ONE OF THE COMMON DIFFICULTIES WITH BATTERY POWERED EQUIPMENT IS THE GRADUAL DETERIORATION IN PERFORMANCE AFTER THE FIRST YEAR OF SERVICE. ALTHOUGH FULLY CHARGED, THE BATTERY'S PERFORMANCE MAY HAVE DROPPED TO HALF THE ORIGINAL CAPACITY, RESULTING IN UNEXPECTED DOWN TIME. WITHOUT KNOWING THE REASON FOR THE FAILURE, THE USER SENDS IN THE EQUIPMENT FOR SERVICE, ONLY TO FIND OUT THAT THE PROBLEM HAS NOT BEEN SOLVED. SERVICE CENTRES HAVE INDICATED THAT HALF OF THE EQUIPMENT FAILURES ARE BATTERY RELATED.
THE FIRST PART OF THIS TWO-PART ARTICLE BY ISIDOR BUCHMANN, FOCUSES ON THE NEEDS OF THE DIFFERENT BATTERY CHEMISTRIES, WHAT APPLICATIONS ARE SUITABLE AND HOW ONE CAM GET THE MOST OUT OF THEM.
Among the rechargeable batteries, the NiCd remains the most popular choice. Some of its distinct advantages over other battery chemistries are:
The NiCd is a reliable worker. Hard work poses no problem. It prefers fastcharge over trickle- charge and pulsecharge over d.c. charge.Improved performance is achieved by interspersing discharge puIses between charge pulses during 'the charging process. This charge method is commonly referred to as Reflex or Reverse Load charge. The brief discharge currents promote the recombination of gases generated during fastcharge. This type of charge method results in a cooler and more effective charge than can be obtained with conventional d.c. chargers. A study by a German battery manufacturer has shown that the Reverse Load charge method adds 15% to the life of the NiCd battery.
The NiCd does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods. In fact, the NiCd is the only battery type that performs best if periodically fully discharged. All other battery chemistries prefer shallow discharges. So important is this periodic full-discharge that, if omitted, the NiCd gradually loses performance due to voltage depression or "memory".
The NIMH has been heralded as the shining star that will solve the battery problems of the 20th century and lead us into the 21st. While some of the claims for are over-optimistic, the NIMH has distinct advantages over the NiCd.
We have had good test results with the Japanese prismatic NIMH cell used by NTT for a line of cellular phones. Stable results have also been achieved with the Motorola NIMH replacement batteries. However, another brand of NIMH cells from the Pacific Rim do not offer the same performance.
Another commonly-used battery is the lead-acid type and here it is the sealed Lead Acid (SLA) which is of interest, rather than the flooded type found in motor cars.
The SLA is commonly used when high power is required, weight is not critical and cost must be kept low. The typical current range of the SLA is 2Ah to 3OAh. Applications that fall into this category are wheel chairs, uninterruptable power supply (UPS) units and emergency lighting. There are also some transportable cellular phones, laptop computers and camcorders that use SLA batteries. During the eighties, the SLA gained ground in the biomedical industry.
The SLA is not subject to memory. No harm is done by leaving the battery on trickle or float charge for a prolonged time. If removed from the charger, the SLA retains the charge for a longer period than the NiCd and NIMH. The SLA is usually lower priced per Ah than other rechargeable batteries.
On the negative side, the SLA does not lend itself well to fast charging. Typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state. A discharged SLA will sulphate within hours. if left in that condition, a re-charge is difficult or impossible. Unlike the NiCd, the SLA prefers a shallow discharge. A full discharge causes extra strain and reduces the cycle count. The number of discharge/charge is limited to 200 to 300 cycles. An additional difficulty of the SLA is a relatively low load current.
The load current is further reduced at lower temperatures. Compared to other@rechargeables, the energy density is low, making the SLA unsuitable for cellular phones and other devices that demand small size. Because of its high lead content, the SLA is not environmentally friendly.
The rechargeable lithium battery is the most talked-about battery chemistry in research labs today. One of the biggest advantages of the lithium battery is its high energy density. Indeed, the lithium weighs less than half of the equivalent NiCd. Equally exciting is the fact that once charged, the lithium retains its energy for up to 10 years.
In several ways, the Lithium shares the same negative characteristics of the SLA. Charge times are a long 8 to 16 hours; the discharge current must be kept low. The cycle count is only 150 to 300 and is related to the depth of discharge.
The rechargeable Lithium battery is fairly expensive and will likely continue to cost more than its rivals. Disposal may cause some problems. The concern is not its toxic metal content but the danger of explosion during corrosion as a direct result of exposure to moisture.
On earlier versions of rechargeable Lithium batteries, stability was a major concern. This difficulty has now supposedly been solved with the use of solid lithium polymer. Battery experts speculate that it will take another three to five years before the rechargeable lithium battery will be readily available.
There is some misconception about the word "memory". Memory is commonly blamed for just about any battery failure known to man. The word .memory" is derived from "cyclic memory", meaning that a NiCd battery could remember how much discharge was required on previous discharges. improvements in battery technology have virtually eliminated this phenomenon. Tests performed at a Black & Decker lab, for example, showed that the effects of the "cyclic memory- were so minute that they could only be detected with sensitive instruments. After the same battery was discharged for different lengths of time, the cyclic memory phenomenon could no longer be detected.
