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BQ24079T: Pre-charge Safety timer setting

Godfrey Chen
Prodigy 60 points
Part Number: BQ24079T
Other Parts Discussed in Thread: BQ25180

Hi Sir,

I want to ask BQ24079T safety timer setting, I saw the RTMR resistor setting range from 18Kohm to 72Kohm in the datasheet

So the max precharge time would be 72kohm x 55s/kohm = 3960 sec, that's the maximum precharge timer is 66 minutes.

Due to our charger input source is the solar panel, it will have a lower charging current when the battery charging in precharge status.

So the precharge timing will easy to trigger the safety timer to make the charger stop charging to the battery in the field side.

Could we set the larger resistor value like 400kohm to increase the safety timer to 6 hours on precharge status?

If we increase the resistor value over 72Kohm, what it will happen in the safety timer setting?

Thanks

  • 0
    TI__Mastermind 22250 points

    Hi Godfrey,

    This is not recommended as it can lead to inconsistent results that vary from IC to IC. Do note that the safety timers are slow proportionally to charge current. 

    You may want to look into our newer device that we're currently sampling, the BQ25180. It's an I2C powerpath charger that features a configurable safety timer (fast charge up to 12 hours) where precharge is 25% of the fast charge safety timer. You can also enable the 2x slow factor where if the charge is reduced for any reason, the safety timer is doubled. 

    Best Regarfds,

    Anthony Pham

  • 0
    Prodigy 60 points

    Hi Anthony,

     Thanks for your feedback, I just want to know why we need a safety timer on the charger IC, could we disable it?

    If disable the safety timer of BQ24079T, will it have any risk to impact battery and charger IC?

  • 0 in reply to Godfrey Chen
    TI__Mastermind 22250 points

    Hi Godfrey,

    It can help prevent overcharging of the battery and can be a sign that the battery health is deteriorating if the battery is charging for longer than it should be based on the charge current. 

    You will not have any risk to the charger IC but you do risk damaging the battery. You can disable the safety timers by connecting the TMR to VSS.