Marriott Hotels, based in Bethesda, is installing wireless charging stations for smartphones and tablets in the “Greatroom” lobbies of 29 properties this month, the company announced on Tuesday. Marriott will use Kube Systems’ portable charging stations, which work when a device is placed on top. Marriott said it is responding to concerns expressed by guests who say one of their issues as travelers was finding places to plug in their mobile devices to keep them running. A Marriott survey showed that the average business traveler is carrying two or three devices. The Kube Systems station can handle up to six devices at once, though that involves plugging in phones or tablets to the Kube charger.