Is a B&D Simple Start safe to use?

[Supervinnie40]

I've got a Black and Decker Simple Start batterypack, and I was thinking about putting it in my 'emergency kit' for the car.

The way it works is that it contains a big battery (that should be charged).
When you battery fails and you run out of electricity, you connect the battery-pack via the aux-socket. Because the current in the batterypack is bigger than that of the car battery, it will slowly charge it, giving you (after a few minutes) the possibility of starting your car again.

This way you don't have to get out of the car in a battery-related emergency.

But it says very clearly in the manual that you should check with your manufacturer if the simple is compatible with this.
Does anybody know anything about this??

I would hate to blow my ECU because I'm trying to charge my battery after I left my lights on....