A lead-acid battery is a rechargeable energy storage device that is constructed with a lead negative electrode (anode), a lead oxide positive electrode (cathode) and a sulphuric acid electrolyte. Invented in 1859 by French physicist Gaston Plante, lead-acid batteries are the grandfather of rechargeable batteries and remain a popular technology to this day. Although lithium based battery chemistries have gained enormous attention in the past decade, lead-acid batteries will maintain a strong position in applications where low cost-per-watt ratios and/or outstanding cold-temperature performance are critical.
[H2]Electrochemistry[/H2]The electrochemical process in a lead-acid battery includes an oxidation half-reaction at the lead (Pb) anode, which releases electrons; and a reduction half-reaction at the lead dioxide (PbO2) cathode, which accepts electrons. As a battery discharges, the lead anode reacts with sulfate ions in the electrolyte to produce lead sulfate (PbSO4) and electrons (e⁻). Electrons generated in this reaction flow out to provide electric current through an external electric circuit. In the cathode half-reaction, the electron flow (created at the anode) allows the oxygen molecules to be released from the lead dioxide. Hydrogen (H+) breaks its bond with the sulfate molecules in sulfuric acid (H2SO4) of the electrolyte and combines with oxygen to create water (H2O). The now free sulfate molecules combine with the available lead molecules at both electrodes to create lead sulfate. When a battery is being discharged, the electrolyte is depleted of dissolved tetraoxosulphate (VI) acid and turns to water. Charging the battery reverses the electrochemical process. The lead sulfate and water are converted to lead for the anode, lead dioxide for the cathode, and sulfuric acid.
[H2]Performance and Life[/H2]Lead acid batteries have a relatively short cycle life (200-700 cycles) and a lifespan of 5 to 15 years depending on construction, usage and temperature conditions. Their specific energy and energy density are 30-40 Wh/kg and 50-90 Wh/l, respectively. The charge efficiency is typically 50% and charging time is 10 to 24 hours. Lead acid batteries have a self-discharge rate of 3-4% per month which is much better than nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries and is comparable to lithium ion (Li-ion) batteries. The performance and cycle life of a lead-acid are dependent upon its positive electrode, whereas the negative electrode affects the cold-temperature performance of the systems.
[H2]Flooded Lead-acid Battery[/H2]Lead acid batteries are divided into two main categories by construction type: flooded and sealed. Flooded lead-acid batteries, sometimes referred to as vented lead-acid batteries, use a liquid electrolyte and the electrodes are completely immersed in the electrolyte. The electrolyte solution is typically made of 30-35% sulphuric acid and 70-65% water. During charging of flooded batteries, the oxygen created at the positive electrode and the hydrogen created at the negative electrode must be vented to prevent excess pressure from the buildup of these gases. Also, the room or space where the battery is placed must be vented, since a concentrated hydrogen and oxygen atmosphere presents a potential explosion hazard. Meanwhile a loss of water is to be expected with the release of hydrogen and oxygen gases. This necessitates regular maintenance (addition of water) to compensate for lost hydrogen and oxygen. Therefore flooded batteries have removable caps for the routine top up of the electrolyte in the cells.
[H2]Sealed Lead-acid Battery[/H2]Sealed lead-acid (SLA) batteries are sealed to the atmosphere and have a safety vent to release gas in the event of excessive internal pressure buildup. Hence they're also called valve regulated lead-acid (VRLA) batteries. These have essentially the same chemistry as vented batteries, but in addition to a sealed design the electrolyte of VRLA batteries is immobilized by some mechanism such that the batteries have less electrolyte freeing problems compared to flooded batteries. During charging process, hydrogen and oxygen gases produced from water are recombined into water inside the container. Since very little hydrogen and oxygen gases are lost and the water content of the electrolyte remains unchanged, the VRLA battery requires no periodic topping up with distilled water and, therefore, the battery is also referred as maintenance-free battery. Oxygen and hydrogen will escape from the pressure relief valve in an overcharge condition (as is typical of any type battery). The electrolyte of VRLA batteries is immobilized by either soaking it in a separator consisting of matted glass fibers, as in absorbed glass mat (AGM) lead-acid batteries, or by forming a gel through the addition of silicon dioxide, as in gel lead-acid batteries.
[H2]Flooded vs. Sealed[/H2]Lead acid batteries with flooded electrolytes are favored in many applications for their advantages of low initial cost, high cold cranking amps (CCA), good tolerance of improper recharge voltages, and the ability to be charged more rapidly with high current. However, the lower cost of flooded lead acid batteries must be balanced against their disadvantages. High maintenance frequency, high freights due to limited shipping options, low cycle life and short shelf life translate to high secondary costs. Flooded batteries do not work on the recombination principle. Thus they can only be installed upright otherwise liquid electrolyte can spill and cause corrosion if tipped or punctured. When they're left in a discharged condition due to sulfation or continually over-discharged due to active material shedding, flooded batteries lose capacity and become permanently damaged. Acid stratification which causes shortened calendar and cycle life and reduced charge and discharge performance is another issue for conventional wet cells.
