2x what Jim Higgins said!

I worked for a Automotive Salvage Yard and at one time the owner of the junk yard allowed the employees to keep all the used batteries that did not have a full charge or that had a hole in them.
I myself and my friend would bring these batteries home - in the trunk of a Lincoln Continental and stack them on pallets until we got enough to make a trip to the scrap yard, where they would pay $1.50 each for the used batteries.

I had at times patched some of the batteries - with everything from Bondo - not a good idea, to JB Weld, to Blue Goo - Blue or Red Permatex.

I had a sewage problem at my QTH - home septic not city and I took the tops off of several batteries and poured the acid inside of the commode and when I was done, the porcelain shined like new. It did a good job of cleaning out the pipes.

We once needed a battery, short term for a demolition derby car and didn't want to use one of our good ones, so we found a car battery with a dead cell and we took a 3 lbs dead blow hammer and we beat on the bottom of the battery and eventually it broke up the sludge on the bottom and broke the short and held a charge.

Marine Battery - 8 years old, PLLLEEEESSSSE give me a break.
Quit being so cheap and go buy a new battery.
As a matter of fact, if the two batteries are ganged in parallel, you need to buy two new batteries, else the weaker of the two will rob power from the newer battery and the old battery will kill the new battery!

Only a moron would post a battery question in an antenna forum.

My guess is that the boating people gave you the same answer, you just didn't want to listen.

If you have no Denero - then you aren't going to be going boating this year with two dead batteries.

The folks that gave the advice that the batteries does not like to be stationary is dead on.
Maybe you could build a battery tray in the trunk of your vehicle and haul them around while connected to a battery isolator all winter, and you could connect something to them, maybe a car stereo or something, that way the batteries would cycle each time you drove the vehicle and it would keep them from going dead prematurely.

That is what I like about people.
When the sun shines, they don't think about their batteries, then when winter comes, they drag their battery out of their boat and they put it on a float charger and they think that they can perpetuate it by charging it once in a while. Eventually they forget about the battery for a month or two, or they place it directly on the cement floor and it goes dead and then in the spring they run out to their shed or their basement and go to grab their battery from last year - and it is dead!
Then they cry, moan and complain until they pony up the bucks to go buy new ones!

You would be better off to buy a vehicle that has a battery tray large enough to hold those batteries and rotate them on a schedule in your vehicle and use them once every 3 months for a month to keep them charged then you would just setting them on your work bench on a charger.