In my experience, a "relief charge" is the person who will work in place of the "charge" person, much like a "vice-president" works in place of the president when he/she is unavailable for duty. I added the word "suplente" to the already good translation for the rest as provided by Monica Algazi.

Here is an example of a job that includes both relief charge and charge nursing.

http://www.staffpointe.com/nurses_flight.htm
Oregon
Hospital in beautiful Oregon oceanside setting, acute care facility, seeks staff RN for ICU/Med/Surg unit. Majority of hours in ICU. Some relief charge nurse work. 4 bed ICU with 1:2 ratio. Hours 2245-0715. Salary range is $24.59-$35.97/hour plus 19% of base for night shift and 10% for charge nurse work. Also need 32hr med-surg RN day shift, and night Charge RN on med-surg.