Battery Capacity Math

Runtime is capacity divided by average current. The subtlety is “average”: the beacon’s draw swings from microamps in sleep to hundreds of milliamps during transmit.

The basic formula

runtime (hours) = capacity (mAh) / average current (mA)

An 18650 rated 2500 mAh at 100 mA average lasts 25 hours. The rating is itself a moving target: capacity drops as the current rises, and cells age (see 18650 Battery Guide).

The duty cycle average

The beacon spends most of its life in deep sleep. The average current is the sum over one full beacon cycle:

avg = (I_sleep x t_sleep + I_tx x t_tx + I_gps x t_gps) / cycle_time

Worked example: standard beacon

With GPS always on and a 10 s sleep interval:

avg = 30 mA (GPS) + 120 mA x (2/12) + 0.02 x (10/12)
    = 30 + 20 + 0.02 = 50 mA

With a 2500 mAh cell: 2500 / 50 = 50 hours. With GPS off and manual coordinates: about 20 mA average, roughly 125 hours. The Power Budget and Runtimes page lists the measured table.

Derating

Real cells deliver 80 - 90% of rating at moderate current, and the beacon should not run the cell to zero anyway (see Undervoltage Protection). Plan on 70 - 80% of the theoretical number as your honest estimate.

Field check

The firmware’s battery monitor reports voltage, not remaining mAh. The honest field rule: the 3.3 V rail holds until the cell sags; the Battery Monitor Details page explains how to read the warning states.

Related pages

• Power Budget and Runtimes for the measured table
• Current Draw by Mode for the numbers
• Battery Selection for choosing the cell