Florence police say Tanjula Harley, 27, left her two-year-old son home alone at her apartment on South Dargan Street, Thursday morning.
Major Carlos Raines says neighbors called 911 because the child had been on the balcony in his diaper for at least 30 minutes.
Officers went to the home to check on the child.
Raines says when Harley showed up, she told officers she went to a store to get milk and orange juice and walked to the library to see what time it opened.
The child was removed from the home and placed in emergency custody with the South Carolina Department of Social Services.
Harley is charged with unlawful conduct toward a child. She's being held at the Florence County Detention Center.