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Battery Gauges Selection Questions

John Griffith
Mastermind 21501 points
Other Parts Discussed in Thread: TPS61099, BQ27Z561, BQ35100, BQ34Z100-G1, BQ27Z746

Hello,

My customer has a LTO battery cell that they want to be able to discharge as low as possible. I have two questions:

  • What gauge in our portfolio has the lowest VDD? This is not something that is sortable on the parametric table so it isn't easy to find. 
  • Do we have any gauges in our portfolio where you can separate the VDD of the gauge and the analog front end that measures the battery?
    • This would allow a small boost converter, for example the TPS61099 which can run down to 0,7V to keep the gauge powered through a deeply discharged battery

Thanks! 

John

  • 0
    TI__Guru** 103035 points

    Hello John,

    The lowest voltage would be the newest gauges (BQ27Z46, BQ27Z561) with a low voltage supply of 2V.

    If you want to use separate power supply most of our gauges have separate pin for VDD that is not a sense line, just be aware of any other parameters in the DS that may be linked to the VDD like pull-ups or thresholds.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Mastermind 21501 points

    Hi Wyatt,

    Have you seen applications where a customer has successfully used a boost converter for the supply pins and then gauges a cell to lower voltages without any issues in accuracy? This could open up a number of gauging opportunities if this is possible. 

    John 

  • 0 in reply to John Griffith
    TI__Guru** 103035 points

    Hello John,

    As long as the gauge is powered and the chem ID is matched with the cell for lower voltages, then the accuracy wouldn't be affected. The main gauges I'm referring to with separate REGIN (VDD) inputs are the BQ34Z100-G1 and BQ35100. Any of the gauges with protections it may not be possible since we use VDD as a reference for some of the gate drive controls. But top of stack or single cell gauges with separate VDD and sense lines there should be less of an impact on the operation.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Mastermind 21501 points

    Hi Wyatt,

    The customer was looking at our newest BQ27Z746 device for a 1S application with a cell that can discharge completely without damage. Looking at the capacity curve, the amount of usable energy below, let's say 0.7V, is very little. But being able to go from even 2.0V (our minimum VDD) to 1V would enable them to use a decent more of the battery's capacity.

    Looking at the ABS MAX table for the BQ27Z746, it doesn't look like any of the other pin voltages are dependent on the VDD and the EVM has a jumper that would make it easy to have a separate supply. Do you see issues with this device being used this way? I was thinking of offering a boost converter EVM, and the BQ27Z746EVM and having them prototype the idea - but before doing that I want to understand if we see an issue with using the BQ27Z746 device this way.

    Thanks as always!

    John

  • 0 in reply to John Griffith
    TI__Guru** 103035 points

    Hello John,

    I think this could work, it is a fairly new part so we don't have any examples showing this use case. The only thing I see that it may affect is the zero volt charging feature which is hardware based and uses the VDD for reference, so with this kind of configuration the zero-volt charging may not be supported.

    Which chem ID are they looking to use down to 0.7V? Or are they wanting the battery to be characterized?

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Mastermind 21501 points

    Hi Wyatt,

    Its a custom LTO cell and we are trying to figure out CHEM ID or if one needs to be made. Thanks for pointing out the zero volt charging will not work, I didn't realize this feature is enabled off of a threshold on VDD-VSS and not VBAT-VSS. I am not sure if the LTO cell needs any special zero voltage charging but will relay this concern to my customer. I will close this thread now. 

    John