Astronauts at work inside the floor compartment of Zarya to replace batteries. Photo: NASA

Russian engineers are troubleshooting another possible problem with one of the new batteries installed aboard the Russian Zarya module of the international space station. NASA managers are hopeful the problem can be resolved, although the Atlantis astronauts may have to do a bit of unscheduled repair work later today, before the station is sealed off prior to undocking Friday evening.

Four of Zarya's six batteries - Nos. 1, 2, 3 and 5 - have been replaced, along with chargers known by their Russian acronym, PTAB (pronounced pah-TAHB). Batteries 1 and 2 are fully charged and on line while battery 3 is in the process of being charged up. Battery 5, the fourth replacement, will be charged later today.

"We've completed all the equipment changeouts," said space station flight director Paul Hill. "Most of the new equipment we've installed has already been checked out and is looking nominal. Tonight, we have five active batteries for the first time in six months and by tomorrow, we'll have six active batteries on the main bus and be back up to full strength."

But Hill said engineers have seen "an anomalous signature" on battery 3. It is possible one of the wires running between the battery and its PTAB is not fully seated or that a pin in one of the connectors is bent and not making good contact.

"Moscow will be looking at that overnight while we continue charging," Hill said. "The problem that we have doesn't keep us from charging the battery. It does, however, cause some unstable current fluctuations in the other batteries when we put that battery into normal operation."

Depending on what the Russian battery experts determine, "it's likely we'll send (the astronauts) back under the floor (of the Zarya module) to check those connections to the PTAB and to the battery in slot 3," HIll said. "And depending on what we see, we have some options to take some of the equipment we've changed out and put it back into slot 3 like we talked about yesterday when we were looking at battery 2."

Astronauts float inside the Unity connecting node. Photo: NASA

Battery 2 began discharging unexpectedly two hours after engineers began charging it up. It later was determined that disconnecting battery 3 prior to its replacement affected the charging cycle of battery 2. The battery was, in fact, healthy and it is now fully charged and on line. Before the issue had been resolved, however, Hill said if worse came to worse, battery 2 could be replaced with the battery taken out of slot five. And that again is an option for battery 3. If the power pack turns out to be defective, the astronauts likely will be told to replace it with the original battery 5.

Hill said in a previous briefing that engineers are not even sure there's anything wrong with the original battery 5 and that it was being replaced just to play it safe.

"We're still kind of early in looking at the telemetry and trying to assess what the problem is and by this time tomorrow, we could have gotten back under the floor and reseated these connectors and battery 3 could, in fact, be looking normal," Hill said. "We'll know a lot more about it tomorrow (later Thursday). Meanwhile, the battery is charging up fine and we'll still have it ready and fully charged up tomorrow morning when we start up on battery 5."

All in all, he said, "I'm still very happy with all the new equipment and how it's performing. We're definitely getting what we came for with the lifetime extension and all the refurb equipment that's going into the spacecraft. So I'm very comfortable that when we leave on this flight we're still going to have a very robust spacecraft and we'll be very ready for service module docking in the two months."

The service module is a critical Russian component scheduled for launch in July that will take over many of the power and propulsion chores currently provided by the Zarya module. The unmanned linkup will be accomplished primarily by remotely maneuvering Zarya to the service module for an automatic docking.

"In the very worst case, if battery 3 is not usable at all, we're still one battery better off than we were one week ago," Hill said. "And we also know we can pull off a normal service module docking with four batteries and we know how to do it with three. So we're not standing on the edge of a cliff even without battery 3. But I'm very hopeful we'll come in tomorrow and battery 3 will be on line and looking normal."

Meanwhile, Atlantis skipper James Halsell and pilot Scott Horowitz fired the shuttle's small steering jets 27 times over a period of 56 minutes and 30 seconds Wednesday evening in the second of three maneuvers to boost the international space station's altitude by about 27 statute miles. During the first such maneuver Tuesday, the station was boosted 9.7 miles. This time around, the lab's orbit was raised an average of 9.25 miles, giving the station an apogee, or high point, of 225.9 miles and a low point, or perigee, of 223.7 miles. A third and final reboost maneuver is scheduled Thursday evening.