First of all -- most used boats will have knackered batteries. Batteries are consumables like filters and belts, and hardly anyone selling a
boat bothers to keep the batteries in good condition, hence they die. The absolute most basic items to do on a new to you
boat will be replacing all the batts, full
service of engines on board,
cleaning fuel tank, etc. That's just the very beginning of course
Just because batteries were discharged to zero a few times doesn't necessarily mean they're ruined. It depends a lot on what kind of batteries they are. I had a new set of Vartas which were knackered after a
single incident of being run out and left like that. I got less than two years out of them. My
current batteries are Trojans. As a result of various mishaps, they have been run out dead flat twice and left like that for several weeks. After being brought carefully from the dead both times, they are still good as new. Literally -- I regularly measure the capacity with an Argus
battery tester, which shows that they are still holding more charge than when they were new (before being broken in), despite those two incidents, and being almost two years old. That's the difference between a car-type
battery (even if it's
sold for
marine purposes, as a "semi-deep discharge" or "leisure" battery) and a battery with thick plates and heavy internal construction.
That being said, being left dead for a few weeks is probably the limit. Few batteries will survive being left dead for months, as so often happens to boats which are
for sale.
So if I were you, I wouldn't faff about with it -- just go buy a couple new batteries and move on.