[Engineering Feature]
Keep Current With New Battery Technologies
Whether you’re talking micropower reflow-solderable thin-film batteries or lighter, safer batteries for traction motors, groundbreaking advances continue to energize the field of chemically generated electrical power.
While current lithium battery makers are challenged to provide a performance improvement of 7% or so, Deuber said that by leveraging manufacturing efficiencies, ZPower’s silver-zincs might achieve something closer to 10% per annum for several years. The ability to recycle exhausted batteries hinges on the technology used recover silver from used X-ray film. Deuber says that element of the plan is now in place, though batteries won’t need to be recycled for at least a year after the first laptops are sold.
Looking at applications from handheld devices to electric vehicles, PowerGenix’s nickel-zinc batteries are chemically very similar to nickel cadmium (NiCd). But they lack the environmental problems of cadmium while offering a 0.4-V boost that brings the open-circuit full-charge voltage to 1.74 V. Both use an alkaline electrolyte and a nickel electrode. The discharge reaction is:
H2O + Zn + 2NiOOH = ZnO +2Ni(OH)2
With the additional 0.4-V per cell advantage over NiCd, an inherent value of the nickel-zinc cell lies in the reduced cell count required for a multicell battery. For higher-voltage applications, the advantages associated with fewer cells are quickly apparent through a smaller footprint, lighter weight, and lower impedance battery.
Using zinc presented some challenges to PowerGenix, though. The company took a patented electrolyte formulation that reduces zinc solubility to eliminate dendrite shorting problems. Dendrites are famously the cause of “memory effect” that plagued NiCd batteries. PowerGenix also developed advances in both the positive and negative electrode composition that eliminate the need for heavy metal elements.
The batteries, fabricated in AAA, AA, and D sizes, fit into packs for heavier-duty applications. This lets the company take advantage of the current alkaline battery supply chain and have the batteries manufactured on existing NiMH lines.
Last year, PowerGenix showed a rechargeable D-cell battery pack for hybrid electric vehicles (HEVs). The company explained that the display model, installed in a Prius, could deliver 30% more power and increased energy density than the similarly sized NiMH battery packs that come standard in the car. The company also said that nickel zinc can be easily integrated into existing hybrid vehicle designs at about one-half the cost per watt-hour.
Mature Battery Types Ubiquitous alkaline and carbon-zinc cells produce approximately the same electromotive force (EMF) of 1.5 V. For NiCd and NiMH cells, the EMF is 1.2 V. Higher electrochemical potential changes give lithium cells EMFs of 3 V or more.
NiCd cells have an anode made of cadmium hydroxide and a cathode of nickel hydroxide, immersed in an alkaline electrolyte comprising potassium, sodium, and lithium hydroxides. The cells are rechargeable and deliver a voltage of 1.2 V during discharge. NiCd is essentially dead these days. The European Union banned cadmium for most uses in 2004, though Saft in France continues to make them, presumably for military purposes. (The company uses cadmium from recycled NiCd batteries.)
While NiCd batteries have been around since the 19th century, NiMH batteries were first developed around 1980. Nickel-metalhydride anodes are made of a metal alloy capable of absorbing and desorbing hydrogen. Their nickel-hydroxide cathodes are immersed in an alkaline electrolyte solution of potassium, sodium, and lithium hydroxides. NiMH cells are rechargeable and deliver a voltage of 1.2 V. They have very similar properties to NiCd units and share the same manufacturing processes and most components. They also have excellent energy density by volume (up to 140 Wh/L).
Lead-Acids Old and New Lead-acids that you top-up with distilled water are called flooded batteries. “Low-maintenance” lead-acid batteries, also known as valve-regulated lead-acid (VRLA) or recombinant batteries, use less acid than flooded batteries and offer better power density and cranking (short-duration power delivery) performance. “Recombinant” implies that at high recharge currents, some of the oxygen generated at the positive plates recombines with hydrogen from the negative plates, making it unnecessary to add water manually.
Despite the lack of battery caps, they aren’t really “sealed.” Although they won’t spill if tipped, there’s a safety valve that vents hydrogen if necessary. There are two kinds of VRLAs: gel batteries and absorbent glass mat (AGM) batteries.
Gel batteries use an electrolyte in which the sulfuric acid is mixed with silica, producing a gel. Alternatively, in AGM VRLA batteries, a glass fiber mat soaks up the electrolyte. Instead of an array of parallel plates, some AGM batteries are built like capacitors, with long, thin cells, wound into spirals.
Firefly Energy has developed a carbon foam-based lead-acid battery with an energy density of 30% to 40% more than the battery’s original 38-Wh/kg density.
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