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BQ25628: Change the charging and discharing temp range

Kevin San Diego
Prodigy 120 points
Part Number: BQ25628

Tool/software:

I am using a small LiFePO battery with a 10K thermistor. How do you Change the charging temp range such that the battery will not charge if the temperature is below 0C or above 45C? Conversely how do you keep it from discharging if outside of -20C to 60C? Also, do TS_TH_OTG_HOT and TS_TH_OTG_COLD only apply to using the boost converter for OTG application via the PMID pin? because I am not using OTG or the boost converter. However, these two properties seem like the only place where you can set the min and max discharge temps. 

  • 0
    TI__Mastermind 40375 points

    Hello,

    Please use the equations provided in the datasheet at "8.3.5.4.2 TS Pin Thermistor Configuration", along with the thermistor resistance at 0C and 45C, to solve for the upper and lower resistors in the TS network. The BATFET will connect to System if no input is present as long as the events described in "8.3.10.5.3 Thermal Protection in Battery-Only Mode" are not observed. TS_TH_OTG_HOT and TS_TH_OTG_COLD only apply to using the boost converter for OTG application.

    Regards,

    Mike Emanuel

  • 0
    Prodigy 120 points

    For the thermistor we are using the resistance at 0C and 45C degrees are 28.08K and 4.86K respectively, but I do not know what VTS_COLD and VTS_HOT are supposed to be. The datasheet shows these as percentages, not actual voltages so I am confused. The datasheet does not say what they are in the example either. All I know is that we cannot charge outside of 0C to 45C and cannot discharge between -20C and 60C and I do not know how to set these limits using the information in this datasheet. 

  • 0 in reply to Kevin San Diego
    TI__Mastermind 40375 points

    Hello,

    Equations 1 and 2 in 8.3.5.4.2 TS Pin Thermistor Configuration detail how to calculate RT1 and RT2. RTH_0degC is the thermistor resistance at 0C for your calculation, RTH_60degC is the thermistor resistance at 45C for your calculation.

    VTS_HOT and VTS_COLD which are used for VTS_60degC and VTS_0degC respectively are directly taken from the Electrical Characteristics Table. They record the TS% as reference to REGN. Because REGN can vary slightly, it is not a hard-coded voltage. Apply the percentage as a number less than 1. For example 60% apply as 0.60.

    Please see "BQ2562x Calculator Tool" <https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1209815/faq-bq25620-bq2562x-calculator-tool> for a sample calculation.

    From the datasheet: "The value of RT1 and RT2 are determined from the resistance of the thermistor at 0 and 60ºC (RTH0degC and RTH60degC) and the corresponding voltage thresholds VTS_0degC and VTS_60degC (expressed as percentage of REGN with value between 0 and 1). For the most accurate thermistor curve fitting, use the rising threshold for VTS_COLD at 0ºC and the falling threshold for VTS_HOT at 60ºC, regardless of the actual register settings for TS_TH1_TH2_TH3 and TS_TH4_TH5_TH6."

    As mentioned previously, you can use Section 8.3.5.4.3 Cold/Hot Temperature Window in OTG Mode to monitor the discharge in boost mode, but this will not prevent discharge to the system.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 120 points

    According to the datasheet the Battery manufacturer sent me for the 10K thermistor they put in the battery, the 0C and 45C resistance values are 34K and 4.3K respectively. Please see what I placed in the calculator below and verify that it looks correct. Also, how can I be sure that the BQ25628 will not charge the battery above 45C degrees? A VTS_HOT of 1.26V is not really a value that means anything to me. 

                

  • 0 in reply to Kevin San Diego
    TI__Mastermind 40375 points

    Hello,

    The values look correct. The numbers in blue correspond to a ratio of the TS voltage to the REGN voltage. These ratios hold independent of the REGN voltage. These ratios are explained in  8.3.5.4.2 TS Pin Thermistor Configuration. The ratios are documented in THERMISTOR COMPARATORS (CHARGE MODE) and THERMISTOR COMPARATORS (OTG MODE) in the datasheet.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 120 points

    What about discharge? The charging and discharging temperatures are not the same for my battery. only one value for VTS_COLD and VTS_HOT will not work for me. 

  • 0 in reply to Kevin San Diego
    TI__Guru 57651 points

    Hi,

    The battery discharge current is dependent on the SYS load. The charger does not limit discharge current unless it triggers battery overcurrent protection which is independent of thermistor temperature. Please refer to 8.3.10.1.2 Battery Overcurrent Protection for the details.

    Thanks,

    Ning.