I bought one of these purely for peace of mind. I hoped I would never have to use it.
According to the box it will start anything from a motorcycle to an lorry.
I needed it for a transit minibus which had insufficient power to start off it's own battery. Ensuring it was fully charged, I connected it, switched on, and tried to start as per instructions.....nothing, it just discharged!
Eventually I used a jump lead off a spare battery and it fired up in seconds. I took the unit back and got a refund.
Today I had a flat battery as I had left the interior lights on all night. The local garage came out with one of these booster packs, I watched with amusement as a click was all that was heard and the leads began smoking. Again' jump leads and off we went.
The only use I can see for this yellow box would be for camping. The compressor to inflate the airbed, the light to see the plug where you use the 12v outlet to charge the mobile phone. Unlike a good set of jump leads, It's not something you can rely on, but I suppose it CAN start an argument.