I have used a £20 auto charger from Maypole for many years. It has 2 modes, one will top up to and hold the battery at 14.4V (same as an alternator) the other will hold the battery at about 12.85V (maintenance charge). So you don't need to spend a fortune to keep batteries happy. I use this on the RIB, my ride-on lawnmower, vehicles I'm not using, and various other batteries that I have lying around. Sold my 13 year old Discovery a few months back with its original battery in it, so seems to be effective!
I have 4 Maypole chargers from a trickle charger built in to a plug up to a 30A boost charger, and they are all good, both quality and price.
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
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