Unless a battery is kept in a charged state constantly, it deteriorates. I doubt if your battery will ever take a charge, let alone hold it if you do happen to get it to charge a little.
When a battery discharges, the plates in the cells become lead sulfate, by a chemical reaction with the sulfuric acid in the electrolyte. The strength of the acid lessens because the acid is absorbed by the chemical reaction with the cell plates. That's normal, and if the battery is recharged right away, no problem. If it's allowed to sit in a discharged condition, even for a month or two, the sulfate turns from a spongy substance to a hard crystal. At that point, it becomes very difficult to dissolve the crystals by recharging. The longer the battery sits, the thicker and harder the sulfate crystals become.
Here's a trick I use occasionally to try to to reclaim a sulfated battery. Rob an ounce or so of acid from each cell of a well-charged battery, and put it into the cells of the dead one. This creates enough of an electrolyte situation to allow a boost charger to force a little current flow into the sulfated battery. It will probably require 16 or 17 volts to make the battery take even a little bit of charge. As the sulfate crystals begin to dissolve, the current flow will increase somewhat. After several hours, turn the charger off, and connect a load such as a sealed beam headlight to the battery and totally discharge it. On the first try, this will probably only take a few minutes. Reconnect the charger, and force a charge for another several hours. Keep a close eye on the water level- - - - -don't let the cells boil dry. Don't add any more acid, clean water only from here on. After several charge/discharge cycles, the sulfation should dissolve and at least part of the battery's capacity will be reclaimed. You should check the progress of the charge with a hydrometer, and when the acid strength stabilizes for a couple of hours, that's all the charge you're going to get. Each time the charge/discharge cycle is repeated, the sulfated battery will gain a little more acid strength. Normal specific gravity for a fully-charged battery should be somewhere between 1.265 and 1.300. A discharged battery will have a S.G. of 1.100 or less. A sulfated battery will be very close to 1.000, which is the specific gravity of plain water. All the acid is trapped in the cell plates as lead sulfate.
Good luck!
Jerry
