It takes 48 hours for a battery to pass through the intestinal tract, the journal Pediatrics reports, and it usually happens without incident.

But parents still should try to keep their children from eating them.

Georgetown University's National Capital Poison Center reports on 114 incidents of children and some adults swallowing batteries, most of them button-sized batteries used in hearing aids and watches, but also including six larger, cylindrical cells.

In 14 of the cases, children had eaten batteries from their own hearing aids. Four adult patients had eaten batteries mistaken for pills.

"One guy was walking down the street, reached into his pocket for something for a headache and popped a battery in his mouth," says Georgetown's Dr. Toby L. Litovitz, author of the study. Another patient was taking a heart pill and changing her hearing aid battery at the same time and mixed the two up. "She didn't realize what had happened until her hearing aid didn't work," Litovitz says.

Since the study was written, another 300 cases have occurred, she says.

Usually, the battery corrodes, leaking its alkaline fluids into the body. But the poisons are so diluted by body fluids that they cause little harm; only 10 percent of the patients suffered symptoms including vomiting and nausea.

The chief danger, writes Litovitz, is that the button batteries can become lodged in the esophagus, as happened in one case, requiring removal.

All six of the larger batteries, most of them size AAA, passed through safely.