Added 2020: I have edited this post, removed some unrelated material and somewhat improved the formating. BECAUSE: It is interesting and may be useful but should be treated with care. It contains substantial non standard advice, quite a lot of which goes against standard practice. However - The OP claims that it works (for him at least) and it seems worth looking at. - RM
Sorry, I know this is old question and late answer...
Like all lead/acid batteries (AGM,deep cycle wet, automotive starting batteries,whatever) they want to be kept near full charge and you should know the longer they are not at peak charge means the electrolyte is weaker than normal and that means the sulfuric acid is converted to a lower concentration and is weaker and is now more water and less sulfuric.
So you are telling us you leave the battery this way for a month, but like we said, all lead acid batts need to be near full charge and recharged immediately for best results and lifespan.
So how many times(hours/days/months) were you "actually" leaving the battery one quarter empty, half empty,three quarter empty,etc??,see??
Even though it was "in use" it was technically/basically only putting out the equivalent of a self discharge current.
Best to of had a tiny solar panel helping it along if possible.
I could be wrong but I think you would of been better off using a smaller batt and charge more often ..hope you have had goodluck in the long time since your original question friend
Sorry, i re-read your question and you asked how to possibly revive the battery.
What i do (and have had success doing for 7 years to my four 12ah batteries in my 48volt electric scooter, yes 7 years on original AGM batts that I drain almost dead every ride!!) - what i do is open the batt and open the 6 cells and slightly re-wet the fiberglass mats..do not soak them!! This helps tremendously because even AGM batts try to outgas when recharging.
Obviously this is "wetter" than factory and will bubble more vigorously so I like a knife slot into the rubber valve cap to stop excessive internal pressure and when the batts seem to "run dry", I just add a tiny bit more and I guess I have been lucky to add just the same amount that outgassing each year because it worked for 7years, but cutting the slit makes the outgassing easier so feel free to take a chance not cutting a slit or poking a pin hole in the pressure valve.
I did have one 20ah batt on a different ebike pop a seam on its side due to a valve not releasing pressure so now I make sure to cross slit all my batts caps and i just re-water more often.
I take the battery down to 10.8volts and then wet the cells slightly more than factory, then charge at a slightly higher than suggested amps to re-convert any stubborn sulfation, then i drain the batt at one quarter of its rated Ah and recharge and repeat a couple times. Each time you keeps track of the time it takes to calculate its new Ah rating, which should improve. But most people nowadays don't have the patience
I use a 12volt 300watt xantrex brand inverter and a few incandescent lightbulbs plugged into it to drain the battery at the wattage and amps that I want, and I have 2/8/12amp automatic, intelligent chip, smart desulfate CanadianTire brand charger. I even can fool the smart chip into desulphate mode if I want to with by using a dual element automotive brake bulb and some alligator clips that i wire in series with the charger to increase the perceived internal resistance of the battery. I can use the dim setting (high resistance) or I can use the brightest mode which means low resistance to current and higher amps). I prefer adding the least resistance (higher wattage) bulb filament. :-) not only that but if I feel like having the battery charge longer than the charger "thinks" it should on its own, I can use the 2 different wattages of the bulb in parallel connection to the batt terminals and the charger will sit at top voltage longer by perceiving the battery is still accepting a higher amperage when in fact it is the bulb and battery in parallel. This allows me to boil off excess water if I added too much.
The default shutoff in the charger is for 0.3amps at the 2amp manual setting, 0.8amps at the 8amp manual setting, and 1.3amp shutoff at the 12amp setting.
Each manual setting tells the charger what approximate size Ah battery you are charging and the charger shuts off to respect your wishes. It assumes you know what you are doing. It correctly assumes that a huge battery is finished charging when it only takes 1.3 amps at top voltage, and that a small battery is done charging when it takes 0.3amps at full top voltage. Or you can let the smart charger constantly monitor the internal resistance of the batt on automatic setting and hope for the best.