Li-ion cells follow a simple 2 stage charge regime; CC (Constant Current) and CV (Constant Voltage), cells must not be ‘trickle charged’ at a low current as per older NiMH/NiCd chemistries.
Charge Stage 1: Constant Current (CC)
Constant current charging is the first stage of a Li-ion charge cycle, the battery is charged with a current limited power source up to the desired maximum charge voltage (usually 4.1 – 4.2V/cell). The magnitude of charge current applied to the battery is selected based on the configuration and model of Li-ion cells used within the battery pack.
Many Li-ion cells will have 2 charge current values stated within the datasheet, a ‘standard’ charge rate, and a ‘fast’ charge rate. Any charge value below the ‘fast’ charge value can be chosen, however to extend battery cycle life, the number of charge/discharge cycles that can be expected from the battery pack, it is usually advisable to select a charge rate closer to the ‘standard’ rated charge.
When Li-ion cells are configured in parallel, the datasheet charge values can be multiplied by the number of parallel cells with the battery pack.
Charge Stage 2: Constant Voltage (CV)
Constant voltage charging is the final stage of a Li-ion charge cycle, the battery is charged with a fixed voltage power source until the applied charge current required to hold the battery pack at 4.1 or 4.2V/cell decays to the desired charge termination current.
A suggested charge termination current will typically be defined on the cell manufacturers technical datasheet, but a standard charge termination current of 0.01 -0.1C is usually suitable. Charging to a datasheet termination current will put 100% of the rated cell capacity into all cells within the battery pack, it may however extend charge times beyond an acceptable level. Choosing a higher charge termination current will reduce charge times significantly as most of the cells capacity will be replenished at the higher charge rates. Continuing to charge down to the lower charge current levels will put only a small amount of additional charge into the battery pack but will take a disproportionately longer period of time. In addition to decreasing charge time, choosing a higher charge termination current will usually extend battery cycle life due to cells not being held at the highest charge voltages for extended periods of time.
Charge rate and temperature
Li-ion cells can only be charged within a narrow temperature window, usually 10-45oC. A Li-ion charger should ensure batteries outside the acceptable charge temperature range are not charged. Some Li-ion cells can be charged below the standard charge temperature window but only at a reduced charge rate, to take advantage of this feature, some charger designs are equipped with a variable charge rate option dependent on battery temperature.
Charge Current and C rate
Many Li-ion batteries will be specified with a maximum charge as a ‘C’ rate. A ‘C’ rate is a term used to calculate the charge/discharge current as a multiple of the battery pack capacity:
1Ah Battery at a 1C Charge = 1A Charge
2Ah Battery at a 1C Charge = 2A Charge
3Ah Battery at a 1C Charge = 3A Charge
1Ah Battery at a 2C Charge = 2A Charge
2Ah Battery at a 2C Charge = 4A Charge
3Ah Battery at a 2C Charge = 6A Charge
1Ah Battery at a 3C Charge = 3A Charge
2Ah Battery at a 3C Charge = 6A Charge
3Ah Battery at a 3C Charge = 9A Charge