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BQ40Z50-R1: Use Saftey status COV register to terminate charge?

Abdullah Zaman
Prodigy 70 points
Part Number: BQ40Z50-R1

In our product we have seen a lot of failures occur due to COV which then leads to the SOV permanent failure.  This is because we are charging while using the product for delivering current to our product. The charger is connected in parallel to the battery and load while charging.

The solution that we have come up with is to read the safety status register for COV and then terminate the charge and enable balancing of cells. When cells are balance we go back to charging.

I have two concerns on this:

1. Would this be a good idea to implement or can it be harmful to the cells? 

2. The battery will be still in use as it can still discharge would this be ok ? or is it better if I disable the discharge as well until the cells are balanced? 

  • 0
    TI__Mastermind 24870 points

    1. Not a good idea to do this, keep terminate voltage separate from COV or SOV PF.

    2. I recommend to try this on EVM first prior to actual system. The cells cannot be imbalanced beyond a certain VIMR voltage.

  • 0 in reply to Kang Kang
    Prodigy 70 points

    Is there any good way on how we can fix this problem? We have a lot of battery failures due to this.

    We are looking into setting the Threshold for SOV PF to 4200mV as we have seen from Lifetime readings that it goes to 4000mV and our threshold is set to 3950mV which seems to be a default value set by BMS. We are trying to get the cell datasheet from our manufacturer and will be more confident if this is correct or not. But during testing we did not have any problems when changing the threshold to 4200mV. 

    But if you could suggest any better alternative that would nice. Our battery is based on  LiFePO4.

  • 0 in reply to Abdullah Zaman
    TI__Genius 10370 points

    Abdullah,

    I am lost on the issue. Do you not have the ability to set a charging voltage on your charger? If you set the correct charging voltage the current will taper and the battery voltage will stay lower than the COV threshold. 

    It is never a good idea to over charge the battery voltage or rely on protection to save. In addtion the gauge really needs the battery to charge to taper current (CCCV charging) so it can update gauging parameters correctly. 

    Thanks,

    Eric Vos

  • 0 in reply to Vos
    Prodigy 70 points

    Hi Eric,

    Can you please explain why using COV threshold to terminate charging is bad idea. I am not clear on how this could be bad since we will terminate charge before the safety alert flag goes high?  

    The idea is that if the cell voltage is approximately 3750mV that is close to the threshold for COV (3850mV) we stop charging the battery and only allow for discharging.

    Also we have 4 cells connected in parallel.

    Regards,

    Abdullah

  • 0 in reply to Abdullah Zaman
    TI__Genius 10370 points

    Abdullah,

    The gauge takes certain actions when it established a "VCT" valid charge termination event. if you stop charging at a COV alert you will never allow the gauge to get this VCT. When dealing with LiFeP04 cells this VCT is even more critical because it is nearly impossible to establish a correct state of charge based off voltage due to the voltage profile of the cells. 

    At a VCT event that is where the gauge learns the "Full Charge Point" for your profile. In addition during the CV phase that is when the gauge does it's cell balancing mainly. 

    Regardless. If you set your charger to only apply cellV max the current will taper as the battery's voltage increase while still supplying the needed power to the system.

    Thanks,

    Eric Vos

  • 0 in reply to Vos
    Prodigy 70 points

    Hi Eric,

    Does this mean there is no good software solution to this?  unless our cell datasheet specifies that over-voltage can be set higher than 3950mV. 

    Another query, If we terminate the charge on COV alert we are going to stop the gauge from getting VCT event as you mentioned. But lets say we re-enable the charge after 1 second or so wouldn't the gauge be able to get the correct reads the next time (unless it already fails in that second)

    Regards,

    Abdullah

  • 0 in reply to Abdullah Zaman
    TI__Genius 10370 points

    Abdulla,

    There is nothing stopping you from using COV alert to stop charging. I will say when the gauge calculated your FCC (usable capacity) it will simualte this from Voltage@VCT until termV. If you never allow your battery to taper you will never learn the correct VCT voltage. 

    LiFeP04 cells normally are charged to 3.65V. I am still confused on the ask in general. You should be finding a charger that outputs the charging voltage at 3.65V, battery will not be over charged if you do so.

    I would start here

    http://www.ti.com/power-management/battery-management/charger-ics/overview.html

    Thanks,

    Eric Vos