I'm not sure I'm providing enough information in order to get a meaningful reply, but I'll try anyway: Lets say you have a regulator of some sort, with a LC filter on the output, and some sort of feedback loop with an unknown transient response (i.e. you lack info on the system's phase margin, unity gain, etc). Now if you hook a large battery (with very low internal resistance, if that changes anything) onto the output so the regulator works both as a charger and a power provider to the loads, is it possible to say anything a priori about how this affects the system stability? I understand that if the battery can be seen as a large capacitor the LC cutoff frequency drops, but what I'm looking for is an argument along the lines of "since there's a large battery on the output, stability is guaranteed...". Does such an argument exist? Is it possible to say anything general about a situation like the above?