Nickel-cadmium vs. sealed lead-acid

Nickel-Cadmium vs. Sealed Lead-Acid Facts and opinions to ponder May-June 1998 Recombinant gas lead-acid batteries have made considerable headway into the aviation marketplace in the last several years. It used to be that if you had a...

Cold Temperature Performance
Changes in the specific gravity of the electrolyte in any battery will radically affect the unit's ability to deliver and store energy. When discharged, completely or partially, the specific gravity of VRLAB is diluted by the conversion of sulfuric acid to water. This makes the VRLAB susceptible to performance degradation at colder temperatures.

With a lower specific gravity, the probability of freezing at extreme low temperatures is significantly increased. Because the total volume of electrolyte is limited in VRLAB (by the microporous glass mat separator), all of the sulfuric acid in the electrolyte is consumed during discharge. Overdischarging will turn all of the acid to water, increasing the probability of freezing.

A frozen VRLAB will not accept a charge. The electrolyte found in NCAB does not change significantly whether charged or discharged; therefore, it is not susceptible to freezing under normal circumstances.

Capacity Fade
Reduced capacity, caused by electrode degradation or reduced electrolyte volume/specific gravity, is a fact in VRLAB. Unfortunately, there is no adequate way to measure this reduced performance other than by performing a capacity test.

Some VRLAB manufacturers recommend a capacity check every six months for the first year. Others require a capacity check on a 600-hour basis. In addition, VRLAB manufacturers require that their product achieve only 80 percent of the nameplate rating to be considered airworthy. It is suggested that operators follow the airframe manufacturers' recommendations regarding performance evaluations of NCABs.

Additionally, methods have been developed by NCAB manufacturers to determine the optimum service interval for your particular operation. The minimum acceptable NCAB capacity is 85 percent of the nameplate rating. In some of the more recent designs, intended for today's demanding applications, the requirement is 100 percent.

Should reconditioning be required for a VRLAB, recharge times exceed 13 hours in most cases. Ni-Cad offers three different standard charge modes to accomplish the same reconditioning in as little as five hours. Average recharge time for a completely discharged NCAB is six hours. It is not recommended to completely discharge a VRLAB. In fact, one major VRLAB manufacturer recommends that capacity checks be closely monitored so as not to exceed 48 minutes. This manufacturer warns that discharges in excess of 48 minutes will shorten the life of the unit. If a VRLAB is completely discharged for over 48 hours, it is recommended that the battery be discarded. The chances of recovery from a long duration discharged condition are marginal. A NCAB can be completely discharged repeatedly and not suffer from irreversible damage.

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Sealed lead-acid batteries

Low cost/low maintenance

Interview with Skip Koss VP Marketing for Concorde Battery Corporation

May-June 1998

According to Skip Koss, vice president of marketing for Concorde Battery Corp. in West Covina, CA, "The expense in the Ni-Cad battery is related to two different areas: the initial cost, which is two to three times that of lead-acid batteries, and the continued maintenance costs, which primarily involves deep cycling and sometimes the replacement of cells.

What's the cost?
"Ni-Cad cells are very expensive," says Koss, "Anywhere from $100 to $300 each, and sometimes the battery requires more than three or more cells to be replaced. When looked at from the standpoint of continued maintenance costs, Ni-Cad batteries are very expensive to return to service.

"Quarterly costs of $150 to $200 for deep cycling means that you're spending $800 a year for deep cycling alone. This is almost the replacement cost of a sealed lead-acid battery for a one-year period. And if you use the lead-acid battery wisely, you can extend its life for two or more years, which makes it even more economical with little or no maintenance at all. That's the reason that there are so many people going to them," he says.

Koss explains that most of Concorde's sealed RG batteries have STCs which are required for installation, and there are some batteries which are original equipment now. "Cessna is using our batteries as original equipment on the Excel, Bravo, Citation X, and Cessna 208 Caravan. Also, Federal Express is converting over on an attrition basis from Ni-Cads to sealed lead-acid on their Caravans," Koss says.

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