Li Ion batteries

1865 / 18650 cells

Diameter is 18 mm and length 65.0 mm.

Capacity

Measure it

• Battery Capacity Tester
• Constant Current Load (Multimeter and Resistor)

See the chart at https://www.batteryskills.com/18650-battery-capacity-chart/

Voltage

Note that the chemistry changes and thus these value might be a bit different. Be careful.

• Discharge cutoff voltage (the low voltage limit) is around 2.0V to 2.5V
  • Over discharged < 2.5 V
  • Discharged 3 V
• Nominal voltage is (usual) 3.6 V or 3.7 V.
• Full charge voltage is 4.2 V, which is the maximum voltage

Chemistry:

• Lithium Cobalt Oxide (LiCoO2): nominal voltage of 3.6V and is known for its high energy density.
• Lithium Manganese Oxide (LiMn2O4): a nominal voltage of 3.7V and offer thermal stability compared to LiCoO2
• Lithium Nickel Manganese Cobalt Oxide (NMC): a nominal voltage of 3.6V or 3.7V and provide a good balance of energy density and power capability.
• Lithium Iron Phosphate (LiFePO4): a nominal voltage of about 3.2V with excellent safety characteristics and long cycle life.

Over the upper limit (maximum voltage) battery will cause electrolyte breakdown and thermal runaway.

Below the lower Limit the battery’s chemistry may change irreversibly.

State of charge (SOC) and maximum voltage relation.

Charging

The standard charging voltage for most 18650 Li-ion batteries is 4.20 V ± 0.05 V. But slight charge and discharge will improve the battery reliablity and life cycles.

Slow charging is best (reduces stress and lengthens cell life.). Charge at less than 1 C current whenever possible, and at less than 0.5 C for resuscitating depleted cells.

Reviving over discharged batteries

Do not attempt these methods on:

• Physically damaged cells
• Cells that show signs of leakage
• Cells that heat up abnormally
• Cells with visible corrosion

Step 1. If voltage is under 2.5V, give a battery a brief pre-charge before full charging:

• Set power supply to 3.7V and 500mA current limit. OR Set your charger to 0.1C (e.g., 30mA for a 3000mAh cell).
• Connect discharged cell and charge for 2-5 minutes. OR charge until 2.5 V.
• Check voltage again. Stop when voltage reaches 3.0V.
• This small pre-charge brings the cell voltage up enough for normal charging.

Step 2. Charge depleted cells at 0.5 C or lower (slower is better). Then capacity test cycles to check cell health. Healthy 18650 cells should be:

• 2300+ mAh capacity
• <150-200 mΩ internal resistance
• <2%/month self-discharge

Also, Step 0: the jump start

• A fully charged 18650 battery to be connected to the dead battery
• Connect the positive terminals of both batteries for 1-2 seconds using a wire
• Check voltage with a multimeter
• Repeat 2-3 times until the dead battery reaches at least 2.5V
• Once above 2.5V, your regular charger should accept it

Step 0b: The Controlled Charging (batteries that still won’t take a charge after the jump start)

• Use a power supply or specialized battery charger that allows manual voltage control
• Set a current limit of 0.1C (for a 3000mAh battery 0.1C=300mA)
• Start at 2.0V and slowly increase in 0.5V increments
• Monitor temperature constantly – if the battery gets warm to the touch, stop immediately
• Once you reach 3.0V, switch to a standard charger

References

• https://nuranu.com/how-to-fix-dead-18650-battery/
• https://cnsbattery.com/what-are-the-ways-to-repair-an-18650-lithium-ion-battery-with-a-voltage-of-0v/
• https://secondlifestorage.com/index.php?threads/how-to-recover-18650-cells-safely-and-reliably.315/
• https://www.reddit.com/r/18650masterrace/comments/10dhiel/comment/j4n2dlh/?utm_source=share&utm_medium=web2x&context=3