Lightweight AC adapter with Battery Backup?

My plan is to make a portable lamp which I could unplug and take outside, beyond the reach of AC power. So far, I have envisioned using an old alarm clock's battery backup + AC adapter, coupled with changing the power cord to something detachable.

Does the part I need already exist? I hope to do this with rope lights, which could probably be powered by one or two AA batteries.

Thanks for any help,
E

see "Flashlight"

Why not use a charger circuit instead of an AC adapter and use a recharegeable battery?

Brilliance! This forum has already proved its value. Thanks.