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BQ25895: Charging only to 4.15v maximum instead of specified 4.204V in REG06.

Benjamin Todd
Prodigy 10 points
Part Number: BQ25895

Hi everyone. I am wondering why my lithium ion cells potentially may not be charging to 4.2v? I am controlling the BQ25895 via the PIC16F15324. The code I am using is ported from a different project,  which was originally designed for the BQ25895M. However I am using the BQ25895 in its place as I ordered the wrong part. As far as I have checked in the datasheet there should be no differences that affect setting the voltage maximum in the registers, however the charge termination register reads the charge termination done bit as a 1 at 4.15v instead of 4.204v which is what it should be. I’m using a 1s2p configuration. My board layout is pretty poor (i.e very limited GND vias connecting the GND pins to GND plane). I have added 9 thermal vias to the board. The VSYS capacitors are very close to the chip, however they are connected to a general GND plane and not to the chip itself. The only other issue I can think of is that the REGN capacitor is not close enough to the REGN pin). I am reading a CCV of 4.15v. VSYS tends to read about 4.2V when the charger is plugged in, and then cuts off after about 15 seconds of the charger being plugged in.

I have tried adjusting all of the registers, including VREG, IRCOMP and ITERM, however nothing seems to be making a difference. I have an LED that reads the VBUS_CHRG_STAT register to turn on an LED (I should have used the STAT pin, oops).

As for the register values of interest, I have them set to the correct values. Please check below:

//REG05 uint8_t IPRECHG = 0b0001;
               uint8_t ITERM = 0b0011;

//REG06 uint8_t VREG = 0b010110; //4.192V
               uint8_t BATLOWV = 0; 
               uint8_t VRECHG = 0;

Register read values when charging:

REG0B: 0xb6
REG0C: 0x0
REG12: 0xb
REG13: 0x3a

Register read values when not charging: termination cut-off at 4.15v

REG0B: 0xbe
REG0C: 0x0
REG12: 0x0
REG13: 0x3a

If anyone knows if the M/non-M are interchangeable please let me know, or if I am missing something in software. Happy to share my code/board layout (which is very poor as this is my first PCB design and I am not a very experienced engineer).

Thank you so much,

Ben

  • 0
    TI__Guru 57331 points

    Ben,

    Cell voltages drop after the charge completes. This is often the normal behavior of the cell. A cell’s voltage will rise when it is being charged and will begin to drop somewhat after the charge current has been removed. This is a result of many factors, but primarily due to internal resistance of the cell and the charge characteristics of the chemistry. This is normal battery behavior and does not necessarily indicate that the cell is being discharged or that there is missing capacity.

    Thanks,

    Ning.