I found this useful deep-cycle battery FAQ that included a rough voltage level chart which helps you determine how full your deep-cycle battery is:

State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75

While I haven’t blogged about it, I have a small solar panel setup on my patio which provides enough power to run a few lights in my living room each night for a few hours. This information lets me know how charged the battery system is. Right now my battery is at 12.75 volts.