Battery life formula

Runtime (h) = (Capacity × Efficiency%) / Current draw

Efficiency accounts for losses in the battery, converter, and wiring. A typical value is 80–90% for a fresh lithium battery.

Typical current draw reference

Arduino Uno (active): ~50 mA
ESP32 (Wi-Fi active): ~150–240 mA
Raspberry Pi Zero 2: ~300 mA
LED (5 mm, standard): ~20 mA
Small servo: ~100–300 mA

Battery chemistry comparison

Chemistry	Nominal voltage	Typical cycle life	Common uses
Li-ion (Lithium-ion)	3.6–3.7 V	~300–500 cycles	Smartphones, laptops, power banks
LiPo (Lithium polymer)	3.7 V	~300–500 cycles	Drones, RC vehicles, wearables (flexible form factor)
NiMH (Nickel-metal hydride)	1.2 V	~500–1000 cycles	AA/AAA rechargeable batteries, older electronics
Alkaline	1.5 V	Non-rechargeable	Remote controls, flashlights, low-drain devices
LiFePO4 (Lithium iron phosphate)	3.2 V	~2000+ cycles	Solar storage, e-bikes, high-safety applications

Self-discharge rates

Batteries lose charge even when not in use. Choosing the right chemistry for standby applications is important:

Li-ion / LiPo: ~2–3% per month - excellent for devices used weekly.
NiMH: ~10–20% per month - needs recharging before use after storage.
Alkaline: ~2% per year - long shelf life; suitable for emergency devices.
LiFePO4: ~1–3% per month - very low self-discharge; good for infrequent use.

mAh vs mWh: which matters more?

Capacity in mAh (milliamp-hours) is printed on most batteries and power banks, but it only makes sense when comparing batteries at the same voltage. A 3000 mAh Li-ion cell at 3.7 V holds 11.1 Wh of energy. An AA alkaline at 1.5 V with 2500 mAh holds only 3.75 Wh - less than a third as much energy, even though the mAh figures look similar.

mWh = mAh × Voltage - use watt-hours (Wh) to compare batteries of different chemistries or voltages fairly.