I was going to recommend Ring, but look for battery conditioning as a feature
My battery was completely goosed, before the cold set in I would leave it for 3 or 4 days and it would be very slow turn over - but would always start.
Knowing the cold would make it harder to start I needed to do something,
My brother in law offered his Halfords branded Ring charging that had battery reconditioning, this dissolves the crystals that build up on the plates.
Still sceptical I plugged it in - told to do it twice that took nearly 24 hours, but it has made a huge difference.
I jumped into the Golf yesterday after over a week of parked up and it was was fully charged.