What Is Battery Acid?

Battery acid typically refers to the acid used in lead-acid batteries, though it’s essential to the function of any acid-based battery or chemical cell. Storing chemical energy for eventual electrical use is the basic principle behind batteries, including those that use acid. 

How this electrochemical process works is largely due to the distinct structure of and chemistry behind these devices, but battery acid also plays a crucial role. 

Related: Buy battery acid online

What Is Battery Acid Made Of?

Typically referring to the type of acid used in rechargeable lead-acid batteries, like the ones used in cars, battery acid is made of sulphuric acid (H2SO4) that has been diluted with purified water to a concentration of around 30-50%. In this context, battery acid has an acidic pH of 0.8. This is why it needs to be handled with caution. 

Lead-acid batteries are made of two conductive lead plates called electrodes, that are filled with a viscous gel-like substance. In between these plates is the diluted sulphuric acid solution, also known as the electrolyte. The other key part of lead-acid batteries is the separator, which serves as an insulator by preventing the two plates from directly touching one another, something that would result in a short circuit. 

Vector ball-and-stick model of sulphuric acid
Battery acid is sulphuric acid that has been diluted with purified water

What Does Battery Acid Do?

Put simply, battery acid facilitates the conversion of stored chemical energy into electrical energy. The common battery is usually composed of three essential parts:

  • A negative electrode, also known as the anode, which sends electrons to the external circuit. This is usually made from sponge lead 
  • A positive electrode or cathode, which receives electrons from the external circuit. This is usually made from lead dioxide
  • An electrolyte, which allows ionic conduction to occur. This is usually a liquid solution of a solvent and dissolved salts 

The electrochemical reaction facilitated by the battery acid allows a stable current to flow through the pairs of electrodes as electricity enters the anode and exits the cathode. 

When a battery is either being discharged or used as a power source, the sulphuric acid in the electrolyte reorganises its ions so that it’s less concentrated, and more closely resembles H2O. As this happens, a series of sulphates are released and coat the lead plates of the anode and cathode. This decreases the area from which further electrochemical reactions can occur. This surface area, while constantly giving off ions, will then continue to get smaller and smaller as more of its ions are used up. 

The battery will continue to work until it’s unable to supply enough power, a result of its electrolyte being depleted of sulphate ions. In order to work again, the battery will need to be recharged, a process which is basically a reverse reaction of what we’ve just described. 

In a nutshell, charging a battery converts electrical energy into chemical energy, while using (or discharging) a battery uses the stored chemical energy by converting it into electrical energy.

Why Do Batteries Contain Acid?

Batteries contain acid because it’s fundamental to the electrochemical reaction that takes place. Also referred to as battery electrolyte, battery acid is the medium that carries the electrical flow between positive and negative electrodes. 

However, while batteries need an electrolyte to facilitate the reaction, it doesn’t have to be a diluted sulphuric acid solution. Electrolytes in batteries can be solvents mixed with dissolved acids or alkalis, and typical AA/AAA/D batteries even contain solid, rather than liquid electrolytes. 

How Strong Is Battery Acid?

Battery acid usually has a concentration around 15%-35%, although its strength can be as high as 50%. Any concentration of sulfuric acid in lead-acid batteries is highly corrosive. Improperly handling this chemical may lead to permanent blindness, severe burns, internal organ damage if ingested, or even death. 

Battery acid is also highly flammable and may ignite under intense heat or pressure. Along with these features, it’s worth noting the unique chemical structure of battery acid and how it reacts with other materials to create electricity or store chemical energy.

What Is The Formula For Battery Acid?

Battery acid doesn’t have a specific formula, but it’s usually just composed of sulphuric acid (H2SO4) and water (H2O), with an approximate pH level of 0.8 at a 4-5 mol/L concentration.

How a battery works is by electrons flowing from the negatively-charged lead plate to the positively-charged lead dioxide plate. This movement of electrons makes the sulphates released from the battery acid coat the anode, which releases excess electrons to the device and back to the cathode, thereby facilitating an electrical flow. The formula for this reaction is: 

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

Battery pack in battery room in power plant for supply electricity in plant during shutdow
Battery acid is found in lead-acid batteries, which are commonly used to power sump pumps in case of emergency lighting

What Happens When The Battery Is In Use?

What actually happens when a battery is in use is that negatively charged sulphate ions in the battery acid go to the negative electrode, i.e. the anode. Here, they release their negative charge while the rest of the sulfate ions combine with the electrode to form lead sulphate. This process gives way to the release of excess electrons flowing out the negative electrode and back to the positive electrode, i.e. the cathode.

This, however, makes the electrolyte concentration weaker over time as it produces more lead sulphate and more water in the acid solution. This happens when the hydrogen separates from H2SO4 and reacts with lead dioxide in the cathode, which increases the concentration of water as more SO4 ions will be used to coat the electrode while the battery is in use. This will happen until the battery acid becomes more and more diluted, making it less capable of undergoing further electrochemical reactions because of its lack of sulphate ions.

Basically, the continuous movement of the ions is what creates the stable current in batteries. With the oxygen in the lead dioxide plate reacting with hydrogen ions to form water, and with the separated sulphate reacting with the other plate to form more lead sulphate, what is left is a more dilute sulphuric acid solution that can be recharged by reversing process, i.e. by putting electricity into it and converting it to stored chemical energy


These electrochemical reactions in batteries wouldn’t be possible without battery acid. However, as technological engineering advances, different types of batteries are popping up, and a need to review the technical aspects of how common car batteries work is necessary in allowing innovators to create more functions for batteries, with or without acid.

Disclaimer

The blog on chemicals.co.uk and everything published on it is provided as an information resource only. The blog, its authors and affiliates accept no responsibility for any accident, injury or damage caused in part or directly from following the information provided on this website. We do not recommend using any chemical without first consulting the Material Safety Data Sheet which can be obtained from the manufacturer and following the safety advice and precautions on the product label. If you are in any doubt about health and safety issues please consult the Health & Safety Executive (HSE).