The battery on my 2004 tdi90 was flat so I used a fully charged Halfords portable 2L lithium ion jump starter for the first time. The instructions ask you to connect the positive jump lead to the positive terminal of the dead battery and the negative to a suitable unpainted point on the chassis or engine of the vehicle. I couldn’t find a suitable point in the boot of the car so I connected to the negative terminal on the dead battery but I wonder whether the electronics risk being ‘fried’ or is there a negative terminal other than the one on the dead battery? Incidentally, the portable unit didn’t get the car going but an old school set of jump leads did, thank goodness.