Here's a conversion function that does what you want. It was a little
tricky to have 'aa' after 'z' instead of 'ba' :

def convert(v)
  res = ''
  while(v >= 0)
    res = (?a + (v % 26)).chr + res
    v = (v/26) - 1
  end
  res
end

# test
puts((0..800).to_a.map{ |v| convert(v) }.join(' '))