Cell Discharge Curve

A lithium cell’s voltage does not fall in a straight line: it holds a plateau for most of its life, then falls off a cliff. Understanding the curve explains why the beacon’s battery warnings work the way they do.

The shape of the curve

A discharge from 4.2 V to 3.0 V looks like:

Charge remaining	Voltage
100%	4.2 V
90%	4.1 V
70%	3.9 V
50%	3.7 V
20%	3.5 V
5%	3.2 V
0%	3.0 V and falling fast

The two lessons

1. The plateau: between 4.1 and 3.5 V the voltage changes slowly. A reading of 3.9 V could mean 70% or 50% depending on load, temperature and cell age.
2. The knee: below 3.4 V the voltage drops steeply. The beacon’s warning threshold sits just above the knee because that is where the remaining time becomes minutes, not hours.

Why voltage is a poor fuel gauge

The voltage depends on load (TX bursts pull it down a tenth of a volt), temperature (cold pulls it down more), and cell age (an old cell reads lower at the same charge). The firmware’s monitor handles this with thresholds, not a percentage guess (see Battery Monitor Details).

The practical use

Voltage (resting)	Meaning
4.1 - 4.2 V	Full
3.7 - 3.9 V	Good, most of the charge left
3.5 - 3.6 V	Half or less; plan
3.3 - 3.4 V	Low; the TX burst will sag (see Brownout Protection)
below 3.2 V	End of useful life for this session

Related pages

• Battery Monitor Details for the thresholds
• Battery Capacity Math for the runtime
• Battery Troubleshooting for the measurements