Undervoltage Protection
Edit on GitHubProtecting the cell from over-discharge: the firmware cutoff, the protected cell's circuit, and the thresholds that keep the battery alive.
Undervoltage Protection
A lithium cell below about 2.5 V starts to be damaged: internal shorts form, capacity is lost, and in the worst case the cell becomes a fire risk when recharged. Undervoltage protection is the layered system that prevents this.
The three layers
| Layer | What it does | Speed |
|---|---|---|
| Firmware warning | Reports low battery, warns the user | Soft, advisory |
| Firmware cutoff | Refuses to transmit below the threshold | Seconds |
| Protected cell circuit | Physically disconnects the cell | Instant, hardware |
The firmware’s role
The battery monitor (see Battery Monitor Details) drives two thresholds:
| Threshold | Behavior |
|---|---|
| Warning (~3.4 V) | OLED warning, shorter beaconing |
| Cutoff (~3.2 V) | TX disabled; the device still powers the display for configuration |
The firmware’s cutoff is a software promise: it protects the cell only while the firmware runs.
The protected cell’s role
The protected cell (see Protected vs Unprotected) is the hardware backstop: its circuit disconnects the cell at about 2.5 - 2.8 V, regardless of the firmware. This is the layer that protects the cell from the firmware’s own bugs and from a flat battery left installed for months.
The layered design
| Failure | Which layer catches it |
|---|---|
| Firmware crashes, device left on | Protected cell circuit |
| Cell aged, voltage collapses fast | Firmware warning first, cutoff second |
| User ignores all warnings | Protected cell circuit |
The restart
After the protected cell disconnects, the beacon appears dead until the cell is recharged. That is correct behavior: a disconnected cell is a protected cell.
Related pages
- Protected vs Unprotected for the cells
- Battery Monitor Details for the thresholds
- Cell Discharge Curve for the voltage shape