Specifications

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As this device will be used by a member of the general public with little to no
training or formal instruction on the device, usability is paramount; as such, a
single-use cell is an option. Given the limited voltages in cell sizes such as AAA
and AA, a 9V battery would be the most practical.
However, the continuous cost to the user may be high, and it also introduces an
additional point of failure into the device – a battery door, as well as exposed
electronics that could be stressed and worn over time to eventually lead to a
failed portable unit. It also a relatively wasteful choice, as the user would have to
contribute to the amount of battery waste in the environment. As a principle of
modern engineering, environmental concerns must also take priority; as such, a
rechargeable battery is a wiser choice.
One option for rechargeable alkaline batteries is rechargeable AAs. However, as
mentioned previously, these retain the disadvantages commonly experienced
with rechargeable alkaline batteries. In addition, a non-rechargeable AA
generally produces about 1.5V output. In order to achieve the voltages necessary
for optimal performance of the portable device, multiple batteries would have to
be used. This would increase the weight, complexity, and size of the device.
2.11.2 Nickel Cadmium Battery
Nickel cadmium batteries have been a popular choice for many handheld
electronics requiring portable power. They are widely available in a variety of
formats, and are a mature technology. In addition, the charging technology is well
understood. It offers the advantages of being able to handle a high number of
discharge and charging cycles; this is important so that the battery can be used
as long as possible (as far as recharge cycles) before needing replacement.
In addition, nickel cadmium batteries can withstand deep discharging and
recharging better than most rechargeable batteries. This may prove to be useful
as it is possible the device may be stored for a significant period of time on low or
no battery before being charged. However, with this comes another issue unique
to nickel cadmium batteries. This issue is commonly referred to as the “memory
effect.” While it is not true that the actual capacity of the battery gets reduced if
the battery is discharged and charged to the same level repeatedly, it does set
up a sort of “memory” where the battery voltage will drop rapidly at the point
where it is discharged to repeatedly. This will cause any battery monitoring circuit
to detect a condition of “low” battery, although technically speaking, the
“capacity” of the battery has not changed (if effects due to aging are ignored).
Considering the possible use of the portable device (being charged after only
being used for a short while), this would not be a desirable characteristic. One
possible solution would be to fully discharge the battery occasionally. The issue
with this is that the better functioning cells in the nickel cadmium battery pack will
affect the weaker cells within the pack by essentially reversing their polarity. In
subsequent uses, the weaker cells will deplete first, and when charged yet again,