If all you're looking to do is run a DVD dash, not boomin it up or anything an Optima Yellow top, or any similar deepcycle battery should do the trick nicely.
You should really be looking at a deepcycle battery. As they are designed for a constant draw. Plus you can run a deepcycle into the ground and it will still take a charge. Starter batteries provide higher cranking amps, but the Yellow top for instance has CCA of 650, which is more than fine for a car.
Most, if not all batteries have an AH rating, which means Amp hours. A battery with a ratting of 55 aH means it can put out 55amps continuously for one hour, or 1 amp for 55 hours before hitting a voltage (10.8V? I'm not exactly sure) Of course if you like math you can figure out how long it will last at any load you want.
Some batteries provide a "RC" rating- Which stands for reserve capacity in minutes, which states how long it can output 25 amps until the voltage is around 10.8V, (again with the 10.8V)
I think a single, 55aH or higher deepcycle should do the trick for you.
DVD playback on a radio, even with the radio cranked will not even touch 20 amps. Even with a nice system setup, at a moderate volume for one hour, you would still be fine.
If you wanted to tack on a second battery, I would recommend the batteries being matched. The biggest thing with a second battery is for it to be vented. As with any lead acid battery, when they charge or discharge they emit fumes which can be explosive. (remember when jumping a car you always attach the dead car first?
)
Second would be isolation. Its not really important, if the batteries are matched spec then you can just tie them into each other.
However you can do several things here:
Battery isolator / diode block- Think of it as a Y-Pipe and two check valves. Diodes only let flow in one direction, the alternator charge two batteries, but the two batteries could never discharge into each other.
~The advantage is you can run OEM/starter equipment on one battery of your liking, and all your stereo and aftermarket stuff on the second battery. No mater what you do with the second battery, your car will always start.
~The main disadvantage here is didoes drop 0.7V, so your maximum charging voltage will be whatever it is (14.4V) minus 0.7V. Plus they are fairly bulky.
Battery Relay. This is merely a big honkin relay that you can decide when to isolate two batteries. You can tie it to a manual switch, wire it into the ignition system, turn on wire, whatever you like to make it turn on. You can achieve the same thing as isolator, just may be more involved, depending on ones ability.
~ The advantage here is mainly there is no voltage drop, unlike the isolator. (Also because of no voltage drop, it generates no heat and is smaller)
~ Disadvantage, when engaged you can discharge both batteries with your load. (However you can use both batteries when it is turned on)
Looking to add a second battery for DVD playback
Linear Mode
