Overcharging a battery will b&gger it up just as much as letting it go flat for an extended period.
I've got a little auto battery charger that I run from my garage, made up an extension lead of about 10 metres for the DC side so I can keep the charger in the garage (don't fancy it in the console next to the petrol tank!!). The voltage drop is not significant as I used heavy grade hi fi loudspeaker cable. I stick it on for a couple of hours a month if the boat hasn't been used for a while at this time of year.
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
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