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It has been stated here numerous times - the ELT battery is not chargeable, ELT is completely stand alone unit, it is sealed and not connected with the rest of the aircraft. There are only two wires going outside of ELT to a switch - so a pilot could turn the antenna ON but 787 (or any other aircraft) doesn't supply ELT with any voltage/current. "
Except for 406MHz which also have at least 2 more wires for GPS feeds for position data and a coax cable to an antenna. However yes, you are correct that they are not externally powered.