It can only charge as high as your electrical system voltage goes. They should have put in a cheap PWM/DC Converter circuit to boost voltage to 30V so it would work EVERY TIME.
A 30V capacitor of sufficient FARAD rating can turn over a car with a weak or dead battery every time, and start it if the ignition system is working OK. I have built simple DC converters (boosters) but I'd not use one on it without a voltmeter to check the voltage. DO NOT OVERCHARGE a Capacitor, it will short out internally and become useless or even leak, or possibly Explode.
This COULD WORK if properly charged. I have the standard Mighty Jump, and will find out. I will update the review later.
Bottom Line: No, I would not recommend this to a friend
It's 20 AA single-use alkaline batteries, which are not that safe.