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Lead Acid Batteries

Characteristics

Lead acid batteries were invented in 1859 by Gaston Planté and first demonstrated to the French Academy of Sciences in 1860. They remain 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, Lead acid batteries are generally too big and heavy and they suffer from a shorter cycle life and typical usable power down to only 50% Depth of Discharge (DOD). Despite these shortcomings Lead acid batteries are still being specified for PowerNet applications (36 Volts 2 kWh capacity) because of the cost, but this is probably the limit of their applicability and NiMH and Li-Ion batteries are making inroads into this market. For higher voltages and cyclic loads other technologies are being explored.

 

Lead-acid batteries are composed of a Lead-dioxide cathode, a sponge metallic Lead anode and a Sulphuric acid solution electrolyte. This heavy metal element makes them toxic and improper disposal can be hazardous to the environment.

The cell voltage is 2 Volts

 

Discharge
During discharge, the lead dioxide (positive plate) and lead (negative plate) react with the electrolyte of sulfuric acid to create lead sulfate, water and energy.

Charge
During charging, the cycle is reversed: the lead sulfate and water are electro-chemically converted to lead, lead oxide and sulfuric acid by an external electrical charging source.

 

Many new competitive cell chemistries are being developed to meet the requirements of the auto industry for EV and HEV applications.

 

Even after 150 years since its invention, improvements are still being made to the lead acid battery and despite its shortcomings and the competition from newer cell chemistries the lead acid battery still retains the lion's share of the high power battery market.

 

Advantages

Low cost.

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 prolonged periods.

Wide range of sizes and capacities available.

Many suppliers world wide.

The world's most recycled product.

Shortcomings

Very heavy and bulky.

Typical coulombic charge efficiency only 70% but can be as high as 85% 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 been introduced to avoid deterioration of the active chemicals.

Gassing is the production and release of bubbles of hydrogen and oxygen due to the breakdown of water in the electrolyte during the charging process, particularly due to excessive charging, causing loss of electrolyte. In large battery installations this can cause an explosive atmosphere in the battery room. Because of the loss of electrolyte, Lead acid batteries need regular topping up with water. Sealed batteries however are designed to retain and recombine these gases. (See VRLA below)

Sulphation may occur if a battery is stored for prolonged periods in a completely discharged state or very low state of charge, or if it is never fully charged, or if electrolyte has become abnormally low due to excessive water loss from overcharging and/or evaporation. Sulphation is the increase in internal resistance of the battery due to the formation of large lead sulphate crystals which are not readily reconverted back to lead, lead dioxide and sulphuric acid during re-charging. In extreme cases the large crystals may cause distortion and shorting of the plates. Sometimes sulphation can be corrected by charging very slowly (at low current) at a higher than normal voltage.

Completely discharging the battery may cause irreparable damage.

Shedding or loss of material from the plates may occur due to excessive charge rates or excessive cycling. The result is chunks of lead on the bottom of the cell, and actual holes in the plates for which there is no cure. This is more likely to occur in SLI batteries whose plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area enabling high power handling, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells.

Toxic chemicals

Very heavy and bulky

Lead acid batteries can work down to temperatures below -45 °C, however, like all batteries the discharge rate and effective capacity are reduced at low temperatures. In the case of Lead acid batteries the capacity falls by about 1% per degree for temperatures below +20 °C so that at the lowest temperatures cranking capacity is seriously impaired.

Decomposition of the Electrolyte Cells with gelled electrolyte are prone to deterioration of the electrolyte and unexpected failure. Such cells are commonly used for emergency applications such as UPS back up in case of loss of mains power. So as not to be caught unawares by an unreliable battery in an emergency situation, it is advisable to incorporate some form of regular self test into the battery.

Charging

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 using the V Taper charge control method.

Applications

Automotive and traction applications.

Standby/Back-up/Emergency power for electrical installations.

Submarines

UPS (Uninterruptible Power Supplies)

Lighting

High current drain applications.

