The Odyssey 2150 is a Group 31 battery with 205 minutes of reserve capacity and 94 amp hours. If it's only lasting 3 hours, then it's not starting out with anywhere near a full charge. If you run it until things stop working, you've obviously gone well below the 50% DoD down below 80%. That's OK with AGMs, but it's still better to keep it 50% or higher. In any case, if you draw 45 amps out of there to get it to 50% DoD, then it'll require somewhere around 12-14 hours off the alternator to fully recharge it. Every time it doesn't get fully recharged, sulfation increases and lifespan and capacity decreases. My guess it that's it's been chronically undercharged and chronically discharged too deeply, and the capacity is about half of the original 94 Ah.
If the current battery is severely sulfated and has a diminished capacity, adding a second battery will just kill the second battery in short order. Better to replace the current battery with 2 new ones.
First thing you need to determine is the amp draw of your appliances. Watts divided by volts is amps. If your TV draws 60 Watts, then it's drawing 5 amps from the battery. (Actually, 5.5 because of the 10% loss of the inverter). An xBox draws about 170 Watts of power, which is 14 amps (15.5 with the 10%). Just watching a DVD on an xBox draws 120 Watts (10 amps). I'm guessing, but add another 5 for the TV and another 5 for the computer and you're at a 25 amp draw. That's a lot. So on a fully charged 2150, 3-4 hours is all you're gonna get. Two hours would run it to 50%.
If you want to draw 20-25 amps an hour for 8 hours between charges and stay no less than 50% DoD, you'll need a battery bank of 400 amp hours or more, or a generator to run things.
There should be a tag on all the appliances to let you know how many Watts or amps they draw. The computer, it's probably on the transformer. Report back here what you find.
