Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Won't charge a dead battery

It is a maintainer, not a charger. This unit is great for topping off a battery, and/or maintaining one in storage. No issue with that, but it will not charge a dead battery. At a very low battery voltage like 10.6V, it simply turns on its RED LED and shuts off - no go, unusable. That was on an unusually small car battery, for a Prius. I think it is cooling problem, because in final desperation, I set it up on end, with LED at bottom, and hot transformer at top, then it managed to work. Set it back flat, and it turns off.Disappointed.. risk is that it may not be there for me when I most need it.