I had a cheap one 5 years ago that was able to jump start a car. The instructions said to leave it connected to the battery for 30 seconds before trying to star

I had a cheap one 5 years ago that was able to jump start a car. The instructions said to leave it connected to the battery for 30 seconds before trying to start it, so I guess it's more a portable battery charger than a jump starter. For sure if you disconnected the 12V battery and only connected this it would not work.