Battery Technologies for Embedded and IoT Devices

Why Battery Choice Matters

The battery is often the largest and most expensive component in a portable embedded device. Choosing the right battery type affects the device size, weight, cost, operating temperature range, lifespan, and user experience. A poor choice can lead to a device that dies too quickly, cannot operate in cold environments, or degrades after a few months.

Common Battery Types

Alkaline (AA, AAA, 9V)

Parameter	Value
Voltage	1.5V per cell (drops to ~0.9V when depleted)
Capacity	AA: ~2500 mAh, AAA: ~1000 mAh
Rechargeable	No (single use)
Temperature Range	-18 C to 55 C
Self-Discharge	~5% per year
Cost	Very low

Best for: Low-cost devices, prototyping, remote sensors where replacement is easier than recharging. Use 2x AA (3V) or 3x AA (4.5V) with a voltage regulator.Watch out: Voltage drops continuously during discharge. Your circuit must work across the full voltage range (3.0V fresh to 1.8V depleted for 2x AA).

Lithium Coin Cell (CR2032, CR2025)

Parameter	Value
Voltage	3.0V (very flat discharge curve)
Capacity	CR2032: ~230 mAh, CR2025: ~160 mAh
Rechargeable	No
Max Continuous Current	~3-5 mA
Self-Discharge	~1% per year
Cost	Very low

Best for: Ultra-low-power devices drawing under 50 uA average: BLE beacons, key fobs, RTC backup, simple sensors.Watch out: Cannot deliver high current. WiFi or LoRa transmissions (100+ mA) will cause voltage to collapse. Add a buffer capacitor if you need occasional current spikes.

Lithium-Ion / Lithium-Polymer (Li-Ion / LiPo)

Parameter	Value
Nominal Voltage	3.7V (4.2V fully charged, 3.0V empty)
Capacity	100 mAh to 10,000+ mAh
Rechargeable	Yes (500-1000+ cycles)
Max Current	High (1C to 3C typical, e.g., 1000mA for a 1000mAh cell)
Self-Discharge	~2-3% per month
Energy Density	Highest of common batteries

Best for: Most IoT and wearable devices. High energy density, rechargeable, and can deliver high current for WiFi/cellular transmissions.Watch out: Requires a protection circuit (BMS) to prevent overcharge (above 4.2V), over-discharge (below 3.0V), and short circuits. LiPo cells can swell or catch fire if mishandled. Always use a proper charging IC like the TP4056 or MCP73831.

Lithium Thionyl Chloride (Li-SOCl2)

Parameter	Value
Voltage	3.6V
Capacity	Up to 19,000 mAh (D-size)
Rechargeable	No
Temperature Range	-60 C to 85 C
Self-Discharge	Less than 1% per year
Shelf Life	10-20 years

Best for: Professional IoT deployments that must last 10+ years without maintenance: smart meters, industrial sensors, infrastructure monitoring.Watch out: Low pulse current capability. Not rechargeable. More expensive. Has “passivation” effect where the first pulse after long storage has a voltage dip.

NiMH (Nickel-Metal Hydride)

Parameter	Value
Voltage	1.2V per cell
Capacity	AA: ~2000-2800 mAh
Rechargeable	Yes (500-1000 cycles)
Self-Discharge	~20-30% per month (standard), ~5% (low self-discharge like Eneloop)

Best for: Replaceable rechargeable option. Drop-in replacement for alkaline AA/AAA in many designs.

Battery Comparison

Type	Voltage	Rechargeable	Energy Density	Best For
Alkaline AA	1.5V	No	Medium	Prototypes, disposable sensors
CR2032	3.0V	No	Low	Ultra-low-power, BLE beacons
Li-Ion/LiPo	3.7V	Yes	High	Wearables, WiFi/cellular IoT
Li-SOCl2	3.6V	No	Very High	10+ year industrial IoT
NiMH AA	1.2V	Yes	Medium	Consumer devices, replaceable cells

Battery Management Basics

Charging Li-Ion / LiPo

Li-Ion batteries require a specific charging profile called CC-CV (Constant Current, Constant Voltage):

Phase 1 - Constant Current:
  Charge at a fixed current (typically 0.5C to 1C)
  until voltage reaches 4.2V

Phase 2 - Constant Voltage:
  Hold voltage at 4.2V
  Current gradually decreases
  Stop when current drops below ~50mA (C/20)

  Voltage
  4.2V |           _______________
       |          /
       |         /
  3.0V |________/
       +---------+---------+---- Time
            CC       CV

Popular charging ICs: TP4056, MCP73831, BQ24072.

Monitoring Battery Level

Read battery voltage with an ADC through a voltage divider (LiPo voltage exceeds 3.3V ADC limit):

  VBAT ──[100K]──┬──[100K]── GND
                 |
              ADC Pin

  V_adc = VBAT / 2

  Battery percentage (rough approximation for LiPo):
    4.2V = 100%
    3.7V = 50%
    3.3V = 10%
    3.0V = 0% (stop discharging!)

Protection Circuit

Every LiPo/Li-Ion system needs protection against:

• Over-charge: Voltage above 4.2V damages the cell and risks fire
• Over-discharge: Voltage below 2.5-3.0V causes permanent capacity loss
• Over-current: Drawing too much current generates heat
• Short circuit: Must disconnect immediately

Most LiPo cells sold for hobby/maker use include a tiny protection PCB. For custom designs, use a dedicated protection IC.

Practical Design Tips

1. Calculate your power budget first. Estimate average current before choosing a battery. Do not pick a battery size and hope it works.
2. Account for voltage drop. If your MCU minimum is 2.8V and LiPo drops to 3.0V, you have very little margin. Consider a buck-boost regulator to maintain stable output.
3. Consider temperature. Alkaline capacity drops 50% at -20 C. Li-Ion should not be charged below 0 C. Li-SOCl2 works down to -60 C.
4. Size the capacitor for current spikes. A 100-470 uF capacitor on the power rail helps the battery handle brief high-current events like WiFi transmit bursts.
5. Add a low-battery warning. Monitor battery voltage and alert the user or trigger a graceful shutdown before the battery dies.

Summary

Choosing the right battery is a critical design decision for any portable embedded system. Alkaline cells are cheap and simple for prototypes. Coin cells power ultra-low-power BLE devices for years. LiPo batteries are the standard for rechargeable IoT devices. Li-SOCl2 serves decade-long deployments. Match your battery to your power budget, current requirements, operating temperature, and desired lifespan.