It works on the principle of charging the main battery over 15 minutes at a low charge rate.
After that time I would guess that the unit will be close to totally discharged, so try to start the car with it connected and there won't be that much current flowing in the item.
Do the maths, it has a capacity of 1300mAh, or 1.3 Ah, so IF it is 100% efficient it will contribute about 1.5% to the cars battery, which can be 85 Ah or more.
So will it start a car...:nenau
I doubt it, it might contribute to the voltage available at the battery, so it could help, if the battery is half charged.
Best to use it to charge your mobile phone before you call the RAC or AA
I have used one of those mains powerd starter chargers on the Mav in the early days, you plug it in for 10 minutes, it charges the battery at 20 amps, then I leave it connected to start it. Now that worked very well indeed.
The battery was just unable to turn on the starter at the time, so the extra boost helped.
PS car was still under warrenty, so they replaced the battery FOC.
Edit...
A woman calls the AA to her car as it won't start.
AA to woman... " You have a flat battery miss"
Woman to AA man... "What shape should IT be ?"
Sorry about that!:doh