DN1• 4
INTRODUCTION
With today’s ever changing technologies, more appliances depend on battery
power to enable their use. While this gives us greater freedom, it is often at
the high cost of purchasing portable energy, or batteries, to run our portable
electronic gismos. Consider the cost of energy from our local electric
company, about 8¢ for a KW hour, or about 450,000 joules of energy for a
penny. On the other hand, that 500 mA-H NiCad that you just purchased for
about $1.75 can only supply 2250 joules of energy; that's about 13 joules for
1 cent. So it’s fairly easy to see that energy costs about 35,000 times more
when it’s in a battery.
Nobody likes the idea of throwing all those batteries into a landfill. That's the
reason for the recent emphasis on using “green” rechargeable cells. If a set
of NiCad cells lasts you for a few months, they can save the equivalent
volume of themselves many tens or hundred times in the trash. This is not
only good for the environment, it’s also great for the wallet!
Nicad rechargeable batteries have been around for years, but there are a few
real disadvantages in their use. They usually require a long time (sixteen
hours) to recharge. This “trickle charge” arrangement is quite common
because it is much cheaper for the original product manufacturer to produce
(the entire battery charger is typically a couple of rectifier diodes and a
current limiting resistor), and works well given the draw back of a long charge
time.
Another disadvantage to the “plug-in wall transformer charger” is that the
charging cutoff action is regulated by the heat produced by the cells’
chemical reaction when recharging. If you’ve ever opened up a rechargeable
pack you have probably seen the thermal shutoff “mystery part” connected
and mechanically touching one cell of the battery pack. While this will help if
you leave your appliance charging for several days, notice that it is sampling
only one cell in the pack, and assuming that the rest of the batteries are
“behaving” in the the same manner. Also, since the ambient temperature can
change (i.e.recharging your cordless drill in the cool garage or basement, or
your two way radio on the hot seat in the car), this heat sensing approach
can vary considerably from undercharging your pack to overcharging until
you “cook” the electrolyte solution right out of the battery.
Often times we cannot wait for the full recommended charging time or do not
use the batteries until they’re completely “dead”. When this is repeated, the
uncared for battery or pack can seem to “run out” rather quickly. This effect is
caused by not completely discharging the cell before it is recharged and is
known as the memory effect, since the battery appears to memorize the
amount of energy it is called upon to produce.. By not completing the