Choosing the right battery and managing it properly is essential for reliable emergency operation. This guide covers selection criteria, charging procedures, and power management strategies.

Recommended Battery Types

Primary Recommendation: 18650 Li-Ion

Specification	Value
Chemistry	Lithium-ion (Li-ion)
Nominal Voltage	3.7V
Recommended Capacity	2500-3500 mAh
Discharge Rate	1C minimum (3A continuous)
Protected Cell	Yes (recommended)

Top recommendations:

Samsung INR18650-30Q (3000 mAh) — Best balance of capacity and discharge rate
Panasonic NCR18650B (3400 mAh) — Highest capacity, lower discharge rate acceptable
Samsung INR18650-25R (2500 mAh) — Best for high-drain Emergency mode

Tip

Always buy batteries from reputable suppliers. Counterfeit 18650 cells are common and may have significantly lower capacity than labeled.

Alternative: 21700 Cells

For extended runtime, a 21700 cell can be used with a modified battery holder:

Cell	Capacity	Advantage
Samsung INR21700-50E	5000 mAh	67% more runtime
Molicel P42A	4200 mAh	Higher discharge rate

Note

21700 cells require a different battery holder and minor enclosure modification. The BOM specifies 18650 as the standard.

Battery Discharge Characteristics

Voltage vs. Capacity (18650)

Voltage	Approximate Charge	Device Behavior
4.20V	100%	Full power
4.00V	80%	Normal operation
3.70V	50%	Normal operation
3.50V	20%	Low battery warning
3.30V	5%	Critical — enter sleep
3.00V	0%	Cutoff — device shuts down

The firmware monitors battery voltage through a resistive voltage divider on ADC pin GPIO 36 (ADC1_CH0, SVP) and displays the percentage on the OLED. The divider halves the voltage: a full 4.2 V cell reads about 2.1 V at the ADC input.

Charging

TP4056 Charger Board

The BOM includes a TP4056 USB-C charging board with the following characteristics:

Input: 5V via USB-C
Charge Current: 1A (default, adjustable via Rprog)
Charge Time: ~3-4 hours for 3000 mAh cell
Indicator: Red while charging, blue when complete

Warning

Never charge the battery inside a sealed enclosure without ventilation. While rare, lithium cells can vent gas during fault conditions.

Charging Procedure

Ensure the device is powered off (switch in OFF position).
Connect USB-C cable to the charging port.
Red LED on TP4056 indicates charging.
Blue LED indicates charge complete.
Disconnect charger after completion — do not leave on trickle charge indefinitely.

Power Consumption by Mode

Mode	Average Current	3000 mAh Runtime
Beacon (default)	~45 mA	~65 hours
Beacon (low power)	~25 mA	~120 hours
Search	~80 mA	~37 hours
Config (WiFi)	~150 mA	~20 hours
Emergency	~350 mA	~8.5 hours
Deep Sleep	~10 uA	~34 years (theoretical)

Tip

In Beacon mode, the device transmits briefly every 30 seconds and sleeps the rest of the time. The average current is dominated by the sleep current and the brief TX bursts.

Battery Care and Longevity

Storage: Store at 3.7-3.8V (40-60% charge) for long-term storage.
Temperature: Avoid charging below 0C or above 45C.
Cycle Life: Expect 300-500 full charge/discharge cycles before significant capacity loss.
Self-discharge: Approximately 2-3% per month at room temperature.

Field Tips

Carry a spare charged 18650 cell in a separate pouch.
In cold weather, keep spare batteries inside your jacket.
Label batteries with the date of purchase and last test date.
Test battery voltage with a multimeter before each deployment.
A fresh cell at 4.2V provides maximum runtime and TX power.