Nickel-cadmium vs. sealed lead-acid

"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.