Nickel-cadmium vs. sealed lead-acid

May 1, 1998

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 particular aircraft, the size of the aircraft and the battery capacity requirements pretty much dictated which type of replacement battery you needed. Typically, the larger turbine-powered aircraft required high capacity nickel-cadmium batteries.

Today, technological developments in the lead-acid battery arena have resulted in larger "sealed," "maintenance-free," or "recombinant gas (RG)" batteries that are giving Ni-Cads a run for their money.

As expected, the manufacturers of the respective batteries have taken strong stands on the advantages, safety, cost, and performance of their respective products. Each claims that the cost over the long run is less than the other. Ni-Cad manufacturers claim that their batteries have a longer service life, longer shelf life, and are more easily maintained. The sealed lead-acid manufacturers say their products don't have to be maintained, are a fraction of the cost of Ni-Cads, and their performance is at par or better than the Ni-Cads.

Each has valid research to point to, and each uses solid logic to explain their opinions on the subject. Unfortunately, the water is muddied by the fact that different manufacturers use different specifications and testing parameters to prove their product's performance.

As a result, the end user is faced with taking the manufacturers' claims at face value or actually trying both. The following is intended to give you a leg up on the issues surrounding both types of batteries. It is presented in two parts: In Part 1, John McCoy, from Saft Aviation Batteries, one of the leading Ni-Cad battery manufacturers, reviews facts and opinion about both battery types. In Part 2, AMT interviews Skip Koss of Concorde Battery, one of the leading manufacturers of RG lead-acid batteries. Both articles represent the opinions of the respective parties and in no way represents AMT's viewpoint on the subject.

Sealed lead-acid batteries

What you need to know about Nickel Cadmium and Valve regulated Lead-Acid batteries

By John McCoy Sales Manager, Aviation, for Saft Aviation Batteries

May-June 1998

This discussion outlines the differences between nickel-cadmium aircraft batteries (NCAB) and valve regulated lead-acid batteries (VRLAB).

Construction — Valve Regulated Lead-Acid Batteries
In all electrochemical couples, there is a negative (anode) and positive (cathode) element. In VRLABs, these elements consist of pure lead (negative) and lead dioxide (positive). The positive and negative electrodes are prevented from coming into physical contact by a separator mechanism to prevent short circuits. In this case, the separator is the microporous glass mat filled with a solution of sulfuric acid and water.

Aviation VRLABs do not employ "gelled" electrolyte. Gelled electrolyte combines silica with sulfuric acid and water to limit the fluidity of the electrolyte. VRLABs typically "trap" the electrolyte in a microporous glass mat. The electrolyte in a VRLAB is the same as found in a flooded lead-acid battery, only there is less of it.

Measurement of the specific gravity of the electrolyte is a good indicator for state of charge estimation in lead-acid batteries. Unfortunately, the specific gravity of the electrolyte is not measurable in VRLABs. VRLAB design does not allow access to the electrolyte, therefore, the phrase "maintenance- free." Electrolyte replenishment is not possible in VRLABs.

Electrochemistry — Valve Regulated Lead Acid Batteries
Some within the industry identify valve regulated lead-acid batteries as sealed. VRLABs vent when overcharged. During discharge, the pure lead, lead dioxide, and sulfuric acid in the electrolyte interact to form lead sulfate and water.

During the charge sequence, the lead sulfate component is converted to the original state and sulfuric acid is reintroduced into the electrolyte.

Under ideal conditions, both negative and positive electrodes would have 100 percent charge efficiencies. This is not the case. In the real world, each electrode has different efficiencies.

When the conversion of lead sulfate is complete, on the more efficient (negative) electrode, hydrogen gas evolves as the water in the electrolyte begins to break down. Oxygen evolves from the positive electrode shortly thereafter. If the pressure created by gassing reaches a high enough value, the gases are vented overboard, hence the term "valve regulated." Prior to venting, the hydrogen gas is collected within the container. Venting reduces the overall volume of electrolyte and will eventually result in cell dry out.

A 10 percent reduction in electrolyte volume will translate into a 20 percent reduction in performance. The valve in VRLAB technology is intended to assist recombination of oxygen, thereby reducing the amount of water hydrolyzed during the charge process. It is inevitable that some of the generated gasses will escape, however, because the reaction is not 100 percent efficient.

Electrochemistry — Nickel Cadmium Aircraft Batteries

During charge, the active material (cadmium hydroxide) of the negative plate is converted to pure cadmium. Nickel hydroxide in the positive plate is converted to nickelic oxyhydroxides. The electrolyte is not directly involved in the charge/discharge process but serves as a medium for ionic transfer. Specific gravity of the potassium hydroxide/water solution remains virtually unchanged during these processes.

