Southwest Airlines said Tuesday it will limit passengers to one lithium portable charger on flights and ban them from being stowed in overhead bins to reduce risks of inflight battery fires.The new policy on power banks is effective April 20 and Southwest also said portable chargers are no longer allowed to be re-charged using in-seat power. Last month, the United Nations aviation agency said the use of power banks, or portable, rechargeable battery packs, would be limited to two per passenger as part of new rules for using the devices while in the air.