For what it's worth, I've successfully reanimated a sealed lead acid battery that was kept totally discharged for maybe a year (zero volts), at least it's good enough for what I need it for anyway.
To begin with, the battery was high impedance and wouldn't accept any significant charging current at the proper voltage. What I did is connect it to a current limited laboratory supply, initially set to a low current limit (100mA for a 7AH battery) and I turned the voltage up to about 8 volts per cell. The low current limit is to prevent too much heat from being generated in the battery due to the high voltage. The battery passed only a very small current at first, in spite of the very high charging voltage.
After some hours, the current built up until the current limit was reached and the voltage gradually dropped down to the normal charging voltage, and I was able to put an amp or two into the battery, at which point I set the voltage limit to the rated float charging voltage printed on the battery. I waited a couple of days till the battery had accepted a charge, then I tried it out under heavy load, and it has been satisfactory ever since.
Another time I was unsuccessful in reviving a battery because it developed a shorted cell either before, during or after the revival procedure. Since it was given to me as dead, it is quite possible that it had the shorted cell before it was allowed to get deep-discharged.
Chris