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- Which battery chemistries are best suited
for my application?
- What are the advantages and disadvantages of
the Ni-MH?
- How much can I discharge a battery?
- How often should I exercise and recondition
my battery?
For more detailed information on rechargable batteries, please refer to
WWW.CADEX.COM or WWW.BUCHMANN.CA.
When selecting a suitable battery for a given task, four distinct
battery characteristics must be considered: endurance, size, cost and
maintenance requirements.
Some of the distinct advantages of today's Ni-MH are:
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30%
more capacity over a standard Ni-Cd. |
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Less
prone to memory than the Ni-Cd. Periodic exercise cycles need
to be performed less often. |
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Fewer
toxic metals. The Ni-MH is currently labeled "environmentally
friendly". |
Unfortunately, the Ni-MH also exhibits some negative attributes and
in some aspects lags behind the NiCd. For example:
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Number
of cycles: The Ni-MH is rated for only 500 charge/discharge
cycles. Shallow rather than deep discharge cycles are preferred.
The battery's longevity is directly related to the depth of
discharge. |
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Fast
charge: The Ni-MH generates considerably more heat during
charge and requires a more complex algorithm for full-charge
detection than the Ni-Cd if temperature sensing is not available.
(Most NiMH batteries are equipped with internal temperature
sensing to assist in full-charge detection). In addition,
the Ni-MH cannot accept as fast a charge as the Ni-Cd; its
charge time is typically double that of the Ni-Cd. The trickle
charge must be controlled more carefully than on the Ni-Cd. |
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Discharge
current: The recommended discharge current of the Ni-MH is
considerably less than that of the Ni-Cd. For applications
demanding high power or a pulsed load, such as on GSM digital
cellular phones, portable transceivers and power tools, the
more rugged Ni-Cd is the recommended choice. |
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Self-discharge:
Both Ni-MH and Ni-Cd are affected by reasonably high self-discharge.
The Ni-Cd loses about 10% of its capacity within the first
24 hours, after which the self-discharge settles to about
10% per month. |
The commonly used end-of-discharge voltage for the Ni-Cd and Ni-MH is
one volt per cell. At that voltage level, most of the energy is spent
and the voltage starts to drop rapidly. Discharging a battery further
could damage the battery through cell reversal. Caution should be
exercised when discharging a battery too deeply under heavy load.
Since the cells in a battery pack cannot be perfectly matched, a
negative voltage potential (cell reversal) across a weak cell may occur
if the discharge is allowed to continue below one volt per cell. A N-iCd
can tolerate a limited amount of cell reveral. However, if a high
current is drawn at the same time, the affected cell will likely develop
an electrical short circuit.
The effects of crystalline formation (memory) are most pronounced if
a Ni-Cd battery is left in the charger for days, or is repeatedly
recharged without a periodic full discharge. Since most applications do
not use up all energy before recharging, a periodic discharge to one
volt per cell (known as exercise) is essential for the Ni-Cd to prevent
the buildup of crystalline formation on the cell plates. All Ni-Cd
batteries that are in regular use or on standby mode (sitting in a
charger for operational readiness) should be exercised once per month.
Nothing else needs to be done between these monthly exercise cycles; the
battery can be used with any desired user pattern without the concern of
memory. The Ni-MH is also affected by memory but to a lesser degree--it
only needs exercise once every three months.
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