Your external charger should have enough 'amps' to charge your battery in the time that you want to spend waiting for it. If you have a 35Ah battery and you can wait 5 hours until it is full, then a 7A charger is needed. Charging a battery too rapidly is wasteful and dangerous, so don't stretch this too far (Don't try to charge it in two hours with 18A). The battery should not ever be in a living space because of the acid and danger of hydrogen explosion. A flooded battery seeps acid from its caps, so it must be kept away from children and animals. Make sure you don't let anything get too hot.
I use an old UPS with a car battery the same way that you plan to. It is safe for me, but I can't see if there is anything you might have missed.