While I won't deny that such a system may exist (a battery that switches on and has no power-conditioning built in), I haven't touched one and I'm not sure it would make sense to build that way.
The "brown out" is a perfect example of why. If your circuit is either-or (battery or live) then a brown out will eat power at the same rate as a blackout; and if you *can* run through the battery (there are a couple different paths) I can't image why you would ever not.
Similarly, without conditioning in the system, you are begging for issues with where the switch-over point should be. Set it too high and you'll unnecessarily loose power: set it too low, and your equipment will fail.
But it seems we are "six of one, a half-dozen of the other". If adding power conditioning to a battery turns a UPS into a power-conditioner, then adding the battery to line conditioning turns a power-conditioner into a UPS.
Perhaps like a "receiver", which is a combination decoder, A-D converter, switch, pre-amp, amp, and tuner (to name a few), we need to invent some new words.