Protected vs Unprotected Cells
Edit on GitHubThe difference between protected and unprotected 18650 cells, and which one belongs in a beacon.
Protected vs Unprotected Cells
An 18650 cell without protection is a small bomb waiting for a mistake: overcharge, over-discharge, or a short can start a fire. The protection circuit is the difference between a battery and a hazard.
What protection does
A protected cell has a small PCB under the wrapper that cuts the cell off:
| Protection | Action |
|---|---|
| Overcharge | Cuts off above ~4.3 V |
| Over-discharge | Cuts off below ~2.5 - 2.8 V |
| Overcurrent | Cuts off above the rated current |
| Short circuit | Cuts off instantly |
The cost
| Unprotected | Protected | |
|---|---|---|
| Length | 65 mm | 67 - 69 mm |
| Capacity | Nominal | Slightly lower (circuit space) |
| Current limit | None | 3 - 10 A typical |
| Safety | Depends entirely on your charger and firmware | Circuit-level |
Which for the beacon
Use a protected cell. The beacon’s TP4056 charger does not terminate at 4.2 V perfectly for every cell, the firmware’s low-voltage cutoff is a software promise, and the cell sits inside a plastic case against other metal. The protection circuit is the last line that the firmware cannot replace.
Fitting the longer cell
Protected cells are 2 - 4 mm longer. The beacon case is designed around protected-cell dimensions. If you print your own case, measure the cell with its wrapper before cutting the battery slot (see Case Cutouts Guide).
Spotting a fake
Counterfeit “protected” cells are common on marketplaces. Buy cells from a known battery seller (the brand’s official store or a reputable distributor), check the weight (a real 2500 mAh protected cell weighs 45 - 50 g), and treat suspiciously cheap cells as unprotected.
Related pages
- 18650 Battery Guide for the recommended cell
- 18650 Safety Notes for handling
- Charging and Cell Care for the charging side