I do actually have a car battery charger that is a pretty slick device, but its trickle charge is not enough to power my PC all night long, and in any case my DC-DC power supply/inverter/regulator (whatever the hell it is) monitors my battery power and shuts down the PC when it gets too low. So draining my battery isn't really a concern - it's not going to happen.
However, I DO want to be able to keep that PC on for as long as I want - by plugging in the vehicle. And since the power supply wasn't designed to be hooked up to a battery - it's just supposed to be a source of DC power for whatever bench work you may be doing - I'm not sure how it would interact with the battery in terms of drawing current. What stops the battery from just drawing as much power at the device outputs? Also, is there any likelihood that the battery itself would actually output power to the device?