U.S. officials investigating a battery fire on a Boeing Co 787 Dreamliner will travel to Phoenix, Arizona, for equipment tests at a facility run by Pratt & Whitney, a unit of United Technologies Corp, a spokesman for the U.S. National Transportation Safety Board said on Sunday.

NTSB spokesman Peter Knudson said investigators would carry out functional testing and memory downloads of the auxiliary power unit, or APU, built by Pratt & Whitney Power, known as Hamilton Sundstrand until United Technologies took over Goodrich last year. He said he did not have a date for that trip.

The safety board earlier ruled out excess voltage as the cause of a battery fire on the 787 at the Boston airport this month. It said investigators would travel on Tuesday to Tucson, Arizona, where the maker of the battery charger, Securaplane, a unit of Britain's Meggitt Plc is based.

(Reporting By Andrea Shalal-Esa; Editing by Maureen Bavdek)