18650 Safety Notes

The 18650 is the energy source and the biggest safety risk in the project. Treat it with respect.

Protected vs unprotected

• Protected cells contain a small PCB (usually visible as a slightly longer cell with a ring at the positive end) that cuts off over-charge, over-discharge, and over-current.
• Unprotected cells are raw lithium cells. They are fine in a proper BMS circuit but dangerous when misused.

Use protected cells in this beacon. The TP4056 module has its own protection, but two layers of protection are the difference between a dead battery and a fire in your pack.

Why protection matters here

• The beacon runs until the cell is deeply discharged; protection stops the cell below 2.4 V.
• A shorted wire in a pack can dump 10+ A; protection trips before the cell vents.
• Charging errors (e.g. a damaged TP4056) are caught by the cell’s own protection.

Safe handling rules

• Never carry loose cells in a pocket with keys or coins. Use a plastic case.
• Inspect the wrapper: any tear, dent, or bulge means replace the cell.
• Charge at room temperature, not on a bed or sofa.
• Do not leave charging unattended overnight as a habit.
• Dispose of damaged cells at a battery recycling point, not in the trash.

Cold weather

Standard Li-ion cells lose capacity below 0 C and can be damaged by charging below freezing. For alpine use, consider a LiFePO4 cell (rated to -30 C with less capacity loss) - see the battery guide pages.

Measuring

• Full: 4.2 V. Empty (protection trip): about 2.4-2.5 V.
• Healthy cell at rest: 3.6-4.2 V.
• A cell that reads below 2.5 V at rest is suspect; do not force-charge it at full current.