Excellent source for battery information
http://batterytender.com/faqs.php"3. How can I tell if my battery needs to be replaced?
Referring back to the discussion of how you can calculate the charge level of your battery, we know that about 1.5 volts represent the full range of charge on a 12-volt battery. Now it is possible to over discharge a battery, well beyond its intended design. It is possible to take the battery voltage on a 12-volt battery down to 3 or 4 volts under load. That would constitute a severe over-discharge. Many lead acid batteries will not respond kindly to such abuse. Although if this only happens a few times, the battery voltage may recover to 8 or 9 volts without recharging. There is also a good chance that the battery can be restored to full health provided that it is recharged with a few hours of experiencing the severe over-discharge.
Now we said that 11.4 volts represents 0% state of charge. So the battery is sitting at a negative state of charge relative to its normal use. We usually don’t talk about negative state of charge, but rather that the battery was discharged 120% to 150% of rated capacity. Without knowledge of very recent severe over-discharge conditions, we could make a judgment about the condition of the battery by a voltage measurement. If the battery voltage on a 12-volt battery is only 8 or 9 volts, when measured in a rest state, then there is a very good chance, in fact a very, very good chance that battery is defective. At the very least it is safe to say that the battery has been severely over-discharged."
