The battery fires may be breaking out because of the small space between the lithium ion battery cells.
They are now being widened as Boeing continues testing.
The National Transportation Safety Board says it will ultimately be up to Boeing to show the batteries are safe before they will allow Dreamliners back in the sky.
Fifty Dreamliner aircraft around the world were grounded last month until further notice after a series of battery-related incidents, including several fires.
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