Lead Acid Batteries
Lead acid batteries were invented in 1859 by Gaston Planté andfirst demonstrated to the French Academy of Sciences in 1860. Theyremain the technology of choice for automotive SLI (Starting,Lighting and Ignition) applications because they are robust,tolerant to abuse, tried and tested and because of their low cost.For higher power applications with intermittent loads however, Leadacid batteries are generally too big and heavy and they suffer froma shorter cycle life and typical usable power down to only 50%Depth of Discharge (DOD). Despite these shortcomings Lead acidbatteries are still being specified for PowerNet applications (36Volts 2 kWh capacity) because of the cost, but this is probably thelimit of their applicability and NiMH and Li-Ion batteries aremaking inroads into this market. For higher voltages and cyclicloads other technologies are being explored.
Lead-acid batteries are composed of a Lead-dioxide cathode, asponge metallic Lead anode and a Sulphuric acid solutionelectrolyte. This heavy metal element makes them toxic and improperdisposal can be hazardous to the environment.
The cell voltage is 2 Volts
During discharge, the lead dioxide (positive plate) and lead(negative plate) react with the electrolyte of sulfuric acid tocreate lead sulfate, water and energy.
During charging, the cycle is reversed: the lead sulfate and waterare electro-chemically converted to lead, lead oxide and sulfuricacid by an external electrical charging source.
Many new competitive cell chemistries are being developed to meetthe requirements of the auto industry for EV and HEVapplications.
Even after 140 years since its invention, improvements are stillbeing made to the lead acid battery and despite its shortcomingsand the competition from newer cell chemistries the lead acidbattery still retains the lion's share of the high power batterymarket.
Reliable. Over 140 years of development.
Robust. Tolerant to abuse.
Tolerant to overcharging.
Low internal impedance.
Can deliver very high currents.
Indefinite shelf life if stored without electrolyte.
Can be left on trickle or float charge for prolongedperiods.
Wide range of sizes and capacities available.
Many suppliers world wide.
The world's most recycled product.
Very heavy and bulky.
Typical coulombic charge efficiency only 70% but can be as high as85% to 90% for special designs.
Danger of overheating during charging
Not suitable for fast charging
Typical cycle life 300 to 500 cycles .
Must be stored in a charged state once the electrolyte has beenintroduced to avoid deterioration of the active chemicals.
Gassing is the production and release of bubbles of hydrogen and oxygen inthe electrolyte during the charging process, particularly due toexcessive charging, causing loss of electrolyte. In large batteryinstallations this can cause an explosive atmosphere in the batteryroom. Sealed batteries are designed to retain and recombine thesegases. (See VRLA below)
Sulphation may occur if a battery is stored for prolonged periods in acompletely discharged state or very low state of charge, or if itis never fully charged, or if electrolyte has become abnormally lowdue to excessive water loss from overcharging and/or evaporation.Sulphation is the increase in internal resistance of the batterydue to the formation of large lead sulphate crystals which are notreadily reconverted back to lead, lead dioxide and sulphuric acidduring re-charging. In extreme cases the large crystals may causedistortion and shorting of the plates. Sometimes sulphation can becorrected by charging very slowly (at low current) at a higherthan normal voltage.
Completely discharging the battery may cause irreparabledamage.
Shedding or loss of material from the plates may occur due to excessivecharge rates or excessive cycling. The result is chunks of lead onthe bottom of the cell, and actual holes in the plates for whichthere is no cure. This is more likely to occur in SLI batterieswhose plates are composed of a Lead "sponge", similar in appearanceto a very fine foam sponge. This gives a very large surface areaenabling high power handling, but if deep cycled, this sponge willquickly be consumed and fall to the bottom of the cells.
Very heavy and bulky
Lower temperature limit -15 °C
Decomposition of the Electrolyte Cells with gelled electrolyte are prone to deterioration of theelectrolyte and unexpected failure. Such cells are commonly usedfor emergency applications such as UPS back up in case of loss ofmains power. So as not to be caught unawares by an unreliablebattery in an emergency situation, it is advisable to incorporatesome form of regular self test into the battery.
Charge immediately after use.
Lasts longer with partial discharges.
Charging method: constant voltage followed by float charge.
Fast charge not possible but charging time can be reduced usingtheV Taper charge control method.
Automotive and traction applications.
Standby/Back-up/Emergency power for electricalinstallations.
