The mother, DiJanelle Fowler, kept the car running with the air conditioning on, but the car's battery died while she was inside the beauty shop in Tucker, Georgia, on June 15, said DeKalb County Police spokeswoman Shiera Campbell.
Police believe Skylar Fowler was dead by the time her mother returned to the car a few hours later, Campbell said.
"Instead of calling 911, she called roadside assistance to get her battery jumped," Campbell said in a phone interview.
Fowler concealed the baby from the person who helped restart her car, then went to a hospital and called 911, Campbell said.
The mother told police she herself had passed out from some sort of medical condition, Campbell said. But the child's condition indicated she had died sooner than the time Fowler suggested, the police spokeswoman said.
The high temperature in DeKalb County that day was 92 degrees Fahrenheit at 5 p.m., according to the National Weather Service.
Twenty-three U.S. children have died so far this year after being left in hot cars, up from 20 through mid-July in 2016, according to the nonprofit KidsAndCars.org, which tracks such deaths.
In all, 39 children died in hot cars in 2016. The highest annual number was recorded in 2010, when 49 children died, said Janette Fennell, founder and president of the KidsAndCars.org.
Children ages 3 and younger account for 87 percent of the hot-car deaths the group has tracked, she said.
Fowler left the Atlanta area after her daughter's death. She was arrested late on Monday after turning herself in to DeKalb County Police, who had been searching for her, Campbell said.
In addition to the second-degree murder charge, she also is charged with second-degree cruelty to children and concealing a death, jail records show. Court records did not show whether Fowler had hired a lawyer.