Sealed lead-acid batteries are maintenance-free, leak-proof, position-insensitive. The recombinant design allows the oxygen normally produced on the lead oxide positive plates of sealed lead-acid batteries to be absorbed by the lead negative plate. This also suppresses the production of hydrogen gas at the negative plate. Since hydrogen and oxygen gases are not generated during charging, water is produced. The recombination reaction eliminates the requirement to top up the battery with water while allowing the battery to be mounted and can operate in any orientation without leakage. Sealed lead-acid batteries also suffer less from electrolyte stratification. However, sealed batteries have a more expensive upfront. AGM lead-acid batteries, for example, are approximately three times higher than the wet lead acid batteries. Sealed batteries require more careful charging and automatic temperature-sensing at the battery. Incorrect charging and or poor thermal management increases the risk of thermal runaway. A self-discharge rate of up to 30% per month means the sealed batteries must be regularly recharged when in storage.
[H2]SLI and Deep Cycle Batteries[/H2]Lead-acid batteries can be divided into two categories by discharge type: SLI (Starter, Lighting and Ignition) batteries which deliver a short burst of high power, and deep-cycle batteries which deliver a lower, steady level of power over longer periods of time. SLI batteries have high momentary current capability and are well suited to applications such as engine cranking and supporting large electrical loads. Deep-cycle batteries are designed to provide continuous output to power electrical equipment but are not designed for maximum current output for short durations. The life of the battery depends on the depth of each discharge. The shallower the average discharge, the longer the battery life. Deep cycle batteries are discharged to less than 50% (SLA) or 80% (flooded) of the rated capacity. Thicker lead plates are used to ensure that the batteries can be discharged and recharged many times without degradation. Deep-cycle batteries require longer charging times.
[H2]Applications[/H2]Despite its bulkiness, low energy density, short cycle life and falling voltage profile during discharge, the lead acid chemistry remains a competitive technology. There are few other battery chemistries that deliver bulk power as cheaply as lead acid and provide consistent performance in cold environments. All SLI (engine starting, vehicle lighting, and engine ignition) batteries for internal combustion engine (ICE) vehicles are currently lead-based. Lead acid batteries have been a common storage option for standby and emergency backup applications, minor micro-grids or grid-independent electrical power systems, and for driving wheelchairs, forklifts, golf cars, sailboats, and electric scooters and bicycles. Lead-acid battery technologies are expected to remain in consideration for use in plug-in hybrid-electric vehicles (HEVs) and full electric vehicles (EVs) since they are a cost effective solution.
[H2]Electrochemistry[/H2]The electrochemical process in a lead-acid battery includes an oxidation half-reaction at the lead (Pb) anode, which releases electrons; and a reduction half-reaction at the lead dioxide (PbO2) cathode, which accepts electrons. As a battery discharges, the lead anode reacts with sulfate ions in the electrolyte to produce lead sulfate (PbSO4) and electrons (e⁻). Electrons generated in this reaction flow out to provide electric current through an external electric circuit. In the cathode half-reaction, the electron flow (created at the anode) allows the oxygen molecules to be released from the lead dioxide. Hydrogen (H+) breaks its bond with the sulfate molecules in sulfuric acid (H2SO4) of the electrolyte and combines with oxygen to create water (H2O). The now free sulfate molecules combine with the available lead molecules at both electrodes to create lead sulfate. When a battery is being discharged, the electrolyte is depleted of dissolved tetraoxosulphate (VI) acid and turns to water. Charging the battery reverses the electrochemical process. The lead sulfate and water are converted to lead for the anode, lead dioxide for the cathode, and sulfuric acid.
[H2]Performance and Life[/H2]Lead acid batteries have a relatively short cycle life (200-700 cycles) and a lifespan of 5 to 15 years depending on construction, usage and temperature conditions. Their specific energy and energy density are 30-40 Wh/kg and 50-90 Wh/l, respectively. The charge efficiency is typically 50% and charging time is 10 to 24 hours. Lead acid batteries have a self-discharge rate of 3-4% per month which is much better than nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries and is comparable to lithium ion (Li-ion) batteries. The performance and cycle life of a lead-acid are dependent upon its positive electrode, whereas the negative electrode affects the cold-temperature performance of the systems.
[H2]Flooded Lead-acid Battery[/H2]Lead acid batteries are divided into two main categories by construction type: flooded and sealed. Flooded lead-acid batteries, sometimes referred to as vented lead-acid batteries, use a liquid electrolyte and the electrodes are completely immersed in the electrolyte. The electrolyte solution is typically made of 30-35% sulphuric acid and 70-65% water. During charging of flooded batteries, the oxygen created at the positive electrode and the hydrogen created at the negative electrode must be vented to prevent excess pressure from the buildup of these gases. Also, the room or space where the battery is placed must be vented, since a concentrated hydrogen and oxygen atmosphere presents a potential explosion hazard. Meanwhile a loss of water is to be expected with the release of hydrogen and oxygen gases. This necessitates regular maintenance (addition of water) to compensate for lost hydrogen and oxygen. Therefore flooded batteries have removable caps for the routine top up of the electrolyte in the cells.
