On Sun, 14 Jun 2020 11:47:38 +0100, Jim GM4DHJ ... wrote:

https://www.ebay.co.uk/itm/OBD2-Auto...Saver-OBD-12V-
Battery-Replace-Tool-Extended-Cable/292763616836?ssPageName=
STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649

anybody use one of these ?...are they safe?

Donno, I just made up a open end to lighter plug made sure the
lighter socket was powered and connected a 12 V SLA to the open ends.
With a modern car one might have to be quick changing the main
battery as they have a habit of going to sleep after a while and
might switch off the lighter socket...

I guess the ODBII power line is less likely to be turned off. Or find
other unswitced/secure +V and Gnd points, obvoious place is the
around the battery terminals but there is a chnace that one
connection might get dislodged whilst faffing about with the
battery...

--
Cheers
Dave.