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BQ25306EVM: charging time is almost same even changing charging current

Chase Jeong
Genius 4670 points
Part Number: BQ25306EVM

Hi all,

Now I am using BQ25306EVM for charging LiFePO4 battery.

the problem is that even I change charging current, charging time is almost same.

the condition is as below;

case1:

charging voltage : 3.6V (changed R5 to 455K)

charging current : 2A (JP10 jumper installed, JP9 is not installed)

overall charging time : about 18 min (from completely discharged to end charging)

CC charging time : about 2 min

CV charging time : about 16 min

case2:

charging voltage : 3.6V (same as case1)

charging current : 3.3A (changed R11 to 12K, under same jumper setting as case1)

overall charging time : about 16 min (from completely discharged to end charging)

CC charging time : about 17 second

CV charging time : about 16min

and here is the spec. of battery;

nominal voltage : 3.2V

nominal capacity : 360mAh

charging voltage : 3.6V

output capacity : 10C

as shown above, CC charging time is much different, but CV charging time is almost same.

but overall charging time is important to me, i thought that there is much difference between 2A charging and 3.3A charging.

anyway I want to make charging time much more shortly, about less than 5 minutes.

is there any settings that I should change?

Please check this issue. Thanks.

Best regards,

Chase

  • 0
    TI__Mastermind 24075 points

    Hi Chase,

       This is a function of battery charging property itself and not so much the charger. To ensure accurate charging to the battery full charge regulation voltage, we charge in CC (constant current) and CV (constant voltage). In CC, charge current is regulated at fast charge current setting, until VFB rises to 1.1V. After this charger moves to CV phase, where VFB (equivalent to charge regulation voltage of battery) is regulated at 1.1V, and charge current tapers down until it hits its termination threshold, after which termination is detected.

    CV phase is more dependent on your test setup:

    • If you have impedance in between battery and where battery voltage is sensed, this causes an IR drop which causes the charger to enter CV earlier
      • In your end system, the BAT pin of the charger is a remote BATSNS pin which can be tied directly to battery. This allows charger to sense true battery voltage and increase time spent in CC.
    • Battery impedance/battery pack protector FET itself. Older batteries have higher impedance

    As you can see, without CV phase, and only CC phase, your battery would not be charged to its true 100% capacity, which is why we need CV phase.

    CV phase will always be the longer phase, which is why most end applications that spec fast charging will spec the time in terms of time spent in CC to charge up to ~80%-85& capacity and not full 100% capacity

  • 0 in reply to Kedar Manishankar
    Genius 4670 points

    Hi Kedar,

    Thank you for your reply.

    I read your reply, still I don't understand few things clearly.

    1. I understand that it needs CV phase after CC phase, but what I wonder is why the time of CV phase is almost same between 2A charging and 3.3A charging.

    the reason why we use 3.3A charging instead of 2A charging is finally to reduce time to charge the battery. but the time is not much different.

    i think it means that we don't need to use 3.3A charging to reduce charging time.

    2. if I use recently-designed and produced battery, can I reduce the charging time? or should I design the PCM module to reduce time? what is the most important factor of pcm to reduce charging time?

    3. is it impossible to reduce time by changing the values of EVM?

    I think the questions are pretty long but my purpose is only one; reduce the charging time.

    please check this issue. Thanks.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 24075 points

    Hi CHase,

     Termination time is fixed at 10% of ICHG so the CV loop will terminate at a higher charge current in the 3A setting vs 2A. Please refer to my above explanation of how and what influences CV and time in CV.

    Having the BAT pin tie directly to your battery will also help reduce time spent in CV.

  • 0 in reply to Kedar Manishankar
    Genius 4670 points

    Hi Kedar,

    1. do you know how to increase CC time or decrease CV time by changing the value of resistor or capacitor on EVM?

    2. can I remove 10uF capacitor or add more capacitor on BAT pin? what will affect to the charging time?

    3. our battery is LiFePO4, and has 3.6V charging voltage. is it harmful increasing charging voltage to 3.7V or 3.8V?

    Please check these issues. Thanks.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 24075 points

    Hi Chase,

    1.   You can change the charge current in CC, but there is no setting to decrease the time in CV as it is dependent on your system (see above: Battery age etc). The remote BAT sense will help as described above.
    2. This will not affect charging time, as the capacitor is to be added for loop stability. No issue with adding extra 10uF, but also no issue with keeping 10uF and saving on one passive component for your BOM
    3. Recommended maximum charging voltage for LiFePO4 is 3.6V. Check with your battery manufacturer with regards to overcharging your battery. Generally this is not recommended.