i have seen posts here for turning computer power supplies into car battery chargers.

What i was wondering is if it is possible to hardwire something like this directly to the battery either one made from a pc power supply or something i can buy(if there is something like this) and use it when the car is not running so i can run my car pc and other equipment without draining the battery.
i am thinking of using this when going camping so i can just plug my car into an outlet and run some of the devices inside without worrying about the battery being drained.
i am not sure if this would have a negative effect on my car battery.

please let me know if there is someone out there who has done this
any help is greatly appreciated.