The problem with the modern NiCd battery is not the cyclic memory but by the effects of crystalline formation. (When we refer to memory, we are referring to the formation of crystals.) The active materials (nickel & cadmium) of a NiCd battery are present in crystalline form. When the memory Phenomenon occurs, these crystals grow, forming spike or tree-like crystals that cause the NiCd to gradually lose performance. in advanced stages, these crystals may puncture the separator, causing high self-discharge or an electrical short.
The crystalline effect on the NiCd cell plates is similar to the crystal formation during use of calcium chloride to clear roads in the winter. As the salt absorbs water, crystal formation can be observed.
If no exercise is applied for several months, the crystals engrain themselves, making it more difficult to dissolve. In such a case, exercise is no longer effective in restoring a battery and "recondition" is required. Recondition is a slow, deep discharge that drains the cell to a voltage threshold below one volt must be discharged to at least 0.6 volts per cell to dissolve the more resistive crystalline build-up.
Not all batteries respond well to recondition. Let's examine a battery that has been in service fbr over a year and had not been exercised regularly. Its capacity reading may have dropped to a low 20%. Even with repeated recondition cycles, the battery may not improve to an acceptable capacity.
An older type battery may even get worse with recondition. If this occurs, the battery is a candidate for retirement. This type of battery may be compared to an old man to whom a vigorous exercise is harmful. On the other hand, there are older NiCd batteries that, to everybody's amazement, recover to near original capacity. Caution should be exercised when re-hiring these old-timers, as they may exhibit high self-discharge.
The NiCd, and in particular the NIMH, have a relatively high self-discharge, if left on the shelf, a new NiCd looses about 10% of its capacity in 24 hours. A problem arises if the self-discharge causes the battery to drain within a day.
A high self-discharge is caused by a damaged separator. The separator is a thin insulator that isolates the positive and negative cell plates. Once injured, the separator can no longer be repaired. External forces that harm the sensitive separator are: uncontrolled crystalline formation due to lack of exercise, poorly designed chargers that boil the battery and plain old age.
Let us examine briefly the construction of the NiCd cell. The negative and positive plates are rolled up together and placed into a metal cylinder. The positive plate is sintered and filled with nickel hydroxide. The negative plate is coated with cadmium active material. The two plates are isolated by the separator that is moistened with the electrolyte.
To check the self-discharge of a battery, a battery analyzer should be used. The self-discharge is obtained by first measuring the capacity of the fully charged battery by applying a discharge. The battery is then stored for 24 hours and the capacity is measured again. If the capacity loss during the rest period is more than 30%, the battery should be discarded.
A battery analyzer is useful not only to measure self-discharge, but more importantly, to exercise and recondition batteries to prolong the service life. In addition, an analyzer helps to identify and "weed out" weak batteries.
In the last few years, the technology of battery analysers has advanced significantly. No longer are we limited to analog units with fixed charge and discharge currents and programs that do nothing more than apply one or two discharge-charge cycles.
The modern battery analyzer (see photograph) evaluates the condition of a battery and implements the appropriate cycles to maximize its performance. A recondition cycle is applied automatically if a set minimum capacity cannot be reached. Battery chemistry, voltage and current rates are user- programmable. These parameters are stored in cups or cables and configure the analyzer to the correct function when connected. Over 400 battery specific cups are available.
It will also identify battery packs that contain shorted, mismatched or soft cells. The derived battery capacity readings are organized into residual, intermediate and final capacities. Problems, such as insufficient capacity reserve at the end of the work shift, can easily be identified and corrected. An analyzer is able to charge and discharge the battery at user-defined currents. This feature allows testing the battery under true field conditions. A charge and discharge rate of 1C* is up to four times faster than that of a fixed current analyzer. in addition, the capacity readings are more accurate.
Modern analysers offer well defined software that allows safe charging of all batteries, including those that are known to be faulty. Damaging overcharge on a pack with mismatched cells, for example, is eliminated.
User-selectable programs are include on some analysers to address the different battery needs. For example, PRIME prepares a new battery,for field use, AUTO reconditions those batteries that are Unable to reach a set target capacity. In addition, CUSTOM allows the operator to set a sequence of cycle modes composed of charge, discharge, recondition, trickle charge or any combination thereof. Rest periods and repeats can be added as required.
Easy to use, the battery analyser provides, for example, capacity readouts in percentage rather than milliampere hours (mAh) which is more convenient as the operator does not need to remember the mAh ratings of each battery tested. In addition, simple 'pass/fail" lights help to distinguish good batteries from unserviceable ones at a glance.
In addition the Cadex C4000 battery analyzer offers a printer and computer interface. With printer, service reports and battery stick-on labels showing time and date can be generated. An RS232 port allows a computer to store and process battery test results in a data base.
Designated charge algorithms and specific cycle programs for different battery chemistries, such as the NiCd, NIMH and Lead Acid are also included. As new chemistries are introduced, EPROM upgrades should be made available to enable servicing of these batteries.