Bipolar VRLA Technology
Atraverda has achieved bi-polar VRLA technology manufacturing that simplifies lead acid battery technology for expanded uses and unparalled performance. This technology fits perfectly as the solution for electrified vehicle and renewable energy applications.
Conventional Lead-Acid Battery Technology
Conventional lead acid batteries are comprised of two electrodes: a positive electrode made of lead dioxide (PbO2) and a negative electrode made of sponge lead (Pb). Both the lead dioxide and sponge lead materials are pasted onto lead grids that act as the current collector. Two half reactions occur on the electrodes during charge and discharge that are described by the well known double sulphate theory for lead acid batteries:
Lead battery design consists of lead alloy grids (current collectors), pasted with active electrode material to make plates (positives and negatives), which are in turn alternately stacked with separators between each plate in sufficient number to produce the desired battery size. Tabs on the plates (lugs) of common polarity are connected to a lead alloy strap (called cast-on-strap). This stack (commonly referred to as a cell) is then inserted into an individual plastic compartment. Each cell operates at approximately 2.0 volts and the desired voltage of the battery is achieved by connecting a sufficient number of cells together, each in their own plastic compartment, through the strap, one cell to the next in series. For example, 6 cells, at 2.0 volts each, connected in series to produce 12 volts. The strap of the end cells in the series string is connected to the battery terminals by a lead post to provide the positive and negative terminals of the battery. This basic construction scheme has prevailed for 100 years, and in spite of significant investment in research, little improvement in basic energy density, power density or number of duty cycles has been achieved in the last few decades. Today, lead battery technology is mature, offering little differentiation among products from different producers. And although lead batteries are forecast to maintain their position in established markets, their performance is not likely to meet the needs of many new high growth applications such as HEVs, which are currently being served by nickel based rechargeable batteries. Figure 1 is an illustration of a conventional lead battery with the stacking concept of separators, electrodes and grids (ref: Eurobat).
Conventional Lead Battery Construction
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 Valve Regulated Lead Acid (VRLA) Batteries
Bipolar VRLA batteries replace the stack of multiple layers of separators, lead grids and electrode paste materials used in conventional designs with a single layer of conductive material, the Ebonex® Bipole Element, which serves as both the support structure and divider for the positive and negative electrode paste material. The use of individual plastic compartments for cells, straps and complex inter-cell connection schemes is also eliminated. Each Ebonex® Bipole Element, when assembled with electrode paste material, forms a 2 volt cell. Through a dramatic reduction in the number of components and amount of raw material used in the battery, weight and size are reduced, overall energy densities are improved and low cost is maintained. Figure 2 is a schematic representation of a bipolar 12 volt battery comprised of 6 cells.
Bipolar 12 Volt Battery Schematic (6 cells)
Bipolar design elements
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.
Advantages of Bipolar Battery Design
The lead-acid industry has long been aware of the theoretical advantages available from bipolar batteries—particularly bipolar lead-acid batteries.
Bipolar construction:
- Shortens the current path between the positive and negative terminals of the battery. This reduces the battery’s internal resistance to current flow and improves power delivery.
- Creates a uniform current distribution and load potential over the entire surface of the electrodes, which provides more efficient utilization of paste materials.
These two design concepts are shown in animated form in Figure 3 where the long, complex path for current flow in a conventional battery can be seen in the picture.
Figure 3. Conduction Path inside a Lead Battery - 4 Volt Conventional (Monopolar) Design vs. Bipolar
- Can be used to produce high voltage batteries in a small amount of space, an advantage since high voltage is a benefit in the operation of most electrical devices.
The battery industry uses ampere-hour (Ah) capacity as a measure of the basic size of a battery. Bipolar batteries can also be assembled in parallel strings to increase Ah, in much the same way lithium-ion batteries are assembled to power notebook computers.
Environmental Benefits
Ebonex®: Doing its bit for the planet
Ebonex® doesn't just give improved performance. It is better for the environment.
Compared to conventional lead-acid products, Atraverda’s environmentally friendly solution offers:
- Fits into the well-established recycling stream of lead acid batteries (around 98% are recycled)
- Displaces nickel
- Like for like, uses 40% less lead than conventional batteries lead acid
- Ebonex® originates from titanium, a metal abundant in the Earth’s crust, meaning supplies are plentiful.
- Simpler and fewer manufacturing processes reducing the environmental footprint
- Reduced use and exposure of employees to lead
- Reduced freight costs (more batteries per shipment)
Charging Bipolar Batteries
Atraverda bipolar batteries can utilize commercially available VRLA chargers. Charging algorithms need to be optimized for best results.
Contact Atraverda with your application requirements on +44 (0) 1495 294 026 or info@atraverda.com
