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Which gas guage to choose

Other Parts Discussed in Thread: BQ34Z100, BQ76925, BQ40Z50, BQ78350

Dear forum members,

I would like to know which fuel gauges are appropriate for a battery with 4 LiFePO4 cells in series with a capacity ranging from 60Ah to 200Ah. Low power consumption is very important. It should be very low when the battery is not in use (sleep mode when the current is low). Balancing is taken care by the MCU, so no need for the Gas Gauge to support this. The MCU must be able to measure or request the cell voltages such that it can do end of charge balancing. Also protection FETs to stop charging or discharge are not necessary. The two IC’s that seem to be most appropriate are the BQ34Z100 (in combination with the BQ76925) and the BQ20Z45. My questions are the following:

-          Which gas gauges are appropriate and which is the best?

-          Does the impedance track version matter for LiFePO4 batteries with their shallow discharge characteristic? If so, which chip has the newest version?

-          I have read something about a new chip, the BQ20Z50. Is it much improved (and is it really coming; I only read it somewhere but can’t find it anymore)?

From the datasheets I have retrieved the following information regarding the BQ34Z100 and BQ20Z45. It is not that I want to hold on to one of these, if there is a better solution then please let me know.

BQ34Z100:

- Datasheet mentions support for LiFePO4.
- Support for high capacity and current batteries. However, the X10 method does not change anything in the Impedance Track algorithm, so this should be possible with the BQ20Z45 as well.
- Lower power consumption in normal mode, but about equal in sleep mode (when it is most important since the load current is small in this mode).
- The IC itself is cheaper, but the needed extra components make the difference small.
- Another IC, like the BQ76925 needed to measure cell voltages and provide the power supply.
- Only measures the impedance of the stack. Not necessarily a disadvantage.
- Higher power consumption in shutdown mode.

BQ20Z45:

- Datasheet does not mention support for LiFePO4, but the chemistry ID should be available.
- The BQ76925 has hysteresis on the regulator shutdown voltage; do the regulators inside the BQ20Z45 have this feature as well?
- Lowest power consumption in shutdown mode, this gives a longer storage period.
- Measures all cell voltages and gives the impedance of the individual cells.
- More of an all-in-one solution with internal voltage regulators.
- Supports SBS, which might be an advantage, however the BQ34Z100 provides more than enough information.
- No routing of analog signals to the MCU needed. All analog signals are digitized in the BQ20Z45, whereas the BQ76925 is merely an analog multiplexer.
- Charge/discharge FET control inside. These will not be used; can they be turned off in software to reduce power consumption? Or is it possible to just not the connect these?

I am looking forward to answers and ideas.

With kind regards,

Roland

  • Hello Roland,

    A helpful resource for selecting a Fuel Gauge is the Battery Management Guide http://www.ti.com/lit/slyt420 which is a part of the Power Management Guidehttp://www.ti.com/lit/pdf/slvt145

    For the BQ20Z50, it looks like the Z50 versions of this device were not implemented, or perhaps they where designated for proprietary use. There is the BQ40Z50 though.

    If you go with a two chip solution, your current consumption will always be higher.

    For the hysteresis, if you are referring to the BQ76925 current threshold for shutting down the regulator, these can be set in the BQ20Z50 Flash.

    Using BQ20Z45 with LiFePO4, the ChemID can be changed and this should set the Vavg to 3.2.

    • The BQ20Z45 has overprotection and under protection that will shut off the FETs. You might as well use them, even as a double redundancy. In fact, you might as well use all the features available and reduce the MCU overhead. That will also reduce your current consumption.
    • The FETs can be turned off in Shutdown Mode or through FETControl settings

    You can also download the BQ20Z45 Technical Reference Manual sluu313 http://www.ti.com/lit/pdf/SLUU313 for more details on the device.  The TRMs for the devices provide more details than the datasheets.

    Thanks,

  • Roland, 

    Greenja makes some good points in his post. The z50 device that you are referring to is the bq40z50 and I would go with it, if you are looking for an IT gauging algorithm. I would go with the bq76925 AFE and bq78350 Gauge, if you are looking for a CEDV gauging algorithm. 

    Tom

  • Hello Greenja en Tom,

    Thank you for helping me finding the right device, I will take a look at the mentioned documentation and especially the bq20z45 and bq40z50.

    With kind regards,

    Roland van de Wetering