The other option I looked at was a setup that had a red led that the operator could position/see on their gun. The light would light when the pack went close to 9v.
Downside...you usually end up with a flickering light as the pack gets down to the lower end. It'll dip a fair bit with sustained fire. Not bad as a warning light I suppose. The other "hard" part was wiring it so you could see it. Not too bad for handguard positioned batteries...not as noticeable for stock mounted batteries. Basically extension wires with an LED that you'd stick on your gun. Very Red/Green.
Variations of that were units that chirped...but guys found that annoying as it got low and blipped below the threshold.
In the end...I decided just to periodically check them with the plug in tap. I run them in the stock and usually the tap is easily accessible.
An alternative is to have a velcro mountable unit (or trick RIS attacheable unit!) that has a long tap extension that runs into the gun to the battery. The velcro type could be mounted on the stock tube, side of receiver or on the PEQ box. The RIS type could go anywhere...even up between sights. Little black out flip cover and you're all tricked out.
Good luck!
