Lead-Acid Car Batteries: How They Work, Types & Why They Fail
Conventional vehicles typically rely on Lead Acid Car Battery due to their high power output and affordability. These batteries use water-based electrolytes and have individual cell voltages that are relatively low. While they offer proven safety, lead-acid batteries have a lower specific energy compared to lithium-ion types. In contrast, hybrid electric vehicles often use nickel-metal hydride (NiMH) batteries because of their long lifespan and ability to undergo many charge/discharge cycles. For a broader look at how all battery types work, see our guide to electric batteries explained.
What is a lead acid car battery?
A lead acid battery is rechargeable and operates using lead and sulfuric acid. The lead is immersed in the sulfuric acid, facilitating a controlled chemical reaction that generates electricity.
Lead acid Battery History
In 1801, French scientist Nicolas Gautherot observed that wires used in electrolysis experiments could produce a small “secondary” current after the main battery was disconnected. In 1859, Gaston Planté invented the first rechargeable lead-acid battery by passing a reverse current through it. The 1970s saw the development of valve-regulated lead-acid (VRLA) batteries.
Major comparative features
Lead-acid car batteries are known for their high discharge rate, making them ideal starter batteries for automobiles. They are typically aqueous or unsealed, requiring low maintenance, with some variants like VRLA (valve-regulated lead-acid) batteries. VRLA batteries are prevalent due to their deep-discharge capability, often used in applications like golf carts. These batteries are commonly referred to as SLI (starting, lighting, and ignition) batteries, reflecting their primary functions in a vehicle.
Types of Lead-Acid Car Batteries
Modern vehicles use one of three main lead-acid battery designs, each optimized for different performance and maintenance needs:
- Flooded (Wet Cell) — the most traditional design, with the plates fully submerged in liquid electrolyte. It’s the least expensive option and provides high cranking power, but it isn’t sealed and can require occasional maintenance to top off electrolyte lost during charging.
- AGM (Absorbed Glass Mat) — a type of VRLA battery where the electrolyte is absorbed into fine fiberglass mats packed between the plates instead of sitting as free liquid. This makes AGM batteries spill-proof and more resistant to vibration, and better suited to vehicles with start-stop technology or heavy electrical loads from onboard accessories.
- Gel Cell — also a VRLA design, but the sulfuric acid electrolyte is suspended in a thick silica-based gel rather than absorbed into a mat. Gel cells resist deep discharge and vibration well, making them common in golf carts and mobility scooters rather than daily-driver cars.
Active circuit parts of a Pb-acid battery cell
The active circuit parts of a lead-acid battery cell include several components. The negative electrode is made of lead (Pb), while the positive electrode is made of lead dioxide (PbO2). The electrolyte used is typically a 5M H2SO4 aqueous solution. A porous glass mat filled with electrolytes acts as a separator between the two electrodes. Both the negative and positive electrodes are connected to current collectors, which allow the flow of electrons in and out of the cell during the charging and discharging processes.
Typical Lead acid car battery parameters
Typical parameters for a Lead Acid Car Battery include a specific energy range of 33–42 Wh/kg and an energy density of 60–110 Wh/L. The specific power of these batteries is around 180 W/kg, and their charge/discharge efficiency varies from 50% to 95%. Lead-acid batteries have a self-discharge rate of 3–20% per month and can endure approximately 500–800 charge/discharge cycles. The nominal cell voltage for these batteries is 2.0 V, and they can be charged within a temperature range of -35°C to 45°C.
Lead Acid Car Battery – Chemistry
Overall: PbO2+Pb+2H2SO4 →2PbSO4 + 2H2O
Discharging
Negative Plate: Pb + SO42 –→ PbSO4 + 2e–
Positive Plate: PbO2 + SO4 2-+ 4H+ + 2e– → PbSO4 + 2H2O
Charging
Negative Plate (reduction): PbSO4 + 2e– → Pb + SO42 –
Positive Plate (oxidation): PbSO4 + 2H2O→ PbO2 + SO4 2- + 4H++ 2e–
Attention!! “Negative electrode is always losing electrons (oxidation) while Positive electrode always
gains electrons (reduction)” is only true at discharging. When charging, the chemical reaction is reversed.
Why is the car battery 12V?
The car battery is typically 12V because it is made up of six 2V battery cells connected in series. Historically, cars used 6V electrical systems and batteries until the mid-1950s. The shift to 12V systems occurred when larger engines with higher compression ratios required more electrical power to start. This change to 12 volts offered the advantage of requiring less copper to transfer power throughout the vehicle. Additionally, advancements in battery technology enabled the creation of 12-volt batteries that were the same size as the previous 6-volt batteries, making the transition feasible without major redesigns.
Why are battery cells 2 volts?
Because the electric potential (voltage) produced by most chemical reactions is around 2V, which is often insufficient for most loads, many battery cells are connected in series in most batteries. For instance, each cell in lead-acid batteries typically generates a voltage of approximately 2V.
