Dumb question: do most battery meters (internal to the device) test voltage? If so, does that mean using this would give you no warning that your batteries are

Dumb question: do most battery meters (internal to the device) test voltage? If so, does that mean using this would give you no warning that your batteries are dying? From the demo, it sounds like they might go from 100% straight to dead, at least from the device's perspective.