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At Omnica we are always
talking about batteries.
We have many requests for developing handheld devices, and most of
them require batteries as a power source. But
there is always a trade-off between duration of operation and size and
weight. For some reason, many of our customers have difficulty
understanding the concept. Except in science fiction, there is no way to
compactly store the massive amounts of power required for futuristic
devices like phasers or hand-held arc welders. Many of those use
replaceable alkaline cells, but for rechargeable applications, there are
basically three choices: Nickel-cadmium,
Nickel-metal hydride, and Lithium-ion.
Lithium-ion cells are
currently considered to have the best battery chemistry.
They are the most widely researched of all portable cell
technologies. Development of lithium batteries started in the 1980's, but
it wasn't until the mid 90's that they were produced and shipped in
commercially available quantities. Early
versions of the batteries had a significant drawback; they could explode
after repeated recharging! It
was a rare occurrence, but it wasn't until their fourth generation that
the safety hazard was eliminated. Today's
Lithium-ion batteries have integral microcircuits (adding cost) that
prevent overcharging and over discharging.
They contain no liquid, and are therefore leak-free, making them
more environmentally friendly than other types of batteries.
Lithium has the highest
electrochemical potential of any metal.
With a 125Wh/kg energy density, they can provide twice the wattage
of a NiM-H and 3 times more than a Ni-Cad. But
energy density comes at a price. These
powerful cells cost about 4 times more than a comparable Ni-Cad.
Li-ion batteries are the
power supply of choice for cell phones and laptops, but they don't provide
the electrical current needed for power tools. As the technology improves,
this problem should be solved. Nickel-metal
hydrides provide high current, so they are the best for high-drain
applications. Their significant drawback is a short
shelf-life, much shorter than the years-long shelf lives of Li-ion
batteries. Ni-Cad self-discharge 40% of their stored energy in three
months. Unused NiM-H cells will discharge completely over a period of
weeks.
Regarding lithium-ion
shelf life, one of our engineers recently met with a battery-house
representative. It has come to their attention that repeated charging and
re-charging of a less than fully discharged Li-ion cell affects the
battery "gas gauge" included with many devices. Over a number of
cycles there is a resultant loss in accuracy. Frequently, the indicated
remaining charge has little relationship to the actual amount of life left
in the cell! Fortunately, according to the battery expert, the gauge will
reset itself if the battery is allowed to completely discharge from
time-to-time.
Li-ion
cells have now been on the market long enough to establish a good safety
record. Prices are still too high
for the batteries to sell in all markets, but for medical devices, where
reliability and longevity are most important, the coming years are
expected to prove Lithium-ions the preferred power source.
As prices drop they will begin to replace the next best cell
chemistries.
Cost
aside, when considering which battery is best for your application, there
is at least one valid reason for choosing a replaceable battery rather
than a rechargeable. Even with all the
exotic, powerful batteries and unique shapes available, we still try to
design products that use a power source that’s easily
replaceable. Common sizes like AA, AAA, and 9
volt types are available everywhere. Have you ever tried to find a
special-sized lithium camera battery while on vacation?
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