IIRC when the rescue service had to come out to my car as the battery wouldn't start it, they had some kind of voltage/current limiting device that allowed them to attach their power source with it all in situ. I didn't get the details though.
That stated I'm not sure why you'd want to keep a battery connected whilst charging. How difficult is it to disconnect and reconnect afterwards anyway ?
normal charging shouldnt cause a problem its when jump packs/ booster chargers are used to start a vehicle it can all go wrong,although to be honest i have never seen it happen.
What old geezer is refering to is a 'surge protector' something we also have to use when mig-welding a vehicle .
Thank you TOUREMAN, OLD_GEEZER and CHAS2008 - I will pass on your comments to the lady.
It is always good to get different points of view on these subjects.