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Original question:

BQ25895M: Charging not consistent

BQ25895M: BQ25895M can not charge full

Star Xu
Genius 9645 points
Part Number: BQ25895M
Other Parts Discussed in Thread: BQ25895

Hi 

The customer design with BQ25895M, fully charged 4.2v, BQ25895M configuration cut-off terminate voltage is 4.208V, the actual charging cut-off voltage sometimes only 3.9v, 4.0v, not close to 4.2v, observe the data BQ25895M will enter the constant voltage charge in advance, and the actual battery at this time Only 3.9v.

1. If Enable Charge is not turned on, and the 0x0E register read by Enable Charge is turned on, the voltage obtained is very different;

2, BQ25895 register configuration as below

Please give some suggestions.

Thanks

Star

  • 0
    TI__Guru** 102465 points

    Hello Star,

    This seems like it may be a trace resistance issue, if your power path is resistive the charging voltage at the actual battery terminal may not be the voltage the charger sees and it cannot regulate to the exact voltage due to the trace/connection IR drop.

    Can you share the layout, or where the battery voltage is being measured on their log? if it's a different location than where the charger is measuring that may be part of the issue.

    Sincerely,

    Wyatt Keller 

  • 0
    Prodigy 30 points

    Hi Star,

    1. It's normal for the voltage to go up when charging. The higher voltage is how you push energy into the battery.

    2. What are the battery parameters (capacity, desired charge currents, etc), charger specs, intended charging config (source type, ILIM config, watchdog config, etc). It's impossible to make a full diagnosis without knowing this.

    3. I assume those are the initial register values you're writing to the IC. But what are the values when you read them out:
      1. immediately after beginning a charge cycle
      2. towards the end of the CC phase of the charge cycle
      3. immediately after it changes to the CV phase of the charge cycle

    4. To rule out the issue Wyatt is referring to, can you confirm that the battery voltage reported by the IC matches the battery voltage measured with an external voltmeter?
  • 0 in reply to Wyatt Keller
    Prodigy 30 points

    Wyatt,

    I understood the OP to mean that the voltages shown in the screenshot are the results of reading register 0x0E, not of external measurements. Correct me if I'm wrong, but I don't think high trace resistance could cause the IC to report a voltage lower than the actual battery voltage. (It would report a higher voltage, right?)

  • 0 in reply to Antonio Tejada
    TI__Guru** 102465 points

    Hello Antonio,

    You're correct, the voltage at the BAT pin of the IC should be regulated to the programmed values, depending on what stage of charge. For this one it looks like 4.2V in CV region. It was not clear from the log image shared if the voltage was reported by our charger or from another source measuring the voltage at a different location (possibly directly on the battery terminal.) At the battery terminal the voltage will be lower due to IR losses.

    Sincerely,

    Wyatt Keller