Sealed battery types available for use in portable equipment.

Costs

Low cost

Flooded lead acid cells are one of the least expensive sources of battery power available.

Deep cycle cells may cost up to double the price of the equivalent flooded cells.

 

Varieties of Lead Acid Batteries

Over the years battery manufacturers have introduced a range of additives such as Calcium, Antimony and Selenium to improve various battery performance parameters. For the same reason, different cell and battery constructions have been developed to optimise various aspects of battery performance.

Lead Calcium Batteries

Lead acid batteries with electrodes modified by the addition of Calcium 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 of Antimony providing the following advantages:

  • Improved mechanical strength of electrodes - important for EV and deep discharge applications
  • Reduced internal heat and water loss due to gassing, however the water loss is still greater than the equivalent loss in Lead Calcium batteries.
  • Longer service life than Calcium batteries.
  • Easier to recharge when completely discharged.
  • Lower cost.

Lead Antimony batteries have a higher self discharge rate of 2% to 10% per week compared with the 1% to 5% per month for Lead Calcium batteries.

Valve Regulated Lead Acid (VRLA) Batteries
Also called Sealed Lead Acid (SLA) batteries.

This construction is designed to prevent electrolyte loss through evaporation, spillage and gassing and this in turn prolongs the life of the battery and eases maintenance. Instead of simple vent caps on the cells to let gas escape, VRLA have pressure valves that open only under extreme conditions. Valve-regulated batteries also need an electrolyte design that reduces gassing by impeding the release to the atmosphere of the oxygen and hydrogen generated by the galvanic action of the battery during charging. This usually involves a catalyst that causes the hydrogen and oxygen to recombine into water and is called a recombinant system. Because spillage of the acid electrolyte is eliminated the batteries are also safer.

AGM Absorbed Glass Mat Battery

Also known as Absorptive Glass Micro-Fibre

Used in VRLA batteries the Boron Silicate fibreglass mat which acts as the separator between the electrodes and absorbs the free electrolyte acting like a sponge. Its purpose is to promote recombination of the hydrogen and oxygen given off during the charging process. No silica gel is necessary. The fibreglass matt absorbs and immobilises the acid in the matt but keeps it in a liquid rather than a gel form. In this way the acid is more readily available to the plates allowing faster reactions between the acid and the plate material allowing higher charge/discharge rates as well as deep cycling.

This construction is very robust and able to withstand severe shock and vibration and the cells will not leak even if the case is cracked.

AGM batteries are also sometimes called "starved electrolyte" or "dry", because the fibreglass mat is only 95% saturated with Sulfuric 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% per month

Gel Cell

This is an alternative recombinant technology to also used in VRLA batteries to promote recombination of the gases produced during charging. It also reduces the possibility of spillage of the electrolyte. Prone to damage if gassing is allowed to occur, hence charging rates may be limited. They must be charged at a slower rate (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged.

Used for UPS applications.

SLI Batteries (Starting Lighting and Ignition)

This is the typical automotive battery application. Automotive batteries are designed to be fully charged when starting the car; after starting the vehicle, the lost charge, typically 2% to 5% of the charge, is replaced by the alternator and the battery remains fully charged. These batteries are not designed to be discharged below 50% Depth of Discharge (DOD) and discharging below these levels can damage the plates and shorten battery life.

Deep Cycle Batteries

Marine applications, golf buggies, fork lift trucks and electric vehicles use deep cycle batteries which are designed to be completely discharged before recharging. Because charging causes excessive heat which can warp the plates, thicker and stronger or solid plate grids are used for deep cycling applications. Normal automotive batteries are not designed for repeated deep cycling and use thinner plates with a greater surface area to achieve high current carrying capacity.

Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).

If batteries designed for deep cycling are used for automotive applications they must be "oversized" by about 20% to compensate for their lower current carrying capacity.

 

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Lead Acid Battery Safety Warning

Source: BCI (Battery Council International)

 

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