To become a Basic Ground Instructor (BGI), what are the minimal requirements from the FAA?
I am especially interested in understanding the minimal certifications (PPL/CPL) that are needed to obtain the BGI.
The answer to your question is found in FAR 61.213
For your convenience I've copied and pasted the entire thing here, but you should reference the official link as regulations change from time to time. Highlighting is mine.
Note that no pilot's certificate of any kind is necessary.
(a) To be eligible for a ground instructor certificate or rating a person must:
(1) Be at least 18 years of age;
(2) Be able to read, write, speak, and understand the English language. If the applicant is unable to meet one of these requirements due to medical reasons, then the Administrator may place such operating limitations on that applicant's ground instructor certificate as are necessary;
(3) Except as provided in paragraph (b) of this section, pass a knowledge test on the fundamentals of instructing to include-- (i) The learning process; (ii) Elements of effective teaching; (iii) Student evaluation and testing; (iv) Course development; (v) Lesson planning; and (vi) Classroom training techniques.
(4) Pass a knowledge test on the aeronautical knowledge areas in-- [(i) For a basic ground instructor rating, Secs. 61.97, 61.105, and 61.309; (ii) For an advanced ground instructor rating, Secs. 61.97, 61.105, 61.125, 61.155, and 61.309; and] (iii) For an instrument ground instructor rating, Sec. 61.65. (b)
The knowledge test specified in paragraph (a)(3) of this section is not required if the applicant:
(1) Holds a ground instructor certificate or flight instructor certificate issued under this part;
(2) Holds a teacher's certificate issued by a State, county, city, or municipality that authorizes the person to teach at an educational level of the 7th grade or higher; or
(3) Is employed as a teacher at an accredited college or university.