If I take two identical lipo cells, say 2 x 10ah lipos both new, for instance one is at 3.4v, the other at 3.9v, now if I connect them in parallel to get a 20ah cell, what will happen to the cells?, will they balance out to the same voltage? will this balancing take time? hours, mins, days?, or will it happen rapidly? or is this dangerous? do the cells need to be accurately at the same voltage first before they are connected together?

So many questions, ive searched high and low for details on this but never found an answer yet never seems to be covered in any faq's.