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BQ34110: Using the remaining charge capacity of my battery pack.

Elder Costa17
Expert 2825 points
Part Number: BQ34110

I need some advice on how and when setting the initial remaining charge capacity and how it is updated along the life of the battery. NiMH battery pack, 12V nominal. The fuel gauge is on board, not in the battery pack. FWIW the pack will be in a floating configuration, as power backup in case of mains shortage. Thus, most if its useful life it will stay fully charged. 

1- My first basic question relates to how to set the initial charge state of the battery pack in the fuel gauge. As the FG is not in the pack, when I plug a pack I want to be able to set the initial remaining charge capacity (even if an estimated one) of the pack as it may be partially charged. I learned when I reset the FG it uses the current voltage of the pack and it uses the data in the DOD table to set this remaining capacity. From that on, it integrates the current, which is OK.

So my first question is if this is the right approach.

Is there a better or more correct one?

Does it use the learned capacity (which must degrade with battery aging) or the nominal capacity?

2- My second basic question relates to long term remaining capacity. The pack will not discharged to its minimum as the system requires a minimum voltage to work. So we are using only around 60% of the full pack capacity (when new). I need to give the system an estimation of remaining capacity so my 100% will be the pack fully charged, obviously and my 0% will be when the pack reaches 40% (give or take) of its full capacity, when the voltages reaches the minimum my system allows.

Would you have any hint or advice on how to determine the new 100% an 0% along the time? Will the fuel gauge adjust to the pack's aging?

Thanks in advance.

  • 0
    TI__Guru* 81034 points

    Hi Elder,

    You are correct about the BQ34110 method for estimating the state of charge initially. Unfortunately, this is the best the gauge can do to estimate capacity even though determining capacity based on the DOD table alone is not the most accurate. 

    The BQ34110 uses a CEDV (compensated end-of-discharge) algorithm which relies on a qualified discharge to a low SOC (typically 7%) in order to update the learned full charge capacity. For an application that will rarely discharge to a low voltage, the learned capacity will not be updated. 

    One possible idea to account for aging in your application. If you can cycle a number of your batteries over a large number of cycles (full qualified discharge so that the FCC is updated), you can characterized how the capacity changes over the number of cycles. Since Learned Full Charge Capacity is in data flash, maybe you can use this characterization data to over-write this parameter after specific numbers of cycles?

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Expert 2825 points

    Hello Matt.

    I clicked on the "resolved" by accident, but I still have some follow-up questions.

    1- Your suggestion about cycling battery pack samples seems a good plan to at least having a hint on how the battery behaves with aging (in this case an artificially accelerated aging). I will try this with a sample.

    2- BQ34110 is claimed as having a rarely discharged module. Shouldn't it work despite the lack of fully qualified discharge cycles?

  • 0 in reply to Elder Costa17
    TI__Guru* 81034 points

    Hi Elder,

    The BQ34110 does have a separate feature for applications where the battery is rarely discharged. This is different from the gauging feature. This feature is called End-of-Service determination and it is really for batteries that are always connected to a charger (like a backup battery for a security system for example). 

    Here is a video that describes this feature: https://training.ti.com/end-service-determination-algorithm

    Keep in mind, this is a separate feature that is independent of the gauging. Gauging accuracy would require an occasional full discharge to update the capacity. 

    Best regards,

    Matt