DIW in the battery only - i wouldn't put in in the cooling system as the process of deioising it uses a high voltage passed through the water via a anode ad cathode, this pulls the metal particles away and leave the water charged -ve . put this in your metal engine and the water will try to reclaim some of its metal content and literally eat away at your water jackets . use normal tap water with the correct mix of antifreeze, usually 50/50 mix on fords.