Operations - Valve Regulated Lead-Acid Batteries

During constant current discharge, the VRLAB exhibits a continuous reduction in voltage. This is dependent on the conversion rate of the active materials in the plates and electrolyte as the discharge continues. Here again, temperature has significant influence on discharge characteristics. At the end of discharge there is a sharp reduction in voltage. Basically, the unit runs out of active materials to convert. Both positive and negative electrodes are reduced to lead sulfate and the electrolyte is reduced to water.

During recharge, an initial sharp voltage rise occurs. This is primarily due to the change in internal resistance as the lead sulfate begins to reconvert. As the charge continues, the voltage rate-of-change slows to a gradual upward slope — as the specific gravity of the electrolyte increases.

At the end of charge, another sharp voltage rise is observed as most of the lead sulfate is converted to its original state and gas begins to form on the surface of the electrodes. Water begins to break down at this point. Substantial gassing occurs as the overcharge continues. The sharp voltage rise and gas production is due to the inequitable charge characteristics of the negative and positive plate.

Operations - Nickel Cadmium Aircraft Batteries
During constant current discharge, a NCAB will provide an almost flat curve, with a slight downward slope. When most, if not all, of the active materials convert, as discussed in the electro-chemistry section, the voltage then begins to drop off rapidly. The electrolyte remains a solution of potassium hydroxide and water.

During recharge, a significant voltage rise is observed, followed by a slow continuous rise to a transition voltage. At this point the negative plate is fully charged and begins to generate hydrogen gas. As the over-charge continues, a rapid voltage increase is observed due to the increased impedance of the cell.

Shortly thereafter, oxygen begins to evolve at the positive plate. Little or no recombination actually takes place. This results in the water of the electrolyte being broken down by hydrolysis. The act of reversing current flow, or charging as it is commonly known, in an electro-chemical couple is endothermic by nature.

Charging cools up to the point of gas generation/recombination. At the onset of gassing the reaction changes from endothermic to exothermic. In other words, the cell/battery begins to generate heat.

Whenever gassing occurs, the temperature increases presenting the potential for thermal run-away regardless of electro-chemistry. The higher the recombination rates, the greater the potential for thermal run-away during charge.

Failure Modes
Thermal run-away (Vicious Cycling) Thermal run-away is a potential problem in any electro-chemistry that involves gaseous evolution and/or recombination. This is especially true at higher temperatures. Oxygen recombination, high temperature operating conditions, and a limited amount of free electrolyte (acting as a heat sink) all contribute to the probability of thermal run-away. Any increase in the temperature of an electro-chemical couple will reduce overall resistance.

In a constant potential (DC) system, a decrease in impedance will result in an incremental increase in current draw. This in turn increases gas generation and the cycle accelerates. Eventually the heat generated will become destructive. Some VRLAB manufacturers limit operation above 84 F (30 C), warning that long-term exposure to temperatures above this value can shorten battery life.

Advanced NCAB manufacturing techniques and improved materials have significantly reduced the likelihood of thermal run-away in today's NCAB. In addition, the significantly larger electrolyte reserves in NCAB provide excellent heat dissipation.

The introduction of temperature sensors and 20 cell batteries in the early 1970s also had a significant impact in reducing the incidence of thermal run-away. True incidents of thermal run-away have been virtually eliminated since then. It is important to note that a hot battery is not necessarily an indication that a thermal event has occurred.

Reduced Performance
All lead-acid batteries are subject to the same physical laws regardless of whether a flooded variety or valve regulated. The chemical process is the same. VRLABs are just as prone to a phenomenon, known as sulfating, as any other lead-acid technology.

To counter-act sulfating, manufacturers of VRLAB stress the importance of boost charging on a regular basis while the battery is in storage. One recommends a boost charge frequency of every 90 days; another stresses the importance of purchasing the "freshest" battery available to ensure the longest possible service life.

Nickel cadmium aircraft batteries do not suffer from extended long-term storage.

VRLABs employ a microporous glass mat to immobilize the electrolyte within the cell. While this provides excellent aerobatic capabilities, it can be detrimental in storage. During storage of VRLABs, the electrolyte stratifies. Stratification increases the probability of sulfating in the area of highest concentration. Performance is, therefore, significantly reduced.

Charging has little or no effect in reconstituting the electrolyte to its original saturation of a VRLAB. NCAB electrolyte will stratify if stored for an extended period of time; however, the electrolyte is not immobilized and can be reconstituted by repeated cycling to restore performance.

