I bought 2 of these units because they had good reviews and I live alone and it seemed like a great idea because I have no one to help me. Well, two days ago it was bitterly cold outside and my car battery lost enough charge to start, and so I thought, perfect, now I can try out my jump starter. Well, I followed the directions perfectly, and waited for it to give my battery enough juice to start, as per the instructions but it didnt work. So I gave it even more time. Same thing...I tried a total of 3 times in the freezing cold and it just couldnt do the job it was designed to do. Fortunately, I belong to triple A and I called them to give me a jumpstart. They arrived, hooked me up and we waited a couple of minutes for a charge to build, and it fired right up... to say the least, Im very disappointed, not too mention the time I wasted in the freezing cold trying to get it to work. Hopefully you will have better results but dont depend on it when you really need it...