Frequently Asked Questions
Which battery chemistries are best suited for my application?
When selecting a suitable battery for a given task, four distinct battery characteristics must be considered: endurance, size, cost and maintenance requirements.
What are the advantages and disadvantages of the Ni-MH?
Some of the distinct advantages of today's Ni-MH are:
30% more capacity over a standard Ni-Cd.
Less prone to memory than the Ni-Cd. Periodic exercise cycles need to be performed less often.
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:
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.
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.
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.
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.
How much can I discharge a battery?
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.
How often should I exercise and recondition my battery?
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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