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.
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.
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.
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.
Meet the Lead Acid Battery Family
Lead-acid batteries have been around for over 150 years, giving scientists and engineers plenty of time to refine the technology. Let’s look at some of the batteries you’ll encounter the field.
Lead Antimony Batteries
Lead antimony alloy electrodes are mechanically stronger and cheaper than pure lead. The presence of antimony also improves deep cycling — the art of fully discharging a battery’s capacity before recharging. However, since lead/antimony alloys are prone to sulfation, the battery should not be left at a deeply discharged state for a long period of time. Lead antimony also increases hydrogen evolution (gassing) during charge, which can lead to water loss in the cell and the accumulation of hydrogen in confined spaces, like on a submarine.
Lead Calcium Batteries
Like antimony adding calcium also increases mechanical strength of the electrode, but it also has the added advantage of reducing gassing and the self-discharge rate. Calcium by itself will not overcome the limitation of the lead-acid design’s ability to undergo a deep discharge. Therefore, a combination of calcium and antimony may be used to gain the benefits of antimony while mitigating gassing.
Flooded Lead Acid Batteries
The conventional type of lead acid battery, flooded cells use a liquid sulfuric acid electrolyte. They provide the lowest cost per amp hour of any lead-acid battery type. These cells are also the most high maintenance, requiring watering, charge equalizing, and terminal cleaning.
Valve Regulated Lead Acid (VRLA)
VRLA’s or sealed lead-acid (SLA) batteries as they are sometimes called, are sealed cells with a regulating valve. When a cell is charged, lead sulfate and water are changed back into lead and sulfuric acid. Additionally, parasitic reactions like electrolysis may occur (especially during high current charge) splitting water in the electrolyte into hydrogen and oxygen which can escape the cell as a gas. VRLA’s prevent that gas from escaping greatly reducing the need to add additional electrolyte or water to the cells over the lifetime of the battery. This is why VRLA’s are often called maintenance free, and why today’s cars no longer require you to maintain the water level of their batteries.
Deep Cycle Lead Acid Batteries
Designed for deep cycling applications, these cells are designed to be discharged to 50% of their capacity or more before recharging. This is in stark contrast to the starter battery in your car which is designed to deliver a short, high current burst. The plates are thicker allowing the batteries to deliver a steady current over a long period of time. Lead antimony batteries are a type of deep cycle lead acid battery. Common applications include electric golf carts, floor scrubbers, fork lifts, and older electric vehicles.
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.