Powerbank fully charged, so gave it a go tonight.
It says in the manual, only suitable as a booster and not suitable for a completely flat battery.
It defines okay if the battery is flat say due to leaving your lights on, and not enough to start.
A completely flat battery it defines as a car left unused for 2 years with a flat battery.
Well for me the ultimate test is NO BATTERY, only the powerbank itself!
So I disconnected the earth terminal from the Bosch S5 fitted to my Stag, and hooked up the powerbank to the disconnected earth lead clamp and positive terminal.
In case it did work, I also disconnected the ignition coil, for with no normal battery connected I didn't want the car to start and then the car alternator pumping a load of amps back into the powerbank alone. I might not like that.
Did it work? Amazingly it did, turning over the cold 3lt V8 just as fast as the Bosch S5!
No fizzing melting leads, no smoking plastic, quite unbelievable to be honest.
Did 4-off 5 second bursts on the starter motor in total, more than adequate to prove its worth, and the powerbank still indicated 75% capacity.
So well happy with the purchase at the price point.