When they get too low the device will flash. It depends on your coils and how much power they use as to when it flashes. I've had it flash with as low as about 15% left up to about 30% or, if the battery indicators get low enough to make you uncomfortable (about 25%-30% for me) swap them out... That's usually how I judge it. Only had it flash a couple times while I was out and trying to push my batteries as they were almost dead anyway and forgot my spares.
"Some people call me a redneck, and some people call me a hick, some people call me an asshole, and some people call me just dick. I'm a hellbilly."
HellbillyRJ