A Sony recall of an additional 3.4 million laptop batteries brings the total number of the company’s recalled laptop batteries to about 9.4 million. Previously, almost 6 million of the rechargeable lithium-ion batteries had been recalled. They’re used in laptops by Apple, Dell, Gateway, and other brands. In the latest recall, about 10 percent of the batteries, or 340,000, were used in laptops sold in the U.S.

Those batteries may overheat and catch fire because microscopic metal particles might have penetrated the battery cell during manufacturing. This happens rarely but when it does, an electrical short can occur. There have been 16 reports of batteries overheating, resulting in scorching and fires. Two minor burns have been reported.

Regardless of which laptop you own, here’s how to prevent it from overheating:

Contrary to conventional wisdom, there’s nothing wrong with leaving your laptop computer plugged in all the time. Nor is it inherently safer to run a computer on battery power than AC current.

If you own a laptop made by any of the affected manufacturers, the Consumer Pro­duct Safety Commission recommends that you contact the company to determine whether your battery is covered by the recall and to re­quest a replacement battery. For a complete list of computer lines and models affected by this recall and for contact information, go to the CPSC’s Web site, at www.cpsc.gov.

Until a replacement battery arrives, you can continue to use your laptop if first you turn it off and remove the battery, then power it using the AC adapter.