I've had battery woes over the years with the 12V twin battery set up on my 95.
After messing about with a solar panel I eventually bought a ctek , a msx 5.0 , white will "recover" a bad battery up to 120 AH or will maintain charged one up to 150 AH.
I managed to get another 5 months use out of 2 pretty knackered batteries , first by reconditioning & then by keeping the charger connected whilst the car was parked up.Highly recommended.
Another thumbs up for CTEK chargers. I have been using one for the last 10 years or so to charge & maintain the batteries on my 70 series, which sometimes spends several months at a time "resting" between trips. Its an MSX 5.0 and like goodoldboy says, will maintain a pair of 75Ah batteries in parallel and keep them in tip top condition. 7 years for the current ones and still going. The ones on the boat lasted 9 years and seemed to thrive on neglect but it looks like guyc holds the record at 13 years. These CTEKs are good all right!
One of my customers has a pedestrian fork lift. Its 1950s vintage built like an outside bog! It has 4 12v batteries and was slowing and not holding charge. I leant him my Ctek about 2 years ago and he gave each battery a couple of days on it. The thing now runs like a gazelle and holds a good charge, 2 years on.
If you have the facility and are prepared to leave your battery connected to a maintenance charger you can keep a failing battery going for years but eventually it will degrade to the point where no amount of “recovery” charging will restore its performance. I could almost certainly have got another couple of years out of the failing battery Ive just replaced by leaving the charger connected when not in use but for an (almost) daily driver it’s a bit of a PITA.