Is this a buying opportunity for the stock of this hydrogen fuel cell producer?Shares of Plug Power (PLUG -13.87%) were sinking after the hydrogen fuel cell producer announced a secondary stock offering to help it raise cash. The downward move in the stock only added to its recent woes, with its price now plunging over 75% in the past year.Let's look at the energy company's secondary offering and why the timing of this announcement likely is not good for the stock.Plug Power needs to raise more cashPlug Pow ...