Agree with Necrovis
The voltage of a battery is a good way to determine the state of charge. Here's a rough guideline off the net:
State of Charge / Voltage
100% 12.7 – 13.2
75% 12.4
50% 12.2
25% 12.0
Discharged 0 - 11.9
If your battery is reading 0 volts, chances are the battery experienced a short circuit. If the battery cannot reach higher than 10.5 volts when being charged, then the battery has a dead cell. If the battery is fully charged (according to the battery charger) but the voltage is 12.4 or less, the battery is sulfated. Sulfation is the natural by-product when the battery discharges. Naturally, re-charging the battery will reverse the sulfation crystals and turn it back into electrolyte, ready to produce power again. But if a battery sat, uncharged, severely discharged, and/or drained for extended periods of time, the sulfation will increase in size and harden onto the plates. This covers the surface area of the plates, removing the chemicals needed to produce power. Sulfation decreases the potential to reach a full charge, and it self-discharges the battery quicker than normal. At this point, charging alone will not restore the battery to a healthy condition. The majority of replacement battery purchases occur when the original battery has reached this point.