SEOUL, SOUTH KOREA — The U.S. Federal Aviation Administration has advised passengers not to turn on or charge Samsung Galaxy Note 7 smartphones on aircraft, after faulty battery packs caused some of the phones to explode while charging.

Due to the battery crisis, Samsung was forced to recall 2.5 million units of the Galaxy Note 7, its new flagship phablet.

The faulty battery packs were made by Samsung SDI, a subsidiary of Samsung. Samsung has indicated it will no longer use Samsung SDI batteries in the Note 7.

According to the Wall Street Journal, roughly 65 percent of Galaxy Note 7 batteries were made by Samsung SDI. Thirty-five percent were made by Chinese company Amperex Technology Limited, or ATL.

The Samsung SDI batteries in the Note 7 were found to have faulty separators, which allow the two electrodes to touch. This redirects energy to chemicals called electrolytes, which help to conduct the current.

When the battery is hot, the electrolytes react with other chemicals to release gas in a feedback loop called “thermal runaway,” which can lead to a fire.