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THE CORRECT BATTERY FOR THE RIGHT JOB


When selecting a suitable battery for a given task, four distinct battery characteristics must be considered, namely endurance, size, cost and maintenance requirements.

For applications requiring a rigorous recharging scheme, the NiCd is by far the best-suited battery. In an industry that runs on three shifts and requires multiple recharges per day, for example, a NiMH would only last about six months, a period that would not even outlive the manufacturer’s warranty period. Applications that may require several recharge cycles within 24 hours are power tools, emergency medical equipment, professional video cameras and portable radios.

Many Public Safety Sectors that have experimented with the NiMH are switching back to the NiCd because the NiMH battery in a portable radio simply cannot provide the cycle count needed. During transmit, a high-power radio draws two or more amperes of current and the NiMH is unable to deliver the energy over the expected service life of the battery. The NiMH may perform satisfactorily when new, but does not allow for any performance degradation that occurs after a few months of rigorous use. For this reason, a large manufacturer of telecommunications devices recommends NiCd batteries for their portable radios and makes the NiMH available on special request only.

Another user group that relies on the high current capabilities, sheer endurance and quick charge acceptance of the NiCd battery are model race car enthusiasts. For this application, no other battery system comes close to the performance of the NiCd.

When calculating the correct battery rating for mission critical applications, a degradation factor caused by aging of the battery should be taken into account. Similarly, a reserve capacity to allow for unforeseen activities should be added. By taking a degradation factor of 20% from the top and permitting a reserve capacity of 20% at the bottom, the real usable capacity is reduced to only 60% (based on a total of 100%). Equipment manufacturers should consider these safety factors when choosing a battery. The best choice is not necessarily an oversized battery, but one that has sufficient safety margin and is well maintained.

For applications such as cellular phones and notebook computers, small battery size and long play time are important. In an effort to sell on compact size, some cellular phones are equipped with batteries too small to handle the job reliably. Some GSM phones (digital cellular phone used in Europe and Asia), for example, are supplied with a small prismatic NiMH battery that must deliver between 1.4 and 1.7 amperes of pulsed current during use. Such a battery cannot provide the required current throughout its expected service life. The battery may function satisfactorily when new but does not allow for declining performance that occurs naturally with age. A larger pack containing cylindrical NiCd or NiMH cells would provide better results for cellular phone applications than the ultra-thin prismatic type.

Most electronics equipment, including cellular phones, feature a battery indicator that reads "low battery" when the battery voltage is low. Depending on the make or model, the cut off occurs when the voltage of a NiCd or NiMH battery cell drops to an average voltage between 1.15V to 1.00V per cell. A cut-off voltage of 1.15V, for example, is high when considering the nominal cell voltage of 1.25V. Adding the heavy current demand of a cell phone, warm temperature that depresses the voltage and the inherently high self-discharge of a rechargeable battery, the usable power of a battery may become rather tight if the cut off voltage is set above one volt per cell.

Equipment that does not make full use of the available battery power by cutting off before the cells are discharged to one volt drastically reduces the available run time. Tests on batteries from a certain brand of cellular phone that cut off at 1.15 volts per cell measured residual capacities of 60% and more. Not only does a high cut off voltage withhold valuable battery energy but it inflicts harm to a NiCd over time because the battery is not being exercised through its normal use. Such a battery should periodically be discharged to one volt per cell by some other means to prevent memory (see 4.6 Battery Analyzer).

Another important aspect when choosing a battery type is its maintenance requirement, an issue that is commonly overlooked . Depending on the chemistry, battery maintenance may consist of a periodic full discharge for NiCd and NiMH batteries and a topping charge for the SLA. If maintenance is omitted, the battery commonly fails to deliver the specified performance over its expected service life.

With proper maintenance, a NiCd should outlive a NiMH by a factor of three in terms of cycle life. When asked if the NiCd is truly superior to the NiMH, many battery users feel that neither chemistry stands up to their expectations. The cause of poor NiCd performance is primarily due to insufficient battery maintenance and poorly designed chargers. The quality of the cell also plays an important factor.

Observation: Using good quality cells and a charge algorithm developed by Cadex, NiCd batteries that are used to test and burn-in the Cadex battery analyzers not only meet the anticipated 1500 cycles but consistently get over 4000 full discharge/charge cycles.

Battery maintenance continues to pose a major concern for manufacturers and users alike. In a desperate attempt to find a so-called maintenance-free battery, some manufacturers went so far as to equip laptop computers and video cameras with sealed lead acid, a battery that does not need regular deep discharge. However, the SLA proved to be unsuitable for the applications intended because of its low energy density and short cycle life. The notion of having to apply regular maintenance becomes an acceptable alternative if, through this procedure, the battery performance is improved and the operational costs lowered.

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