Jules submitted this idea :
(I'm not sure how well a cheap battery charger will stand up to permanent
connection and repeated starts with it connected? I expect - but
don't know for sure - that a proper conditioner is designed to be a
bit more robust in that regard)

They (Lidl unit) will be fine. I have one in use like that for a few
years. My car can at times sit in the garage unused for weeks, so it is
often left with a Lidl unit connected. I have also had it charge flat
batteries up and it gets no more than luke warm.

The only down side, is if the mains goes off briefly - you need to
press a button to restart the charge.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk