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BQ27421-G1: BQ27421 Taper Rate and BIN

Guru Level
Prodigy 55 points
Part Number: BQ27421-G1

Hi,

I am designing a device that monitors battery voltage with BQ27421-G1A. I have 2 questions one related to Taper rate value and another about BIN-input.

1:

I would like to know detailed theory of how to calculate “Taper rate” value correctly.

2:

Other ambiguous thing is control logic of BIN-input. Our product works in such way that when battery is removed, whole system (including BQ27421) get’s unpowered and we cannot satisfy any of the following battery removal requirements from (sluuac5c.pdf page 13):

How the BIN-input should be controlled in the situation where whole system get's unpowered during the battery removal?

  • 0
    TI__Intellectual 1840 points

    1. Take a look at this:

    2. To answer your second question: The feature is available for specifically battery detection. It is useful for systems that have an external system power rail (VSYS). In your case, it is not necessary you fulfill the battery removal requirements. 

    Hope this helps, 

  • 0 in reply to Nabil M
    Prodigy 55 points

    Hello,

    Thanks for answering! Issue #2 is now seems to be clear.

    about issue #1:

    I have an application where charging IC stops charging based on charging time rather that charging current. Final charging current might variate depending on temperature, system load and battery aging. Estimated taper current on my application is 5 mA. So calculations looks like following:

    Taper current (15% tolerace added): 5 mA * 1.15 = 5.75 mA

    Design capacity: 2600 mAh

    Taper Rate: 2600 / (0.1 * 5.75) = 4522

    However BQ27421 allows Taper Rate value to be something between 0 and 2000. What kind of a problems I will have when actual Taper rate is so much higher compared to what can be stored to BQ27421 registers?

  • 0 in reply to Guru Level
    Prodigy 55 points

    Hello,

    Does anyone have comment to taper rate issue?

  • 0 in reply to Guru Level
    TI__Guru 61006 points
    5mA taper current is extremely low for a 2600mAh cell. It's not possible to configure the gauge for the corresponding taper rate.

    There won't be a lot of problems because there's so little capacity pushed into the cell between charge termination at let's say 10mA vs. 5mA that the gauging impact should be negligible (e.g. if it sits there charging for another 2 hours at an average current of 7mA then that's just 14mAh vs. a capacity of 2600mAh).