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Original question:

BQ77915: Accuracy of OV and UV recovery voltage

BQ77915: BQ77915

Di Ma
Intellectual 420 points
Part Number: BQ77915

Hello team,

I'm trying to fully understand the working principle of BQ77915 and I have a question regarding the OV limits.

1: So if 4.2V/cell is a standart voltage for Li-Ion batteries and my charger is set to 4.2V and the OV of BQ77915 is also 4.2V it means that it won't charge the battery since it will enter in OV state? Or are the 7791500 and 7791502 designed to protect Li-Ion batteries charged at voltages below 4.2?

 2: In 9.3.2.1 in the datasheet of BQ77915 I read "In the event of an overvoltage fault condition on a particular cell, the balancing FET corresponding to that cell is turned on until the cell voltage drops to the full charge voltage or until the cell has recovered from overvoltage fault condition, whichever occurs earlier".  So in case of an OV does the CHG fet turn off or the balancing of that specific cell starts?

I'm trying to get maximum capacity from the battery, so I want it fully charged. I would choose BQ7791501 for extra 50mV in the OV treshold but OTC is higher than the temperature allowed for charging for Li-Ion batteries (45 degrees C).

Thanks in advance

Dmitriy

www.rmftek.com

  • 0
    TI__Guru 54715 points

    Hi Dimitriy,

    1. 4.2V is the OV threshold of the BQ7791500, with the tolerance stated in the data sheet.  The charge voltage should generally be a lower value to avoid driving the pack into overvoltage.  Limiting the charge voltage of a cell capable of 4.2V (or higher) charge to a lower voltage is a technique to extend the life of the cell although it does not use the full capacity of the cell.

    2. Section 9.3.2.1 of the data sheet https://www.ti.com/document-viewer/BQ77915/datasheet/operation-in-ov-slusbz73011#SLUSBZ73011 describes operation in OV,  the CHG output turns off.  There is a balancing behavior in OV also if enabled described in 9.3.4 https://www.ti.com/document-viewer/BQ77915/datasheet/cell-balancing-slusbz7478#SLUSBZ7478.  The wording in the text above figure 7 may have been different, since it says the balance FET is turned on.  The OV cell is balanced, but the balance FET cycles in the normal manner balancing and measuring so it can sense when the recovery condition is met.  So in OV both the CHG FET control turns off and balancing on the OV cell begins.

    3. Yes, for maximum capacity from a battery you would want a protector which allows full charge of the battery.  The temperature thresholds are ratios as shown in the electrical characteristics table https://www.ti.com/document-viewer/BQ77915/datasheet/electrical-characteristics-slusbz76382#SLUSBZ76382.   The part sees temperature as a ratio of the RTS_PU and RTS.  You can adjust the temperature trip point by selecting the RTS_PU and/or adding a series or parallel resistor with the thermistor.  RTS_PU will tend to shift the temperature curve up or down, a series resistor affects mostly the high temperature and a parallel resistor to the thermistor affects mostly the low temperature, but a resistance change does affect all the thresholds. .