ANALYZER/RECHARGER CAN EXTEND BATTERY LIFE

An advanced battery analyzer can improve the performance and reliability of rechargeable batteries commonly used in portable oil field equipment.

An automated battery analyzer/recharger developed by Cadex Electronics Inc., Burnaby, B.C., has improved rechargeable battery life and streamlined record keeping.

Light-duty batteries, such as those used in hand-held radios and gas detectors, typically weaken from a lack of regular use, or exercise, and the weakened batteries often fail prematurely. Unexpected downtime and excessive replacement costs are not uncommon by the time a rechargeable battery is 1-year old, said Isidor Buchmann, president of Cadex Electronics.

Cadex Electronics developed the C4000 battery analyzer to monitor rechargeable batteries, identify problems and inefficient use, and then restore the batteries to optimum operating condition.

The three most common rechargeable batteries are the nickel cadmium (NiCd), nickel metal hydroxide (NiMH), and lead acid. NiCd batteries are typically used in the oil industry because they recharge quickly, even after prolonged storage, supply large discharge currents, perform well at cold and hot temperatures, and can be cycled several thousand times.

Without regular maintenance, however, NiCd batteries can deteriorate and possibly malfunction in 1015 months of field service. In contrast, in an ideal operation such as in a satellite, the life span of a NiCd battery is 17 years or 70,000 charge/discharge cycles. This long service is achieved by a monthly discharge to 1 volt per cell.

The longevity of a NiCd battery is a function of its use. The batteries perform best at room temperature and require regular exercise (a discharge to 1 volt per cell) to remain in good condition.

For example, a company had to replace all its light-duty NiCd batteries after only 15 months of service because of a lack of regular exercise.

The company purchased new batteries but delayed implementation of an exercise program by 4 months. An analysis of the new batteries indicated half already had a memory-related capacity loss of 25-35%. (A NiCd battery has a cyclic memory; in other words, it "remembers" the amount of discharge required on the previous use. The batteries then revert to a lower oxide state or memory.) The capacity of the batteries dropped because they did not completely discharge each day. After several exercise and recondition cycles, the batteries were restored to 100% capacity.

Recent improvements in battery design help minimize the memory phenomenon, but the crystalline problem remains. Without proper exercise, the nickel and cadmium crystals grow, forming spikes and tree-like dendrites. The battery loses performance, and in advanced stages the crystalline dendrites can puncture the separator material, discharging the battery or causing an electrical short. Exercise can reverse the memory problem.

In advanced stages of memory, the battery must be reconditioned through a controlled deep discharge to less than the 1 volt per cell threshold.

Fig. 1 shows the effects of recharging four batteries afflicted with varying degrees of memory. Each battery was first fully recharged, then discharged to 1 volt per cell. An additional charge/discharge cycle was applied (dotted line), and the battery capacities were again evaluated.

Battery A responded well to exercise-typical of a battery in service for only a few months or exercised periodically. Batteries B and C required reconditioning (solid line) to recover full performance. The reconditioning of the new battery improved its capacity. After 6 months of field service, all the batteries still had excellent capacity.

ADVANCED ANALYZER

An advanced battery analyzer can reverse the degeneration process in a rechargeable battery and extend the service life through automatic maintenance, according to Buchmann. Analyzers capable of reconditioning can restore memory-affected NiCd batteries to full or near-full capacity.

In Cadex Electronics' C4000 battery analyzer, a reverse load technique allows it to fast-charge NiCd batteries. The reverse load method intersperses discharge currents between charge pulses to promote recombination of gases generated during fast charge. This method is cooler and more effective than conventional charging methods, said Buchmann. The memory phenomenon is also reduced because the battery is stimulated during charging.

A NiCd battery should not be left on trickle charge for prolonged periods because degeneration (memory) may occur. However, a battery on the shelf is subject to significant self-discharge. These two characteristics make it difficult to keep fully charged NiCd batteries ready for an emergency. A battery analyzer should maintain batteries at operational readiness: During trickle charge, the analyzer should automatically apply an exercise cycle every 3b days to prevent memory.

An important criterion for a battery analyzer is the need to accommodate a wide variety of battery types. The interchangeable battery adapters on the C4000 allow it to adapt to a wide variety of battery types. The adapters contain an embedded memory chip that stores battery parameters.

The configuration code. can be programmed into each chip, and the battery parameters are retained if the adapter is removed or if the power is turned off.

SELECTING AN ANALYZER

A battery analyzer should accommodate a variety of battery types, including NiCd, NiMH, and lead acid.
Batteries should not be overcharged by a recharger or analyzer. Some analyzers detect the full charge of a NiCd with the negative slope method only. Batteries with mismatched cells or other defects do not produce an adequate slope to terminate the fast charge reliably. In this case, a battery can be overcharged until it is damaged or destroyed.
An analyzer should be able to recondition batteries affected by memory. Simple charge/discharge cycles may not reverse advanced stages of memory.
The display should indicate the residual and final battery capacity to identify problems such as insufficient reserve capacity at the end of the work shift. The analyzer should also identify a battery with low voltage; the battery may have a shorted cell, yet its capacity reading may appear acceptable.
Nontechnical staff should be able to use it with ease. If the capacity readout is in percentage rather than milliampere hours (ma-hr), the user will not have to become familiar with the ma-hr ratings of each battery type.
An analyzer should have varying charge rates to accommodate standard and fast-charge batteries. Programmable charge rates allow the user to evaluate a battery within the range of the manufacturer's specified charge currents. Programmable units that allow setting of voltage, current, and other parameters are flexible and may not become obsolete if new batteries are introduced.