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 is now the second major U.S. airline to announce it will no longer accept bulk shipments of rechargeable batteries. Delta quietly stopped accepting bulk shipments of the batteries at the beginning of February. The airline says it took the action in response to government testing. American Airlines stopped accepting some types of rechargeable battery shipments late last month. But, the airline is continuing to accept small packages of batteries grouped together into a single cargo container. All three airlines say they will continue to accept bulk shipments of equipment containing batteries or in which batteries placed in the same package as equipment. The shipments are profitable. And so far, there have been no cargo fires aboard passenger airlines attributed to lithium batteries. But as some airlines ban the shipments, it could put pressure on other airlines to follow suit.