Sudden death active material and inter-cell connection corrosion is another issue to be considered when discussing lead-acid technology. Lead-acid batteries work because of the corrosive effects of sulfuric acid on the active materials: lead dioxide and pure lead. The corrosive action of the acid causes the active material to swell on the positive electrode and has been documented to cause shedding of active materials. Corrosion will lead to internal shorts between the electrodes and/or opens at inter-cell connections. A short or an open connector will render the unit inoperable The electrolyte in NCAB has no corrosive effect on the active material or on the plate, tabs, or terminal connections.

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.

"Most aircraft don't require any changes to the system when installing lead sealed lead-acid batteries to replace Ni-Cads. If the aircraft has a 28VDC constant potential system, it is capable of charging the sealed lead acid as well as the Ni-Cads," he says.

Capacity fading
"In fact," Koss says, "constant potential charging systems do not work all that well with Ni-Cads and that's why you have to deep cycle the battery and perform so much maintenance to them. In order for a Ni-Cad to deliver 100 percent capacity, you've got to recharge it to 140 percent. This doesn't happen with a typical 28.5V charging system."

Koss continues, "So what happens with a Ni-Cad is each and every time the battery is discharged on the aircraft, such as for a starting event, and then recharged, it loses a little capacity. Over time, the battery Ôfades' and becomes inefficient as a storage battery for essential power requirements. At some point, they have to be brought in and serviced in the shop where they're deep cycled or totally discharged and then recharged. This deep cycle restores the Ni-Cad to 100 percent capacity for the next service period.

"This doesn't happen with lead acid," he says. "Constant voltage charging in aircraft, just like automotive charging systems, keep lead-acid batteries properly charged and their capacity does not fade as with Ni-Cads.

"A lead-acid battery has nearly the same discharge curve as a Ni-Cad. At a one-hour discharge rate, both Ni-Cad and lead-acid start off at about 25.5V. Then as you impose a C-rate (one hour) load on the battery, the voltage on both types of batteries diminishes to around 22 - 23V almost constantly over a 50-minute time frame. As you get to 50 minutes or so, the voltage goes down to about 21V and by the time you get to the end of the hour, the voltage is down to about 20V — which is the cutoff for both Ni-Cad and lead-acid. At that point, the Ni-Cad drops off drastically to almost zero, but the lead-acid produces power down to 18V before dropping off the charts.

"The lead-acid doesn't follow a 45-degree discharge curve as some would lead you to believe, both types of batteries produce a similar curve within their rating then have a very sharp drop off at the end of the discharge period. The truth is that it's quite surprising how flat the discharge curve is on both lead-acid as well as Ni-Cad," says Koss.

Swapping over
He explains, "Aircraft such as the G-IV have AC systems that have a dedicated Ni-Cad charging system, which delivers 32 volts. These systems are capable of charging and maintaining the Ni-Cad to 100 percent capacity. These aircraft are not limited to using Ni-Cads, however." Koss says Concorde does have an STC which modifies the charging system on the G-IV into a constant potential 28.5V system.

He explains, "The change is actually quite inexpensive because it involves placing fixed resistors in the temperature sensing plug that results in the system providing a 28.5V constant voltage. The plugs are less than $100. The cockpit indications are even still the same and the flight manual doesn't even have to be modified.

"Additionally, because of Mil-Spec requirements, the physical shape and weight of both Ni-Cads and lead-acid batteries has remained very similar. So there is hardly any change ever required to swap batteries."

Thermal runaway
Koss continues, "In the early '60s, there were some fatalities that were attributed to Ni-Cad batteries having thermal runaway. This resulted in the FAA issuing ADs against all Learjets. They were required to install temperature monitoring systems and they had to change the plastic that the cells were made out of from polystyrene to nylon, which was more resistant to combustion. Shortly after that, there were some other fatalities that resulted in an AD which required all Ni-Cads that were used for starting engines to have temperature or current monitoring systems. A while later, they changed the AD to require only temperature monitoring for Ni-Cads.

"The problem was that people were installing Ni-Cads on everything and weren't maintaining them properly. And because of the inexperience with the industry at that time, people were overcharging the batteries and destroying them on the bench.

"Any battery will get warm and has the potential for thermal runaway. The runaway, he explains is a result of a cycle where the battery shorts internally, the voltage in the battery begins to drop, and the charging system increases the current — this becomes a viscous cycle. With Ni-Cads, the substrate material on the plates is generally steel or pure nickel, and when they weld themselves together, there is no melting apart and the short becomes more severe. As that heat builds up, another cell adjacent to the shorted cell will have a failure of its separator and they also will weld themselves together and the current building up just feeds that short. With a lead-acid battery, however, the plates are made of pure lead, and so when they short and the current builds, the lead melts before it does any substantial damage," he says.

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