During the Christmas break I purchased the GB40 for my granddaughter to use away at college. I chose this unit since it was safer than most portable jumpers, smaller to store, and light to carry. She has a 2014 Mazda 6 2. liter. So the engine is small enough to crank with this unit. Shortly after arriving back to school from Christmas break a passenger left the interior light on over night and the battery went dead the next morning. So she pulled out her fully charged GB40 jumper and hooked it up. But nothing happened. She read the manual and it said if the car battery was too low and to push a button on the unit. This time the dash lights came on. So then she tried to start it but the starter only clicked and would not turn it over. The unit was brand new and had been charged the day before. It was stored over night in a warm room prior to use. The ambient temperature of the car was in the low 30's. She removed it, recharged it and tried again. Same result so she got a friend to jump it. The manufacturers description says it can be used up to 6 times on a single charge but would not turn this little engine over once. The unit is located too far away to get it back for a return to Amazon and by the time she returns the warranty will be up. I will now get her my old stand-byjumper the' Stanley Jump-It'. It never fails. But you need to be cautious using it. The GB40 was a good idea.