Deanalot - exactly what I use. but I use it along with the NAZA voltage system.
I use 3s 6600ma Lipos and set the first NAZA level quite high, 11.8v, so you get more warning and the auto land very low 9.9v with a load drop of .8v. This sets the two levels at 11v and 9.1v under load. I than use the Lipo alarm, the same one you show on e-bay, at 3.5v to 3.7v per cell dependent on packs - this is under load so the actual at rest voltage is 3.6v-3.8v per cell. The outcome is that I get a visual warning and about 60 seconds later an very loud and rude audible warning that I need to land. This means I get about 60 seconds to land.
Again my F550 is 2.5Kg so at the top end of it's MTOM and draws .8v under load. Unlike my X650 which only pulls 0.3v to 0.4v and weighs the same and still has the ability to lift another 500g before exceeding it's MTOM.
Although I have gone back to the old firmware because the F550 will not fly at all properly with 3.12: will not hover at 50% throttle, very unstable, and quite frankly has instilled no confidence to me at all.