Has anyone modified UPS to use a car battery?

I have one of those portable car jump starting devices that also includes a compressor, cig lighter outlight, and a 120V electical outlet on it.  During the hurricane I found that applying the car battery post clamps to the battery, and plugging something into the electical outlet did the trick.  You would need to add a trickle charger for the battery, which I have and use pseudo regularly because I live in a location where I end up driving my car less than once a month.  I have to haul the battery inside to charge it periodically.  I'm pretty sure this setup would not be wife friendly except during a hurricane.  I haven't had problems, but I wouldn't discount the advice of having an AGM sealed battery for a "permanent" solution.