Mercury has just authorized/certified the Relion RB-100-HP for Pro SX cranking ability It is an Iron Phosphate Lithium Battery, I just ordered one as this battery can discharge to 20% so you get a lot of hours for running Electronics besides the starting capability, The Oddessy I had was the same size but as would only be used down to 50% discharge so the AH was half of this Relion Battery. If this works as advertised no need for a 5th battery, Time will Tell.