I don't often recommend products but as we all dread the A8 flat battery syndrome, which I experienced a few times on the S8 because of door handles keeping the car awake.
The most recent flat battery pushed the wife over the edge and I purchased a booster battery hoping it would get her out of a jam should she need it.
So I bought
this fully expecting it to be a POS.
Today i was working to rectify the battery drains on the car and in doing so managed to flatten the battery
So I broke out the booster expecting it to confirm my doubts about its ability to start a large engine.
To say I was pleasantly surprised is an understatement, if you want piece of mind then buy one and throw it in the boot you might just be glad you did.
See it in action
here