, the plates change their structure by themselves. Eventually, the battery becomes less efficient and should be charged or changed.

Lead Acid battery Charging

Reaction at the Positive electrode
PbSO+ 2H2O            ———->          PbO+ HSO4  + 3H+ + 2e–    

Reaction at the Negative electrode
PbSO+ H+ + 2e–            ———->          Pb + HSO4

Overall Reaction
2PbSO+ 2H2O          ———->          Pb + PbO+ 2H2SO4     

There are several methods to charge the lead-acid batteries. But we should use the best method to reduce the chance of gassing, to obtain maximum battery service life and capacity. The list of charging methods Given below.

  • Constant voltage:- As a name, this method will provide constant voltage till the current taking by the battery go to zero. It takes a long time.
  • Constant current:- As a name, this method will provide constant current till the voltage reach its defined gassing voltage. It also takes a long time.
  • Multi-step constant current:- In this method the charging current is constant at voltage reaches gassing voltage then-current start to reduced in steps to maintain the voltage below gassing voltage. This charger is complicated to build.
  • Modified constant voltage-current:- In this method battery charged in three stages. The first stage is the constant current stage, In this current applied to the battery till voltage reached its defined gassing voltage. In the second stage, the voltage is constant till the current decreases the value of about 0.1C20 (also known as C20/10 ). The voltage is will reduce to floating voltage ( generally 2.25v to 2.27v) to maintain the battery charged.

Effect of charge level on freezing point

Because of freezing-point depression, the electrolyte is more likely to freeze in a cold environment when the battery has a low charge and correspondingly low sulfuric acid concentration.

Ion motion

During discharge, H+produced at the negative plates moves into the electrolyte solution and is then consumed at the positive plates, while HSO−4 is consumed at both plates. The reverse occurs during charge. Therefore, a liquid-medium cell tends to rapidly discharge and rapidly charge more efficiently than an otherwise similar gel cell.

Measuring the charge level

Because the electrolyte takes part in the charge-discharge reaction, this battery has one major advantage over other chemistries: It is relatively simple to determine the state of charge by merely measuring the specific gravity of the electrolyte; the specific gravity falls as the battery discharges. Some battery designs include a simple hydrometer using colored floating balls of differing density.

Chemical Reaction for Recharging

The chemical reaction that takes place when the lead-acid battery is recharging can be found below.


2e– + PbSO4(s) + H3O+(aq) –> Pb(s) + HSO4 + H2O(l) (reduction)


PbSO4(s) + 5H2O(l) –> PbO2(s) + HSO4(aq) + 3H3O+(aq) + 2e (oxidation)

While recharging, the automobile battery functions like an electrolytic cell. The energy required to drive the recharging comes from an external source, such as an engine of a car. It is also important to note that overcharging of the battery could result in the formation of by-products such as hydrogen gas and oxygen gas. These gases tend to escape from the battery, resulting in the loss of reactants.

Pros and Cons of Lead Acid Batteries

Lead-acid batteries have powerful voltage for their size. Thus, they can power heavy-duty tools and equipment.

They can even power electric vehicles, like golf carts. However, in this case, you’d need to be careful to charge the battery often enough (and without overcharging it). If you don’t, the vehicle will die before reaching its destination, which will also damage the battery.

Additionally, lead-acid batteries are great for starting motor vehicles. They provide an intense jolt of energy to start the vehicle and then they recharge as the vehicle drives.

On the other hand, they are not good for devices you wish to use for long periods of time, like cell-phones. Also, they self-discharge when not in use, which will eventually kill the battery.

In other words, you can’t just leave them sitting around. Thus, they are a bad option for any application that will not be used frequently.

Voltages for common usage

The battery charging is a three-stage charging procedure for lead-acid batteries. A lead-acid battery’s nominal voltage is 2 V for each cell.

Float voltage varies depending on battery type, and ranges from 1.8 V to 2.27 V. Equalization voltage, and charging voltage for sulfated cells, can range from 2.67 V to almost 3 V.

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