Simple stuff such as batteries - types, how to wire in series, parallel, and mixed parallel and series, battery maintenance equipment i.e. DMM, analogue meter, SG measurements. What should should check for at the moment of purchasing batteries.
Fork lift truck batteries, telephone exchange batteries,the whole gammut as basic information would be useful for new and seasoned people on this site and save much repetition.

A sort of basic ready reference base that would not neet to be frequently updated once its assembled.

Problem is it could be work for those who may not want it.