For instance, why can you bring only 3.4 ounces of certain liquids rather than, say, 3.6? But at least one carry-on rule is in place for a reason. For proof, look no further than a recent Alaska Airlines flight from Wichita to Seattle that had to return to the airport when a passenger’s power bank overheated and caught fire at cruising altitude. As a result of this and a string of similar incidents, many airlines are implementing new policies related to lithium-ion batteries in general and power banks in particular. Here’s what travelers should know.