out-of-battery
In battery-powered electronics, devoid of any power.
out-of-battery: when the cells are exhausted and dead
An out-of-battery condition occurs when a rechargeable or primary cell has discharged completely and can supply no usable voltage to the circuit. In battery-powered devices, this is the end state of the discharge curve, where terminal voltage drops below the minimum operating threshold required by the load. For alkaline cells, this is typically around 0.9 volts per cell; for lithium-ion cells in series strings, it may be 2.5 to 3.0 volts depending on chemistry and cell configuration.
The term distinguishes between low-battery states, where warning systems trigger, and true battery exhaustion. A device detecting an out-of-battery condition must be designed to cease operation safely, not attempt to draw further current that could damage the cell through reverse current or internal resistance heating. Many designs include a battery management system that actively disconnects load when voltage falls below a defined threshold, preventing the cell from reaching truly zero volts and suffering permanent damage.
A fully discharged cell does not spontaneously recover capacity through rest or ambient temperature change. However, a secondary (rechargeable) battery in an out-of-battery state will accept charge when the charging circuit provides sufficient voltage and current. Primary (non-rechargeable) cells offer no recovery path and must be replaced. In series packs, an out-of-battery condition in a single weak cell can force the entire string below operating voltage, even though other cells retain charge; this is called cell reversal and degrades all cells in the string.
In field service, an out-of-battery state is often the first suspect when a portable instrument fails to power on. The distinction matters because a device that fails during out-of-battery operation should show no signs of partial function, intermittent operation, or unintended behavior. If a device behaves erratically when battery voltage is critically low, the root cause is usually inadequate brownout protection in the power supply or logic circuits, not the battery itself.