ZHCSJR3B February 2019 – November 2019 BQ25887
PRODUCTION DATA.
Some applications require cell balancing when the user can replace one or both of the cells in the 2S1P configuration. When charging two batteries with different voltages, cell balancing is required, as the cell with the higher voltage is at risk of being overcharged. For extremely unbalanced cells, charging the lower voltage cell as well as fast cell balancing is desired.
The BQ25887 implements a passive cell balancing scheme with a recommended maximum discharge current of 400 mA. Balancing current is limited by external resistors placed between the CBSET pin and the mid-point of the two cells. Low side cell voltage is sensed at MID pin. Cell balancing can be enabled in the I2C registers.
The Cell Balancing current limit resistor, RCBSET, can be calculated as below.
ICB_LIM = VCELLREG / (RCBSET+RDSON_QCBX)
For example, the maximum recommended cell balancing current is 400 mA. For 4.2-V battery cell, RCBSET can be calculated as 9.5 Ω (typical).
Cell balancing status register, CB_STAT, HS_CV_STAT and LS_CV_STAT is active in both automatic cell balancing mode and manual cell balancing mode.
The default setting of the cell balancing parameters are below.
PARAMETER | REGISTER | DEFAULT VALUE |
---|---|---|
Enable Auto Cell Balancing Mode (CB_AUT0_EN) | REG0x2A [6] | 1 = Enable |
Disable Charge for Accurate Cell Balancing Measurement (CB_CHG_DIS) | REG0x2A [7] | 1 = Charge Disable for Cell Balancing Voltage Measurement |
Voltage Threshold Enter Cell Balancing Qualification Mode from Pre-Qualification Mode (VQUAL_TH) | REG0x29h [7:4] | 1111 = Disable Pre-Qualification Mode |
Voltage Threshold Enter Cell Balancing Active Mode from Qualification Mode (VDIFF_START) | REG0x29h [3:0] | 0100 = 80 mV |
Voltage Threshold Exit Cell Balancing Offset from VDIFF_START (VDIFF_END_OFFSET) | REG0x28 [ 7:5] | 001 = 40 mV |
Time Interval between Taking Measurements in Pre-Qualification Mode(TCB_QUAL_INTERVAL) | REG0x0x28 [4] | 0 = 2 min |
Time Interval between Taking Measurements in Cell Balancing Active Mode (TCB_ACTIVE) | REG0x28 [3:2] | 10 = 2 min |
Time Delay between Charge Disable and Cell Voltage Measurement (TSETTLE) | REG0x28 [1] | 10 = 1 sec |