Thanks Herbie, that post is so helpful! I don't really see myself as a mega car techie who is likely to have to help others out, but you never know! I have had a flattened battery twice, mainly as a result of lockdown (having to shield and not go anywhere). But last night I called lexus breakdown at 6.30pm (still daylight) and the guy finally came at 9.30pm (dark, and I had to drive around for an hour to charge the battery). I just don't want to be in that vulnerable position again, and so I want some way to be self sufficient if my battery dies in future. If this gadget does it, I am happy! But of course, if I come across someone in need it also means I can help them too. I have got standard jump leads, but the location of my car at home means they are useless without a second car to jump off. Hopefully, this gadget will solve my problems - if it helps someone else, that's a bonus!
Thank you for your helpful advice - when I retire, I intend to take a course in car mechanics, and with luck I will be a bit less clueless!