SANYO.DEC–sc
Nickel-metal-hydride aims at demanding markets
Emerging battery technology offers higher energy density and smaller
size
BY JOSEPH A. CARCONE Sanyo Energy (USA) Corp. San Diego, CA
Until now, portable microprocessor-based products like notebook
computers and cellular telephones have been greatly handicapped by the
size and energy density afforded by existing battery technology.
Nickel-metal-hydride (NiMH) batteries–with a capacity nearly twice that
of nickel-cadmium (NiCd) batteries–meet the stringent demands of these
products. NiCd batteries use a hydrogen-absorbing alloy as the negative
electrode. They are now available from Sanyo, Gates, and other companies
typically supply run times of 0.5 to 5.0 hours at battery power
requirements of 3.0 to 35 Wh. They typically feature 500 charge-discharge
cycles and have good discharge characteristics at 3C currents (C
represents the battery's nominal capacity). The table lists NiMH batteries
now available from Sanyo. Many applications requiring light weight are
highly suited for NiMH batteries. Early versions are intended as power
sources for communications equipment and laptop computers; but the
batteries will also find their way into home appliances like camcorders
and shavers. Eventually, as NiMH batteries replace NiCd modules in certain
areas, applications will expand. NiCd and NiMH will carve out their own
niches in the market.
Performance of NiMH batteries Compared to Sanyo's Cadnica NiCd batteries
for general use, Sanyo's NiMH batteries have 1.8 times the capacity and have
good discharge characteristics at 3C currents. In addition, they fully
recharge in 1.5 hours, using a special charger. Figure 1 shows the charge
characteristics of NiMH: charging takes place from 0.1 to 1 C (1/10 to the
battery's full rated capacity). As the figure illustrates, larger charge
currents produce steeper rising curves for the cell voltage. At full
charge, though, the unit reaches saturation and then declines. Figure 2
compares the discharge characteristics of Sanyo's HR-AA NiMH battery and
Cadnica NiCd battery. The HR-AA offers 1.8 times the capacity of the NiCd
battery. Because the discharge potentials of hydrogen-absorbing alloys and
cadmium electrodes are nearly the same, both NiCd and NiMH batteries
operate at 1.2 V. NiMH batteries exhibit stable characteristics at room
temperature, even with rapid charging. Unlike their NiCd counterparts,
NiMH batteries do not suffer from dendrite shorting. This is an operating
condition characteristic of NiCd batteries in which needlelike cadmium
growths form at the negative electrode, possibly leading to a short
circuit.
Battery characteristics The NiMH battery's cell structure is completely
sealed to prevent electrolyte leaks and requires no maintenance. While the
voltages of NiMH batteries are identical to those for NiCd batteries, they
now cost twice as much as NiCd batteries. They also discharge faster than
NiCd batteries and have charging systems that are sometimes incompatible
with NiCd batteries.
Field testing and laboratory experiments have shown that extensive
constant-current overcharge at room and elevated temperatures greatly
accelerates limiting failure mechanisms (Author: What are “limited failure
mechanisms”). One solution is to use a pulsed (5% daily duty cycle)
overcharge regimen instead of the constant-current techniques readily used
for NiCd systems. NiMH battery makers are working toward solving these
problems and improve NiMH battery performance. They are looking to develop
hydrogen-absorbing alloys with improved anti-corrosion characteristics.
These alloys blend various metals, such as lathanum and nickel, and misch
metal (a mixture of rare earth elements). One formulation eliminates the
nickel portion in favor of a multicomponent alloy of misch metal, nickel,
cobalt, aluminum, and manganese. These alloys produce batteries with large
discharge currents. Developers are also working on nickel electrodes with
large capacities and new manufacturing methods.
CAPTIONS:
OPENING SHOT: Nickel-metal-hydride batteries like these promise higher
energy density than nickel-cadmium batteries.
Fig. 1. As charging input rises, cell voltage for the NiMH battery rises
more sharply than cell temperature at 0.1C to 1C (1/10 to the battery's
full rated capacity).
Fig. 2. Discharge curves for the model HR-AA NiMH battery are compared
with a NiCd battery, the N-600AA.
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