In home setup

Hi, I am wanting to test some stuff before I install/sell and I would like to do so in the house. I understand that hooking up a car battery charger isn't the best idea because a charging battery will emit hydrogen gas. However, due to the fact that I'm not trying to consistently run a system for long periods of time (maybe one song at a time), can I charge a car battery outside and then bring it in... Then proceed to run the system off of a car battery? Is this safe? Are there better ways to do this without absolutely breaking the bank? I've heard of Xbox and pc power supplies but it doesn't seem they're strong enough and I don't want to spend too much time if I'm only testing inside. Please let me know. Thank you.