WASHINGTON - Two major U.S airlines say they no longer will accept rechargeable battery shipments as new government tests confirm that explosions and violent fires are likely to occur when large numbers of batteries enclosed in cargo containers overheat. Tests conducted last month by the Federal Aviation Administration show that rechargeable batteries, also called lithium-ion batteries, consistently emit explosive gases when they overheat or short-circuit, the Associated Press has learned. Citing safety concerns, United Airlines on Monday informed its cargo customers it no longer will accept bulk shipments of the batteries, which are used to power everything from smartphones to laptops to power tools. 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.