For those of us with Android smartphones, there is a nice, free app in the Google Play store that acts as a virtual gas gauge. It's called Fuel Gauge (https://play.google.com/store/apps/details?id=com.kworthstudios.fuelgauge) and I've discovered it works pretty well. The developer says a future enhancement will allow for a configurable low-level warning and she's open to any other ideas you may have for new features. I was recently in touch with her and she said she was busy with other projects for a while, but is now working on the app again. One tip: if you want to exit the app, but keep it running in the background, use the phone's home button to keep it active. Pressing the back button will completely stop the app.