When to change the batteries?

I've got a Devo 7e with Deviation v5 installed and no hardware mods.

I'm just using 4 throw-away alkaline batteries and right now the voltage display says 4.10V. Actually, it kind of fluctuates between 4.0v and 4.12v. Yesterday I got a brief message saying "can't save config due to low battery", or something like that...I forgot to write down the exact message.

Bottom line: how do I know when to to replace the batteries? I don't want to waste batteries, but I also don't want the Devo to die in mid flight either.

(I realize I can get a lipo and use a charger, but I have a *ton* of these AA batteries laying around and might as well use them.)

At 4.1v, they're very close to dead. Lowest I'd run them is 1.2v/battery, so 4.8v (Assuming they're generic 1.5V batteries). Below that, you risk one battery dropping faster than the others and having a sudden brown-out.

Thanks Arakon; I'll change them now.