UPS (Uninterruptible Power Supplies)
High current drain applications.
Sealed battery types available for use in portableequipment.
Flooded lead acid cells are one of the least expensive sources ofbattery power available.
Deep cycle cells may cost up to double the price of the equivalentflooded cells.
Varieties of Lead Acid Batteries
Lead Calcium Batteries
Lead acid batteries with electrodes modified by the addition ofCalcium providing the following advantages:
More resistant to corrosion, overcharging, gassing, water usage,and self-discharge, all of which shorten battery life.
Larger electrolyte reserve area above the plates.
Higher Cold Cranking Amp ratings.
Little or No maintenance.
Lead Antimony Batteries
Lead acid batteries with electrodes modified by the addition ofAntimony providing the following advantages:
Improved mechanical strength of electrodes - important for EV anddeep discharge applications
Reduced internal heat and water loss.
Longer service life than Calcium batteries.
Easier to recharge when completely discharged.
Lead Antimony batteries have a higher self discharge rate of 2% to10% per week compared with the 1% to 5% per month for Lead Calciumbatteries.
Valve Regulated Lead Acid (VRLA) Batteries
Also called Sealed Lead Acid (SLA) batteries.
This construction is designed to prevent electrolyte loss throughevaporation, spillage and gassing and this in turn prolongs thelife of the battery and eases maintenance. Instead of simple ventcaps on the cells to let gas escape, VRLA have pressure valves thatopen only under extreme conditions. Valve-regulated batteries alsoneed an electrolyte design that reduces gassing by impeding therelease to the atmosphere of the oxygen and hydrogen generated bythe galvanic action of the battery during charging. This usuallyinvolves a catalyst that causes the hydrogen and oxygen torecombine into water and is called a recombinant system. Becausespillage of the acid electrolyte is eliminated the batteries arealso safer.
AGM Absorbed Glass Mat Battery
Also known as Absorptive Glass Micro-Fibre
Used in VRLA batteries the Boron Silicate fibreglass mat which actsas the separator between the electrodes and absorbs the freeelectrolyte acting like a sponge. Its purpose is to promoterecombination of the hydrogen and oxygen given off during thecharging process. No silica gel is necessary. The fibreglass mattabsorbs and immobilises the acid in the matt but keeps it in aliquid rather than a gel form. In this way the acid is more readilyavailable to the plates allowing faster reactions between the acidand the plate material allowing higher charge/discharge rates aswell as deep cycling.
This construction is very robust and able to withstand severe shockand vibration and the cells will not leak even if the case iscracked.
AGM batteries are also sometimes called "starved electrolyte" or"dry", because the fibreglass mat is only 95% saturated withSulfuric acid and there is no excess liquid.
Nearly all AGM batteries are sealed valve regulated "VRLA".
AGM's have a very low self-discharge rate of from 1% to 3% permonth
This is an alternative recombinant technology to also used in VRLAbatteries to promote recombination of the gases produced duringcharging. It also reduces the possibility of spillage of theelectrolyte. Prone to damage if gassing is allowed to occur, hencecharging rates may be limited. They must be charged at a slowerrate (C/20) to prevent excess gas from damaging the cells. Theycannot be fast charged on a conventional automotive charger or theymay be permanently damaged.
Used for UPS applications.
SLI Batteries (Starting Lighting and Ignition)
This is the typical automotive battery application. Automotivebatteries are designed to be fully charged when starting the car;after starting the vehicle, the lost charge, typically 2% to 5% ofthe charge, is replaced by the alternator and the battery remainsfully charged. These batteries are not designed to be dischargedbelow 50% Depth of Discharge (DOD) and discharging below theselevels can damage the plates and shorten battery life.
Deep Cycle Batteries
Marine applications, golf buggies, fork lift trucks and electricvehicles use deep cycle batteries which are designed to becompletely discharged before recharging. Because charging causesexcessive heat which can warp the plates, thicker and stronger orsolid plate grids are used for deep cycling applications. Normalautomotive batteries are not designed for repeated deep cycling anduse thinner plates with a greater surface area to achieve highcurrent carrying capacity.
Automotive batteries will generally fail after 30-150 deep cyclesif deep cycled, while they may last for thousands of cycles innormal starting use (2-5% discharge).
If batteries designed for deep cycling are used for automotiveapplications they must be "oversized" by about 20% to compensatefor their lower current carrying capacity.