WASHINGTON (AP) — Some of the world's largest airlines are banning bulk shipments of rechargeable batteries in the face of mounting evidence of their potential to cause catastrophic in-flight fires. Citing safety concerns, United Airlines on Monday became the second major U.S. airline to announce it will no longer accept bulk shipments of rechargeable batteries, also called lithium-ion batteries, which are used to power everything from smartphones to laptops to power tools. The airline said in a statement that it took the action in response to government testing and concerns raised by its pilots and flight attendants. Placing batteries inside equipment like laptops or in the same package as power tools creates additional buffering and is believed to provide added protection, although safety experts say that theory hasn't been fully tested. Federal Aviation Administration tests over the past year show that when a battery overheats it can cause other nearby batteries to short-circuit and overheat, resulting in a chain reaction. The most recent tests were conducted last month at the FAA's technical center in Atlantic City, N.J.