Bipolar Valve Regulated Lead Acid (VRLA) Batteries

Bipolar Valve Regulated Lead Acid (VRLA) Batteries

Lead battery technology innovations in the last 100 years have mainly been focused on materials and components. The corrosive nature of the electrolyte (sulphuric acid) resulted in many attempts to reduce or eliminate electrolyte leakage. In the 1950s it was discovered that the addition of small quantities of fumed silica to the liquid electrolyte caused it to solidify into a gel which greatly reduced the tendency for spillage. Previously, all designs had been of the flooded type with the sulphuric acid electrolyte in free liquid form. Further research revealed that these so-called “gel” batteries required far less maintenance, or water addition to the electrolyte, than theoretical calculations would predict. It was discovered that in gel batteries the gas generated during charging was being recombined into water, greatly reducing the need to regularly add water to the batteries. In flooded batteries this gas had bubbled through the liquid electrolyte and escaped into the air. By adding a small pressure relief valve to the battery, thus sealing it from exposure to outside air, water additions could be eliminated and the first maintenance free, valve regulated lead acid (VRLA) battery was born. About the same time, engineers discovered that the efficiency of the gas recombination process could be substantially improved with the use of absorptive glass mat (AGM) separators. The AGM design was a major technical breakthrough resulting in long life products that were sealed and would not spill sulphuric acid, even during abusive conditions. VRLA batteries using AGM separators were introduced in the late 1970s and today they dominate the lead battery business. VRLA batteries are spill proof, maintenance free and rival their old flooded counterparts in all aspects of performance.

Bipolar design elements

Atraverda bipolar batteries can utilize commercially available VRLA chargers. Charging algorithms need to be optimized for best results.

Robust Design

Bipolar battery development challenges have been driven by the lack of a suitable material from which to fabricate the bipole element (divider).
Lead sheet, while an obvious choice, is structurally weak and would need to be thick enough to maintain its physical shape through the rigours of processing into a complete battery, thus giving up much of the potential weight savings anticipated in bipolar designs. Lead sheet also suffers from gradual corrosion during normal battery operation and will eventually be penetrated by electrolyte (sulphuric acid) causing a short circuit between adjacent cells resulting in catastrophic battery failure. In addition, lead metal presents a notoriously difficult sealing surface. In current sealed lead batteries, the surface areas of lead that must be sealed are relatively small, but in bipolar construction the entire perimeter of the Substrate must be sealed to prevent electrolyte from bridging the space between the positive and negative active materials. As a result, the surface area of lead to be sealed is literally orders of magnitude greater than in a conventional design creating a significant reliability problem.
Thus, the material used to produce the Substrate must be

• Highly conductive
• Resistant to corrosion by sulphuric acid in a lead battery environment
• Suitable for forming reliable seals to prevent electrolyte leakage
• Formed into a variety of shapes
• Allow good adhesion of the active paste materials
• Mechanically robust
• Able to be manufactured in high volumes
• Cost-effective

Atraverda has achieved a bipolar element to meet these design requirements by combining Ebonex® powder with thermoset resins and lead alloy foil.