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Batteries for small devices

Coin cells, Li-ion and LiFePO4: their voltages, their capacity, and when the capacity on the label actually holds.

Foundation

Read firstWriting a power budget
Nominal voltages of common small-device cells, and how lithium-ion is charged A CR2032 coin cell is 3 V, most lithium-ion and lithium-polymer cells 3.6 to 3.7 V, and LiFePO4 about 3.2 V. Most lithium-ion cells are charged to about 4.2 V with constant current, then constant voltage, and none should be charged below 0 °C at normal rates. Nominal voltage 3 V 3.6–3.7 V about 3.2 V 0 V CR2032 Li-ion, Li-Po LiFePO4 Charging a lithium-ion cell about 4.2 V voltage current constant current constant voltage Not to scale CR2032 coin cell Roughly 220–240 mAh, but only at very low drain. High pulse currents cut the capacity you can use. Li-ion, Li-polymer Most are 3.6–3.7 V nominal, charged to about 4.2 V by constant current, then constant voltage. LiFePO4 About 3.2 V nominal. Less energy per kilogram, but longer cycle life and better thermal stability. Do not charge lithium-ion below 0 °C at normal rates Lithium plating on the anode causes permanent damage. Many charger chips read a thermistor in the pack and pause charging outside a set range.
A CR2032 is 3 V, most lithium-ion cells 3.6–3.7 V and LiFePO4 about 3.2 V; most lithium-ion cells charge to about 4.2 V, and none should be charged below 0 °C.

Key points

  1. A CR2032 lithium coin cell is nominally 3 V with a rated capacity of roughly 220–240 mAh, but only at very low drain; high pulse currents reduce the capacity you can actually use.123
  2. Most lithium-ion and lithium-polymer cells have a nominal voltage of 3.6–3.7 V and are charged to about 4.2 V using a constant-current then constant-voltage profile.4
  3. Lithium iron phosphate (LiFePO4) cells have a lower nominal voltage of about 3.2 V and less energy per kilogram than other lithium-ion chemistries, but longer cycle life and better thermal stability.5
  4. Lithium-ion cells must not be charged below freezing (0 °C) at normal rates, because lithium plating on the anode causes permanent damage. Many charger chips read a thermistor in the battery pack and pause charging outside a set temperature range.67

Where to go next

Sources

  1. CR2032 product datasheet · Energizer · Datasheet
  2. Lithium Coin Handbook and Application Manual · Energizer · Reference
  3. Coin Cells and Peak Current Draw (SWRA349) · Texas Instruments · Reference
  4. BU-409: Charging Lithium-ion · Battery University (Cadex) · Reference
  5. BU-205: Types of Lithium-ion · Battery University (Cadex) · Reference
  6. BU-410: Charging at High and Low Temperatures · Battery University (Cadex) · Reference
  7. BQ2407x USB-friendly Li-ion charger datasheet (SLUS810N) · Texas Instruments · Datasheet