Example Problem: Calculate the OCV. for a battery cell: Lead and lead oxide in sulfuric acid
OCV = Vpositive electrode – Vnegative electrode
Positive electrode: PbO2
Negative electrode: Pb
Vpositive electrode = +1.69 V
Vnegative electrode = – 0.36 V
OCV = + 1.69 V – (– 0.36 V) = 2.05 V
Example Problem: What’s the theoretical specific capacity (Ah/kg) for a Lead Acid Car Battery
Relative mass:
Pb: 207.2
O:16
S: 32.07
Other info:
1. One electron: -1.602*10-19C
Example Problem: What’s the theoretical specific energy (Wh/kg) for a Lead Acid Car Battery
The specific capacity of a lead-acid battery is 83Ah/kg
The specific energy of the lead-acid battery is 2.05V*83Ah/kg=171Wh/kg
Why are we still using Lead Acid Car Battery?
Even with the ongoing advancement of new battery technologies, Lead acid car batteries remain extensively utilized in the automotive industry. Lead acid car batteries are still widely used due to several advantages. They are the lowest-cost option among battery technologies. The technology is mature, with a well-established worldwide infrastructure for manufacturing and recycling. Lead-acid batteries have a high specific heat, making them safe, especially because they use an aqueous electrolyte. They also have a low self-discharge rate.
However, there are disadvantages to consider. Lead-acid batteries have a low specific energy storage capacity, typically less than 40 Wh/kg. If designed for high power output, they may have poor cycle life when subjected to deep discharges. Performance also degrades significantly at low temperatures. Additionally, lead (Pb) is toxic, which raises environmental concerns, especially regarding disposal and recycling.
Why Do Car Batteries Die? Sulfation
The most common reason a lead-acid car battery eventually stops holding a charge is sulfation. During normal discharge, both plates build up a coating of lead sulfate (PbSO4), which is reversed when the battery is recharged. But if a battery is left sitting in a discharged or partially discharged state for too long, that soft lead sulfate coating crystallizes into a hard layer that the normal charging process can no longer break down.
Once a battery is sulfated, it permanently loses some of its ability to accept a charge, which is why a car battery that’s been sitting unused for weeks (or a vehicle that’s driven only in short trips that never fully recharge the battery) tends to die well before its rated cycle life. This is also why a lead-acid battery should always be stored at or near a full charge rather than left discharged.
Lead Acid Battery Application
Most of the world’s lead-acid batteries are used as automobile starting, lighting, and ignition (SLI) batteries, with an estimated 320 million units shipped in 1999. In 1992, around 3 million tons of lead were used in battery manufacturing.
Lead acid batteries power golf carts, as well as the electric motors in diesel-electric submarines when submerged and serve as emergency power in nuclear submarines. Valve-regulated lead-acid batteries, which cannot spill their electrolyte, are used in backup power supplies for alarm systems, smaller computer systems, uninterruptible power supplies, electric scooters, electric wheelchairs, electrified bicycles, marine applications, battery electric vehicles, micro-hybrid vehicles, motorcycles, and electric forklifts, where their weight serves as a counterweight.
Additionally, lead-acid batteries were historically used to supply filament voltage in early vacuum tube radios and in portable batteries for miners’ cap headlamps, which typically have two or three cells.
Frequently Asked Questions
Why do car batteries die?
The most common cause is sulfation, where the lead sulfate that normally forms during discharge crystallizes into a hard coating after a battery sits discharged too long, permanently reducing its ability to hold a charge.
What’s the difference between AGM and a regular car battery?
A regular (flooded) car battery has its plates submerged in free liquid electrolyte, while an AGM battery absorbs the electrolyte into fiberglass mats, making it spill-proof, more vibration-resistant, and better suited to start-stop vehicles and high electrical loads.
Why is a car battery 12V?
A car battery is 12V because it’s made of six individual lead-acid cells, each producing about 2V, connected in series. This became the automotive standard in the mid-1950s when larger, higher-compression engines needed more starting power than 6V systems could provide.
Why are lead-acid batteries still used in cars despite lithium-ion?
Lead-acid batteries remain the standard for starting, lighting, and ignition (SLI) because they’re the lowest-cost option, use mature and widely available manufacturing and recycling infrastructure, and offer a high discharge rate ideal for starting an engine.
Conclusion
In conclusion, despite the emergence of new battery technologies, lead acid car batteries, continue to play a vital role, in various applications, particularly in the automotive industry. Their low cost, mature technology, and reliable performance make them a preferred choice for many. While they do have limitations such as low specific energy, poor performance at low temperatures, and environmental concerns due to lead toxicity, their benefits often outweigh these drawbacks. As we move forward, ongoing advancements and improvements in lead-acid battery technology will likely ensure their relevance and continued use in the years to come.
