My 4 children purchased a Die Hard portable power unit for me on my birthday in September of 2009. It worked great up until about 2 weeks ago. With the unit full charged, it would only work for about 15 minutes and then made a loud humming sound and the ERR mode would come on the screen and the unit would then shut off. If I waited for 15 minutes or so, it would work again for a short time and then do the same thing. I took it to the store and they insisted that I provide a receipt which I do not have nor do my children, but I know it was a... read full review »