What is Lead-Acid Battery?

Invented by French physicist Gaston Plante in 1859, the lead-acid battery is best known as the de facto rechargeable energy storage solution of choice for most cars, trucks, and other vehicles. It is also widely used in boats, submarines, uninterruptible power supplies (UPS), and pretty much any mid-range application you can think of that requires a low cost, rechargeable battery. Let’s take a closer look at the lead-acid battery — the world’s first commercially successful rechargeable battery.

How Does a Lead-Acid Battery Work?

A battery is an electrochemical energy storage device, that uses chemistry to store potential energy (voltage) in the form of electrons. When a resistive load is applied across the positive and negative terminals of a battery, the circuit is completed, and you can extract energy from the battery to perform work (like starting the engine in your car). In lead-acid batteries, this is most commonly accomplished with the following redox reaction under sulfuric acid (H2SO4) solution:

Pb + PbO2 +4H+ + 2SO42- → 2PbSO4 + 2H2O

Which can be broken up into the following half reactions:

Oxidation at the Anode:

Pb → Pb2+
Pb + SO42- → PbSO4 + 2e-

Reduction at the Cathode:

PbO2 → Pb2+
PbO2 + SO42- + 4H+ + 2e- → PbSO4 + 2H2O

Much of electrochemical energy storage is about separating the two half reactions of a redox reaction — in the case of lead acid, the concentration of negative charge at the anode from the positive ions at the cathode. To fully understand how a lead-acid battery works, it’s necessary to dive into the inner workings of a lead-acid cell.

What’s in a Lead-Acid Battery

Your typical 12-volt lead-acid car battery consists of six lead-acid galvanic cells connected in series and housed within a battery case. Remember the two half reactions we discussed in the previous section? Each cell contains two types of electrodes one for each half of the lead-acid redox reaction — a negative lead (Pb) anode, and a positive lead dioxide (PbO2) cathode.

There can be more than one pair of electrodes per cell depending on the cell design. The electrons on the anode are attracted to the positive cathode, but are separated by a micro-porous separator. When diluted sulfuric acid electrolyte is added to the cells, the battery is activated, and ions can line up along the positive and negative regions of the cell. Forming a conductive path between the cathode and the anode allows the electrons to travel to the cathode and discharge the battery. The reaction is reversible, allowing lead-acid batteries to be recharged with an external power source returning the anodes and cathodes to their original state.   

Electrodes
A typical battery electrode consists of active material used in the redox reaction and a solid conductive metal grid to serve as a current collector and provide mechanical support. Since pure lead is soft, additives like calcium or antimony are used to create alloys that enhance mechanical strength and electrical properties of a cell. The grid and active material together form an electrode, which is also called a plate. In the lead-acid design, the positive plate is a lead dioxide cathode, and the negative plate is a lead anode.

Lead Anode
The lead anode is also known as the negative electrode in a lead-acid cell. Its active material is sponge lead, which increases the available surface area for reacting with the sulfuric acid electrolyte.

Lead Dioxide Cathode
The cathode is also known as the positive electrode in a lead-acid cell. The active material on the cathode is lead dioxide which is electroformed from lead oxide powder that must be pasted onto the grid.

Sulfuric Acid Electrolyte
As we mentioned earlier, lead-acid battery electrolyte is a diluted solution of sulfuric acid (H2SO4). Concentrations vary by design, but are generally less than or equal to 40% by weight H2SO4. In solution, the acid exists as negatively charged sulfate ions (SO42-) and positively charged hydrogen ions (H+), which you’ll recognize as key ingredients in the redox reactions we detailed earlier in this article. In some designs, silica dust or other gelling agents are added to the electrolyte to turn it into a thick gel. The advantage of the gel cell design is that it can be mounted in any orientation and does not require maintenance of more traditional designs where water must be added through the top of the battery.  

Separator
The separator’s primary function is to separate the positive and negative electrodes through a porous membrane that prevents dendrites and shedded active material from causing a short circuit. In lead-acid designs, there are two main types — microporous membranes and absorbed glass mats (AGM). The microporous membrane is typically made from polyethylene plastic, in an activated cell the membrane is present in free-flowing electrolyte. The AGM consists of a glass fiber mat that is soaked in electrolyte. The advantage of using an AGM soaked in electrolyte over the conventional microporous membrane submerged in solution, is that the AGM provides the added stability of avoiding spills and stratification. Acid tends to sink in solution, concentrating charge and wearing out the electrodes along the bottom of the cell.

The Future of Lead Acid Batteries

Lead acid batteries have been around for so long that it’s easy for them to be outshined by newer flashier battery technologies like lithium ion. However, there’s good reason lead-acid batteries have lasted since the 19th century — they’re cheap, safe, durable, and dependable. Simple and inexpensive to manufacture, with a global supply chain that’s unlikely to go anywhere any time soon, it’s likely that lead acid batteries will continue to stay relevant as a dependable low-cost power source for applications where space isn’t a premium, and you just want more kilowatt hours for your buck.