Duracell 4D batteries are dead?

For Lead Acid batteries, they need to be recharged when they drop to ~75% or less State of Charge... Below ~75% SoC, they begin to sulfate more quickly (sulfation being a permanent loss of battery capacity).

More or less, flooded cell deep cycle lead acid batteries stored at 77F need to be recharged (roughly) 1 day out of every 30 days. (flooded cell FLA deep cycle batteries have a faster self discharge rate)

AGM version of lead acid batteries stored at 77F need to be recharged (again roughly) 1 day out of every 6 months. (AGM batteries have a lower self discharge rate).

If the batteries are stored cool/cold, they will self discharge slower (i.e., 18F cooler, self discharge is 1/2 as fast). If stored hot (i.e., 18F warmer than 77F, they self discharge 2x faster).

The battery chargers are assumed to be large enough to recharge the batteries in 12-24 hours.

Sulfation [corrected. -BB] is, in general, a permanent conversion of fluffy gray Lead Sulfate (part of battery charge/discharge chemistry) slowly turns into hard black crystals which removes lead (and sulfur) from the battery charge/discharge chemistry. A battery sitting below ~75% State of charge--The conversion can start in as little as a few hours. Batteries that are "actively cycling", have very slow sulfation process.

There are people and hardware that claim they can desulfate Lead Acid batteries--That has been a bit something that has strong opinions on both sides... From what I understand (I am not a battery engineer), the "pro-desulfators" believe that high frequency electrical current can "vibrate" the crystals back into lead and sulfur (into electrolyte). I believe that the best that can usually be done is to "equalize charge" the battery (higher voltage over charging) which causes gassing and can shed the "insulating" black sulfate crystals from the plates and expose "virgin plate" material... Again, as I understand...

For cars that sit around a lot (extra cars that don't get driven much in these days of $6.00 a gallon gasoline and/or has small loads like GPS, LoJack, vehicle computers, etc.), I use Battery Minder and Battery Tender brands of "float chargers" to keep the batteries relatively healthy.

They seem to work well and don't "boil the battery dry" (over months of "float charging") like some of the cheap 1 amp cube float chargers from the auto parts stores.

https://www.amazon.com/s?k=battery+minder
https://www.amazon.com/s?k=battery+tender

Some of the charge controllers claim to "desulfate"... Some will cycle charge (absorb through float)... But from what I have seen, you want something like 13.6 volts or so float charging at room temperature. If you have a charger that "floats" at >13.8 to 14.4 volts or greater, they tend to make the batteries "gas" (producing hydrogen and oxygen--And slowly losing water).

-Bill