In preparing for the next leg of the trip, from Hawaii to Phoenix, Arizona, the team discovered that parts of the plane's huge batteries had suffered "irreversible" damage. The batteries store solar energy, and keep the plane aloft at night. The first sign of trouble came on the first day of the trans-pacific crossing, when the ground team noticed that the batteries were overheating. The problem was extra insulation around the battery compartment, which kept heat in. With nowhere to stop for repairs over the ocean, they had to keep an eye on the problem until the plane landed.