The first mistake you made was to check the condition of the battery incorrectly, this voltage can generate 12V even with a low level of electrolyte, this cell regardless of size will give the same voltage until it is discharged. The problem is that you probably dropped the active mass, in short, the lead plates fell and lay at the very bottom, especially to the level of the electrolyte, or even lower, cooking only increased their already advanced decay. Without lead plates, the acid battery loses its meaning. It is worth checking the electrolyte level, the color of the electrolyte, the appearance of the plates and the voltage, and after charging, the density of the electrolyte after adding water and charging (you have to wait about 20 minutes after adding water for the plate to soak if it was below the upper edge of the plate). The color will tell a lot about the condition of the battery plates, brown so that with very strong light you can't see the edges of the plates - a significant fall of the active mass, so there is nothing to play with such a battery. The shape of the plates will tell you how much longer such a battery will last, corrugated plates resembling a harmonica - to be thrown away. Just wait until it shorts out. The voltage says whether it is worth resuscitating it, lower than 10V for a long time = strong sulphation of the plates, all in all, it's not worth playing, now you can buy a battery without a problem. The lower the voltage, the shorter the answer what to do with it.
