A lithium-ion battery is considered effectively "dead" or critically over-discharged when its voltage drops below a specific threshold, typically 2.5 volts per cell. While some protection circuits might allow discharge down to 2.75 volts, going below this range can cause severe, often irreversible, damage to the battery.
Understanding the "Dead" State
Lithium-ion batteries are sensitive to deep discharge. Unlike some other battery chemistries, discharging a Li-ion cell below its safe voltage limit is highly detrimental.
- Critical Threshold: The generally accepted over-discharge voltage limit for a single lithium-ion cell typically ranges from 2.5 to 2.75 volts. Once the voltage drops below this point, the cell enters a critical state.
- Irreversible Damage: When a lithium-ion battery is discharged below this threshold, it undergoes chemical changes that can cause permanent damage. This damage often manifests in several ways:
- "Sleep" Mode: The battery may enter a deep "sleep" state, making it very difficult for standard chargers to detect and initiate a charge.
- Inability to Charge: In severe cases, the battery may become so dead that it completely loses the ability to accept a charge.
- Capacity Loss: Even if a deeply discharged battery can be revived, its overall capacity will likely be significantly reduced, making it impractical for use.
- Safety Risk: Over-discharged batteries can also pose a safety risk if an attempt is made to forcefully recharge them.
Why Avoid Over-Discharge?
Maintaining the voltage above the critical threshold is crucial for the longevity and safety of lithium-ion batteries.
- Preserving Battery Health: Most electronic devices with lithium-ion batteries incorporate a Battery Management System (BMS) or protection circuit. This system is designed to cut off power to the device when the battery voltage reaches the lower safe limit (e.g., 2.5V or 3.0V, depending on the manufacturer's specification) to prevent deep discharge and protect the cell.
- Maximizing Lifespan: Consistently preventing deep discharges helps to maintain the battery's internal chemistry and structure, thus preserving its cycle life and overall lifespan.
Key Voltage Ranges for Lithium-Ion Cells
To put the "dead" voltage in context, here's a general overview of typical voltage ranges for a single lithium-ion cell:
| Voltage Range | Condition | Description