[H2]Sealed Lead-acid Battery[/H2]Sealed lead-acid (SLA) batteries are sealed to the atmosphere and have a safety vent to release gas in the event of excessive internal pressure buildup. Hence they're also called valve regulated lead-acid (VRLA) batteries. These have essentially the same chemistry as vented batteries, but in addition to a sealed design the electrolyte of VRLA batteries is immobilized by some mechanism such that the batteries have less electrolyte freeing problems compared to flooded batteries. During charging process, hydrogen and oxygen gases produced from water are recombined into water inside the container. Since very little hydrogen and oxygen gases are lost and the water content of the electrolyte remains unchanged, the VRLA battery requires no periodic topping up with distilled water and, therefore, the battery is also referred as maintenance-free battery. Oxygen and hydrogen will escape from the pressure relief valve in an overcharge condition (as is typical of any type battery). The electrolyte of VRLA batteries is immobilized by either soaking it in a separator consisting of matted glass fibers, as in absorbed glass mat (AGM) lead-acid batteries, or by forming a gel through the addition of silicon dioxide, as in gel lead-acid batteries.
[H2]Flooded vs. Sealed[/H2]Lead acid batteries with flooded electrolytes are favored in many applications for their advantages of low initial cost, high cold cranking amps (CCA), good tolerance of improper recharge voltages, and the ability to be charged more rapidly with high current. However, the lower cost of flooded lead acid batteries must be balanced against their disadvantages. High maintenance frequency, high freights due to limited shipping options, low cycle life and short shelf life translate to high secondary costs. Flooded batteries do not work on the recombination principle. Thus they can only be installed upright otherwise liquid electrolyte can spill and cause corrosion if tipped or punctured. When they're left in a discharged condition due to sulfation or continually over-discharged due to active material shedding, flooded batteries lose capacity and become permanently damaged. Acid stratification which causes shortened calendar and cycle life and reduced charge and discharge performance is another issue for conventional wet cells.
Sealed lead-acid batteries are maintenance-free, leak-proof, position-insensitive. The recombinant design allows the oxygen normally produced on the lead oxide positive plates of sealed lead-acid batteries to be absorbed by the lead negative plate. This also suppresses the production of hydrogen gas at the negative plate. Since hydrogen and oxygen gases are not generated during charging, water is produced. The recombination reaction eliminates the requirement to top up the battery with water while allowing the battery to be mounted and can operate in any orientation without leakage. Sealed lead-acid batteries also suffer less from electrolyte stratification. However, sealed batteries have a more expensive upfront. AGM lead-acid batteries, for example, are approximately three times higher than the wet lead acid batteries. Sealed batteries require more careful charging and automatic temperature-sensing at the battery. Incorrect charging and or poor thermal management increases the risk of thermal runaway. A self-discharge rate of up to 30% per month means the sealed batteries must be regularly recharged when in storage.
[H2]SLI and Deep Cycle Batteries[/H2]Lead-acid batteries can be divided into two categories by discharge type: SLI (Starter, Lighting and Ignition) batteries which deliver a short burst of high power, and deep-cycle batteries which deliver a lower, steady level of power over longer periods of time. SLI batteries have high momentary current capability and are well suited to applications such as engine cranking and supporting large electrical loads. Deep-cycle batteries are designed to provide continuous output to power electrical equipment but are not designed for maximum current output for short durations. The life of the battery depends on the depth of each discharge. The shallower the average discharge, the longer the battery life. Deep cycle batteries are discharged to less than 50% (SLA) or 80% (flooded) of the rated capacity. Thicker lead plates are used to ensure that the batteries can be discharged and recharged many times without degradation. Deep-cycle batteries require longer charging times.
[H2]Applications[/H2]Despite its bulkiness, low energy density, short cycle life and falling voltage profile during discharge, the lead acid chemistry remains a competitive technology. There are few other battery chemistries that deliver bulk power as cheaply as lead acid and provide consistent performance in cold environments. All SLI (engine starting, vehicle lighting, and engine ignition) batteries for internal combustion engine (ICE) vehicles are currently lead-based. Lead acid batteries have been a common storage option for standby and emergency backup applications, minor micro-grids or grid-independent electrical power systems, and for driving wheelchairs, forklifts, golf cars, sailboats, and electric scooters and bicycles. Lead-acid battery technologies are expected to remain in consideration for use in plug-in hybrid-electric vehicles (HEVs) and full electric vehicles (EVs) since they are a cost effective solution.
