Astronaut Scott Altman floats through the tunnel to the international space station with a new battery in hand. Such a transfer isn't planned during Discovery's upcoming flight. Photo: NASA

As NASA readies shuttle Discovery for launch Thursday on a space station assembly mission, Russian engineers are troubleshooting yet another perplexing battery failure in the station's new Zvezda command module.

Launched July 12, Zvezda provides the lab's initial crew quarters, computer control and propulsion. It is equipped with eight batteries to provide power when the station is in Earth's shadow, three of which were installed during a shuttle visit last month.

One of those just-installed powerpacks - battery No. 5 - cannot hold a charge, possibly because of a poor connection somewhere in the wiring connecting the battery to its charger, known as a PTAB, and a voltage controller, known as a BUPT.

Battery No. 5 has been off line since the shuttle Atlantis departed last month.

Russian engineers took a second battery off line Monday when it failed to operate properly during a charging cycle. It is not yet clear what might be wrong with battery No. 4.

Despite the failures and uncertainty about the ultimate health of Zvezda's remaining batteries, the station's first full-time crew - commander William Shepherd, Yuri Gidzenko and Sergei Krikalev - remains on track for blastoff Oct. 30.

Robert Cabana, a senior station manager and veteran shuttle commander, said today a spare battery charger and a new controller will be launched to the station aboard an unmanned Progress supply ship in mid November.

In the meantime, he said, "the Russians are evaluating what caused battery four not to function properly. They haven't determined what it specifically is right now."

"But the plan would be to fly up a spare BUPT and PTAB and between batteries five and four come up with a good system between them, once the crew got up there," Cabana said. "The loss of battery four is no impact to launching the hardware for (the next two shuttle flights), it just decreases our redundancy in the service module.

Astronaut Ed Lu and Morukov hard at work beneath an area of Zvezda's floor. Photo: NASA TV/Spaceflight Now

"We still have six good strings, we can go down to five and still meet our requirements," he added. "So the plan right now is to get the crew up there and allow them to troubleshoot the problem with spare parts and fix it."

Cabana said Shepherd's crew can be launched as long as Zvezda has five operational batteries and possibly as few as four, depending on the nature of the failures and the work reqired to make repairs.

The Russians have had numerous problems with batteries, PTABs and BUPTs in recent months and NASA carried up replacement parts for all six powerpacks aboard the Russian-built, NASA-financed Zarya module.

But Cabana said the Russians are "fairly confident" the remaining batteries in the Zvezda module will work properly. And in any case, Shepherd's crew could live aboard the station with just four operational batteries if necessary.

At the Kennedy Space Center, work to ready Discovery for blastoff Thursday night continues on track despite torrential rain from a mass of tropical air slowly moving north across Florida.

Forecasters continue to predict a 60 percent chance conditions will improve enough by late Thursday to permit a launch at 9:38 p.m. NASA plans to make two back-to-back launch attempts Thursday and Friday, if necessary, before standing down a day to give the launch team a chance to rest.

The goal of the 100th shuttle mission is to deliver an alternate shuttle docking port and a massive 18,300-pound truss that houses four stabilizing gyroscopes and the station's main communications equipment.

NASA animation shows the Z1 truss being attached to the international space station's Unity node. Photo: NASA TV/Spaceflight Now

The Z1 truss also will serve as the temporary mounting point for a huge solar array segment that is scheduled to be attached in December.

Discovery's crew plans to stage four back-to-back spacewalks to connect the Z1 truss atop the U.S.-built Unity module and to hook up the new docking adaptor to Unity's downward-facing hatch.

"The shuttle missions that have gone to station so far have been extremely successful in that we've built a little, tested a little and provided a lot of equipment and supplies to get ready for permanent flight crew presence in space," said shuttle program manager Ronald Dittemore.

"This next mission, STS-92, takes the level of complexity up a notch over the last two missions. It's one of the most complex station assembly flights that we will conduct."

The Discovery astronauts have no plans at present to enter the Zvezda module, much less do any battery troubleshooting. But Cabana said engineers are studying the timeline to determine if such work might be possible.

"The ops team is looking at that right now," he said. "So far, our Russian partners have not asked us to do that. In fact, there is some feeling on their part that they'd just as soon wait until they got the (full-time) crew up there.

"But we are coordinating with each other to see if there was time we could work into the timeline to have the crew ingress in ... to gain a look at the problem and do some troubleshooting. But right now, it's not planned."