Originally Posted by yvrr
The accompanying chart lists 12.2v as 50% and 11.9v as 25%. So, basically, discharging the battery to below 12.2v will ultimately mean that it will no longer hold a charge for as long as when new.
Keep in mind that these voltages are for when the battery is unloaded. When it is pulling a load it will drop the voltage a bit depending on how large the load is. For example, your battery may be at 12.3V with no load, but will drop down to 12.2 when the fridge kicks on. After the fridge is done cycling, the voltage may pop back up to 12.3 after a brief period. The 12.3v is your reference point for approximate state of charge, not the 12.2.
In a nutshell, yes - deeper discharges will shorten the life of your battery. Nine years is a pretty good service life, especially if you've been doing deep discharges often.