The type that you clip to the battery terminals to start a dead battery?
Are they supposed to start the battery immediately, using the power from the box or, are you supposed to let the charge that is in the box drip into your car battery for a bit - eventually enabling the flat battery to start?
There are two types. There are ones that are just a battery, so these have to be charged up beforehand (can take several hours via USB). If they aren't charged (or have insufficient charge) then they won't work. If you have one of these, it's a good idea to charge it up every few months, because sod's law says it'll be flat when you need it. These should enable your car to start straight away.
The other type use gizmos called supercapacitors which store charge but not for very long. These don't need to be charged beforehand, they charge themselves from whatever's left in your car battery as soon as you make the connection. One of these may need a few seconds to charge itself before you can start the car (they usually have some kind of display to tell you when they're ready).
Neither type charges the car battery, they work in parallel to provide the oomph to start the car, and you then need to keep the engine running in order to charge the battery using the car's generator.
