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BQ24261EVM-079: charger does not follow CCCV characteristics

syt93
Prodigy 70 points

Part Number: BQ24261EVM-079

Hi,

I have a setup where I am charging a 1500 mA-Hr battery with a max. current of 700 mA (I_charge) and a regulation voltage of 4.2 V (V_batreg). I also have a battery fuel gauge - BQ27441 in the loop which reads the battery terminal voltage and charging current. This setup is interfaced with a Raspberry Pi module in which I am logging the V-I data (with 5s sampling time). 

It's expected that battery would stay in CC mode (I_charge = 700 mA throughout) until battery voltage reaches to 4.2 V and after that the voltage would remain at 4.2 V and current slowly starts to drop from I_charge (here 700 mA) to I_term (here 100 mA).

Now, what I observe is that the current starts to drop just when the battery voltage reaches to a value less than set V_batreg (typically ~150 mV less than V_batreg). Moreover, as the current drops, the voltage keeps on increasing with a lower rate. I have set the V_batreg to 4.00 V, 4.15 V and 4.20 V with I_charge = 700 mA and observed the same behavior in all cases. I was wondering if somebody has come across such case? Would appreciate your help.

Thanks!

  • 0
    TI__Guru**** 166566 points
    Hi,

    What is the voltage at the charger's BAT pin? There is some resistance between the BAT pin and the battery pack, for example due to the gauge's sense resistor, the connection from battery to battery pack, etc. There will always be some voltage drop across this resistance which causes the voltage at the BAT pin to be higher than the voltage at the battery pack. If that voltage is the CV voltage, the charge current starts to taper slowly down from ICHRG=700mV to ITERM=100mA as the battery pack's internal cells complete charging.
  • 0 in reply to Jeff F
    Prodigy 70 points
    Thanks for a quick response!

    Yes, there is potential difference between the battery terminal voltage and V_bat which totally explains the CC to CV transition. However, it's still unclear why the battery voltage slowly increases even after switching to CV.
    Note that, when current starts to decrease (CV mode), V_bat is almost equal to V_batreg (few mV off) and remains constant but battery voltage keeps on increasing towards V_batreg.
  • 0 in reply to syt93
    TI__Guru**** 166566 points
    Think of the resistance from BAT pin to pack and "capacitive" battery cells as an RC circuit with a long time constant.
  • 0 in reply to Jeff F
    Prodigy 70 points
    Okay, that makes sense. Thank you.
    So is the wire resistance causing the trouble (very unlikely)? How to counter this problem in an effective manner?
  • 0 in reply to syt93
    TI__Guru**** 166566 points
    If reducing the resistance is not an option, then you could write a host program that raises the charger's VBATREG above 4.2V and then slowly lowers it down to 4.2V as the pack voltage gets close to 4.2V. Ideally, you would also monitor the pack voltage with an ADC to prevent overvoltage. The bq25896 has a similar feature integrated, called IRCOMP.

    The bq25898 has a battery voltage sensing pin, different from BAT, called BATSNS. BATSNS allows the charger to regulate the voltage at a remote location closer to the pack.
  • 0 in reply to Jeff F
    Prodigy 70 points
    BQ25898 seems like a perfect solution.
    Thanks very much for your help!