Association of European Automotive and Industrial Battery Manufacturers

Lead-acid technology is the most widely used electrochemical system, used in numerous applications from back-up for uninterruptible power supplies and grid energy storage, to traction in battery electric vehicles and for starting, lighting and ignition (SLI) in conventional combustion engine vehicles.

The lead-acid battery is based on:

  • Lead dioxide as the active material of the positive electrode,
  • Metallic lead, in a high-surface-area porous structure, as the negative active material,
  • Sulphuric acid solution.

Lead-acid technology is composed of several sub-technologies distinguished by battery design and manufacturing process:

  • Flooded lead-acid batteries,
  • Valve-Regulated Lead-Acid (VRLA) batteries with electrolyte immobilized by a gel,
  • VRLA batteries with the electrolyte immobilized in an absorptive glass mat (AGM)

Flooded lead-acid batteries

In flooded lead-acid batteries, the positive plate (electrode) is comprised of lead dioxide and the negative of finely divided lead. Both of these active materials react with a sulphuric acid electrolyte to form lead sulphate on discharge and the reactions are reversed on recharge. Batteries are constructed with lead grids to support the active material and individual cells are connected to produce a battery in a plastic case. There are, however, major differences in battery construction depending on the duty cycle and application. 

Chemical reactions 

Charging 
Total material conversion: 2 PbSO4 + 2 H2O 2 H2SO4 + PbO2 + Pb 
Discharging 
Total material conversion: 2 H2SO4 + PbO2 + Pb 2 PbSO4 + 2 H2O

Valve-regulated lead acid batteries (VRLA) with electrolyte immobilized by a gel or an absorptive glass mat (AGM)

A secondary battery in which the cells are closed but have a valve that allows the escape of gas if the internal pressure exceeds a predetermined value, valve-regulated lead acid batteries (VRLA) have a starved electrolyte either on Glass fibers (Absorptive Glass Mat, or AGM) or as a Gel (Gel technology) which allows for internal gas circulation. Water loss from overcharge is reduced to less than 10 % through recombination. VRLA can be installed in a free orientation and there are no leakages because of the absence of liquids. The construction of these batteries means that they do not require maintenance, making them especially advantageous for remote area installations.

Typical applications for AGM batteries include use in motorcycles due to their safety in the event of an accident, in auto racing due to their resistance to vibration and in fixed position applications in extreme cold environments where their lack of a free electrolyte means the battery is less likely to crack and leak.

Gel VRLAs can be found in application in wheelchairs due to their suitability for use indoors. 

Vented Lead-Acid Batteries 
Vented lead-acid batteries are covered secondary cells with an opening through which the products of electrolysis and evaporation are allowed to escape freely from the cells. Vented lead-acid batteries have a liquid electrolyte. The battery is closed by a vent plug and has a gassing rate more than 4 times higher than valve regulated batteries. Water loss by electrolysis during overcharge results in the production of hydrogen and oxygen gases. Vented lead-acid batteries are a well established technology and are economical to produce. Maintenance of water refill depends on design features and application (reduction of refill by recombination plugs or custom refilling systems). The state of charge and age can be checked very easily in vented lead-acid batteries.

Vented lead-acid batteries are commonly found in various traction applications.