I dont use my boat much anymore so it usually sits 1-3 months between each use. This has happend to me twice now with 2 different sets of batteries.

I went over yesterday to pull off the cover and clean. Go to raise the hatch and nothing happens. Batteries are dead. Go to hook up the charger and it wont charge because they are so dead. I keep a spare small marine battery on my boat for emergencies so I jumped it to the main battery so the charger would register volts and it would charge. After about 2 minutes I removed the jumper battery and the charger dropped to only 10.2 volts and it was up to 14.2 within just 5 minutes. Took charger off after only 5-10 minutes. Go back today, batteries are at full charge. The starters spin the engines like they are new. Its like the batteries go into sleep mode. This happened before to my 6 year old gel cell Kinetik batteries and I just assumed they were old. Now I have lead Interstate batteries that are almost 1 year old and it did the exact same thing.

What am I not seeing here? Its literally like I had to wake them up. Is this normal?

Prob the internal lead plates of the battery were sulfated badly, Charging and or put a charge on the battery removes the sulfation from the lead plates bringing back the voltage

On one year old batteries? Do they recover 100%?

The disconnects switces are always off when not in use. Its the medium size Schuler hooked directly to the terminals. They dont charge if they dont detect any voltage so I use a jumper from a good battery so it detects voltage.

Agree voltage coming up that quick on charge sounds like sulfated battery. If that's the case battery won't have a good deep charge, not much reserve. I think I would go back to basics. Fully charge both batteries, disconnect all cables from batteries. Check and record voltage on batteries. Come back in couple days or a week and check voltage again to see if they are discharging internally. Are tops of batteries clean? I could see one bad battery sucking down the other, but not if the disconnect switch is shut off and nothing else is providing a path between the two.

Just curious, did you find the problem?