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BQ24074: Safty Timer

Chris Boehm
Prodigy 30 points
Part Number: BQ24074

Tool/software:

Hey,

I am thinking about how to set up my safty timer. I am using  a 1S -Lipo (3.7 V, 0.150 Ohm). Right now i am charging at ~900mA and I_TERM is set to default. The Lipo is fully charged after~ 2hr. The fast charging ands after 1hr.

Do you have any experience what a good value for a safty timer would be? Should i set it to something like 1hr + X?

As far as i understood it: When the battery gets older, the max capacity gets less. does it mean the fast charging end earlier?

Is the safety timer somehow affected during charging if the BQ24074 is also supplying power to another device?

Thank you very much!

Best,

Chris

  • +1
    TI__Mastermind 26385 points

    Hi Chris, 

    The safety timer feature is intended to function as a backup method to stop charge and should be set to a sufficiently longer time duration that the time to charge the battery in your system. 

    The minimum time duration safety timer can be set for is typical 2.4 hours via R_TMR = 18 kohms.  If TMR is left floating default timer duration is typical 5 hours. 

    Chris Boehm said:
    As far as i understood it: When the battery gets older, the max capacity gets less. does it mean the fast charging end earlier?

    Yes when Li-Ion battery gets older the max capacity gets less, but in general an older battery will take longer to charge compared to a new battery due to increase in their internal resistance. 

    Chris Boehm said:
    Is the safety timer somehow affected during charging if the BQ24074 is also supplying power to another device?

    Please refer to datasheet section 9.3.5.6. Certain events, such as system load activating DPPM loop will cause timer to be extended. At a high level if supplying power results in charge current less than ISET value the safety timer will be extended. 

    Best Regards,

    Garrett