LEAD ACID TAKES THE LEAD

A new battery based on thin lead foil and spiral wound plates could help designers of portable equipment

A Thin Metal Film (TMF) battery technology, developed at Bolder Technologies in the US and distributed in the UK by Romsey-based DMS technologies, represents a substantial performance improvement over conventional lead acid battery technology.

The construction of the TMF batteries consists of a thin layer of active paste that is applied to both sides of a solid lead foil. The pasting operation leaves a thin strip of uncoated foil along one edge. The uncoated area forms the contact to the cell terminal.

When compared to conventional flooded and sealed lead acid batteries, the design produces a 16 to 19 times increase in the ratio of plate surface area to active material, and a 20 to 80 times decrease in the path length of the electron flow. The basic cell design is illustrated in Figure 1.

The positive and negative elements in the battery are wound together in a spiral wrap configuration with a thin glass fibre separator. The cell is wound in a manner to permit the uncoated strip on the lead foil collector of each element to protrude slightly from the end of the wound cell. Thus, uncoated areas of each plate polarity protrude from opposite ends of the wound cell (similar to capacitor designs). The lead terminals are cast onto the full length of the exposed ends of the spirally wrapped cell. The castings serve as a seal, current collector and manifold for electrolyte distribution. The cell design also incorporates a low pressure, Bunsen valve vent.

Using the full length of the plate for current collection, through the cast-on-cell terminals, produces a uniform current distribution over the electrode surfaces. This method of construction results in a low resistance path for current flow that substantially minimises the voltage drop during discharge.

The discharge characteristics are shown in Figures 2 and 3. They depict typical discharge curves for a 1.2Ah cell at 15A and 80A. The discharge voltage is very flat, even at the 80A discharge rate.

The low current density (due to the large surface area) and the thin layers of active mass result in high efficiency of use of the active materials in the cell. This efficiency produces cycle life characteristics of over 500 high rate deep discharges and over 15,000 shallow discharges.

TMF cells are also tolerant of overcharging and, in fact, show higher capacities and longer cycle lives with moderate to high overcharge amounts. Using a high rate current charger, a battery can be recharged in as little as five to ten minutes.

TMF batteries have been tested in a number of applications. A 12V TMF battery weighing less than 1.3lb successfully started a 3.0 litre automobile engine 20 times in a row without recharging. A normal car battery weighs 40lb.

TMF batteries can replace nickel-cadmium batteries in portable and cordless devices (Figure 3) with lower cost and higher power. And it is claimed that a TMF battery can provide 70% more work compared to a nickel cadmium battery in power tools.

Figure 1: The thin metal film construction of the cell

Figure 2: 15A and 80A discharge curves

Figure 3: Where the new technology fits in

{{DMS technologiesTel: Romsey (01794) 830111Enter 440

Bolder TechnologiesTel: +1 (303) 422 8200Enter 441}}