These types of mini-jumpstarters are only good for slightly weak batteries to give them a little extra boost. They are not strong enough to fully jump a dead or nearly dead battery. This is one of those occasions where bigger is better. For a regular car I would recommend getting a real jumpstarter rated to at least 450A or better, the smaller 350 ones are more suited for economy 4 cylinders in ideal conditions, i.e, if they're fully charged, the weather isn't too cold, and the main battery isn't too far discharged.

I'm guess the 30V might be because they're relying on a high voltage, low aH battery to limit the amount of charge. Kinda like hooking up 100 AA's to a car battery, it might be 150V but the voltage will drop to about 12V immediately due to internal resistance you won't get a significant amount of energy out of it.