I was recently caught out with a flat battery. I want to buy a jump battery to make sure it doesn't happen again. My car is a BMW 3 litre, 6 cylinder diesel that will need somewhere around 500-600 cranking amps. All the jump batteries I can find here have only 300 cranking amps. If I connect two of those batteries to my car at the same time, will that give me the 600 cranking amps I need?