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BQ25150: Battery charging time

Sara Glick1
Prodigy 130 points
Part Number: BQ25150

I'm testing the BQ25150 with several different LiPo batteries, and the CV phase of the charge time seems excessive. I have the pre-charge set to 77.5mA, the fast charge set to 500mA, and the termination charge set to 50mA (10% of fast charge). With several batteries now in the 900mAh to 1000mAh range, I see that the CV phase of charging lasts more than 3 hours. I expected the entire charging process to be around 3 hours, not just the CV phase. Is this expected behavior? What can I do to speed up this process? 

  • 0
    TI__Genius 12485 points

    Sara,

    Are you performing this test on an EVM or a custom board? Do you happen to have a register dump of all the registers so that we can diagnose what is going on?

    Any plots on the VBAT voltage/ current will help to see if it was a smooth transition into CV phase. 

    If the current is getting limited, I would expect any of the loops (VINDPM/ VDPPM/ ILIM or thermal regulation) to have triggered. Are you seeing true charging currents out of the device?

    Regards,

    Gautham

  • 0 in reply to Gautham
    Prodigy 130 points

    Hi Gautham, 

    This is on a custom board. It doesn't seem like the charge current is being limited, it's still tapering off slowly to 50mA, but it takes a really long time to reach 50mA. I just did a register read of 0x0 while the battery is in the CV phase, and it returned 0x41 (no loops active). Here are the rest of the relevant registers:

    registers while charging:
    0x0: 0x41 
    0x1: 0x0
    0x2: 0x60
    0x3: 0x61
    0x4: 0x0
    0x5: 0xE0
    0x6: 0x0
    0x7: 0x0
    0x8: 0x0
    0x9: 0x71
    0xA: 0x0
    0x12: 0x3C
    0x13: 0xC8
    0x14: 0x9F
    0x15: 0x14
    0x16: 0x0
    0x17: 0x92
    0x18: 0x42
    0x19: 0x7
    0x36: 0x0
    0x37: 0x0
    0x61: 0x34

    Thanks,

    Sara

  • 0 in reply to Sara Glick1
    TI__Genius 12485 points

    Hi Sara,

    I checked the register dump above and none of the register looks suspicious. I noticed that you are setting the termination to 10% of FastCharge which is around C/20 for the battery size. 

    We might have to look into the plots of VIN/ VBAT I/V plots. Is the system pulling any current during the charging process?

    Regards,

    Gautham 

  • 0 in reply to Gautham
    Prodigy 130 points

    Hi Gautham, 

    The system idle current is ~30mA, so I would expect it to draw 30mA from USB during charging. Attached is the schematic of the BQ25150 on my board. We tested our register settings on the EVK and the charging time was much faster (and it line with what I expected). Do you see anything in the schematic that's a red flag? 

    Thanks,

    Sara

  • 0 in reply to Sara Glick1
    TI__Genius 12485 points

    Sara,

    I see the issue! (well hopefully) The IMAX value you seem to use is limiting around 201mA of current. Which explains the +2x long charge time. Now do you actually see the current being around 200mA at 3.5VBAT? 

    Can you try swapping the IMAX resistor with the value on EVM (10kohm) and give it a go?

    Rest of the schematic looks cool. Assuming I2C is pulled up to VIO as well. 

    Regards,

    Gautham

  • 0 in reply to Gautham
    Prodigy 130 points

    Hi Gautham, 

    I do see the battery fast charge at 500mA when in CC mode. My understanding is that if the Rimax value is 5.9kOhm, register 0x13 is set to 0xC8 (ICHG code = 200), and register 0x14 is set to 0x9F (ICHARGE_RANGE = 2.5mA step), the fast charge value will be 500mA. The datasheet says that the Rimax value determines the maximum ICHG_CTRL register value (decimal), which comes out to 201 in my case. So that means that the ICHG code cannot be higher than 200 (decimal). Is that understanding correct? Based on that logic, I don't think that changing the resistor value will change anything (but I will try tomorrow when I'm back in the lab). 

    Thanks,

    Sara

  • 0 in reply to Sara Glick1
    TI__Genius 12485 points

    Sara,

    Correct. Are you also making sure the watchdog disable bit is written? If we have a plot of IBAT through the charge profile, that might give us some clues. 

    Also possibly thermal regulation since you are seeing it on the custom board. Can we take a look at the layout? If you are okay with reaching on email- let me know. i have your info. 

    Regards,

    Gautham

  • 0 in reply to Gautham
    Prodigy 130 points

    Hi Gautham,

    Yes, we've disabled the watchdog timer by setting register 0x17 to 0x92. I'll work on getting the IBAT plot for you. I didn't see any of the FLAG bits that looked suspicious for thermal regulation.

    I can share the layout over email (please send me a note, I don't think I have your email anymore). 

    Thanks,

    Sara

  • 0 in reply to Sara Glick1
    TI__Genius 12485 points

    Hi Sara, 

    Just sent you a note. :)