Is there an easy small portable circuit i can make that lights an LED when a voltage of BELOW 12v is put across it?

A while back i made some portable speakers using a 12V Solid Lead Acid battery and a small car amp with some speaker coned placed in a home-made MDF box. After doing a little mor research, i've read it's bad to discharge SLA batteries, especially deeply, so i was wondering if there was a small circuit i could build that will light an LED when the battery drops below a certain voltage? not necessarily automatically, it could be a seperate device that needs to be connected across the batteries terminals. As the speakers are portable, it owuld need to be kept in the bag with the speakers, and so be reasonably small.

Thanks guys, any more info i can provide just ask! 

Ya, just google for " Low battery indicator circuit" . Any kind of comparator or Op-amp ICs will do the job for you.
Here are a few simple circuits.

Just do an internet search for low battery indicator circuit and you'll come up with allot of helpful sites.