Boeing's (NYSE:BA) 787 Dreamliner just returned to the skies after a four-month grounding, but already there's trouble. The director of the Airline Pilots' Association of Japan, Toshikazu Nagasawa, said that pilots weren't satisfied with the changes Boeing made to it's lithium-ion battery and are concerned that they won't receive appropriate in-flight warnings if there's an issue. Some scientists and battery experts are also expressing concern about the safety of Boeing's battery.

Officials at All Nippon Airways and Japan Airlines said they're satisfied with Boeing's changes and have resumed flights, but investors may have cause for concern knowing that that the pilots -- the people whose lives depend on the safety of Boeing's battery -- aren't satisfied. Here's what you need to know. 

Boeing PictureSource: H. Michael Miley, Wikimedia Commons. 

Are the issues in the past?
In March, Japanese pilots raised 30 safety concerns about the Dreamliner. One of their major concerns was that they didn't think Boeing had provided enough proof that the 787 would be safe to fly if the batteries failed. They also expressed concern that the warning indicator for a battery malfunction didn't indicate the severity of the problem.

More recently, the pilots' group expressed concern that Boeing didn't figure out what caused the problems with the batteries in the first place and is now downplaying the battery's necessity for flight. Consequently, the group challenged Boeing to conduct flight tests without the lithium-ion battery to prove its safety.  

Boeing's vice president and the chief project engineer for the 787, Mike Sinnett, addressed these issues by saying the Dreamliner has backup systems that allow it to continue flying if the batteries fail, and that the warning indicators were ranked by severity, but these assurances have done little to quell the pilots' concerns.

Scientists add fuel to the fire
Battery experts and scientists have also publicly questioned the safety of Boeing's battery. Elton Cairns, a Lawrence Berkeley National Laboratory professor and battery technology expert, said: "I'm shocked that Boeing was willing to stake its reputation on these batteries. Even with the modifications, the individual cells of the battery are crammed too closely together and feature an internal chemistry that's far too volatile."  

More pointedly, Michel Armand, a professor of chemistry at the University of Picardie and a research director at the French government's Centre National de la Recherche Scientifique, told Barron's: "Using these batteries in planes makes no sense, with all the lives potentially at stake. These batteries are unpredictable and prone to thermal runaway and fires."  

Fires, fires, everywhere
Boeing's 787 Dreamliner isn't alone when it comes to problems with its lithium-ion battery. In 2006, Meggitt (LSE:MGGT) subsidiary Securaplane Technologies -- which incidentally builds the charger for Boeing's battery -- suffered millions in damages when a lithium-ion battery exploded during testing and burned an Arizona facility to the ground. 

Further, in both 2010 and 2011, 747 cargo planes carrying pallets of lithium-ion batteries crashed because of a fire, killing the crew.

In neither case could authorities determine a cause for the fire.

In March of this year, Mitsubishi Motors reported two incidents involving a lithium-ion battery. These batteries were manufactured by Mitsubishi's venture with GS Yuasa, the maker of Boeing's lithium-ion battery, although the batteries themselves are not the same. Mitsubishi reported that the problem was caused by contaminants getting into the battery cells during the screening process. 

Fires ahead for Boeing?
Separately, these incidents are scary but manageable. But when you combine these accidents and factor in the pilots' concerns, you get the feeling that lithium-ion batteries are anything but safe. Underscoring this point is the decision by EADS' Airbus to pull the plug on putting lithium-ion batteries in its A350 plane, citing "immaturity of lithium-ion-battery technology." If the Dreamliner's battery really is fixed, this plane could prove to be quite profitable for Boeing. However, given all the issues it's had in the past, and the uncertainty of the battery, investors would do well to keep an eye on the Dreamliner -- especially considering Boeing's stock has risen and fallen right along with the Dreamliner.

Fool contributor Katie Spence has no position in any stocks mentioned. Follow her on Twitter: @TMFKSpenceThe Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.