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Battery Gauge Terminology

B-A
Intellectual 720 points
Other Parts Discussed in Thread: BQ28Z610

Hello,

I was reading through the TRM for the BQ28Z610 gas gauge and I came across a section that talks about the Full Charge Capacity (FCC) and the maximum chemical capacity (Qmax). These are my understanding of the two parameters:

Qmax: the maximum chemical capacity of the battery pack.

FCC: the amount of charge passed from a fully charged state until the battery reaches its termination voltage. (It seems that the value calculated for FCC would equate to the Qmax)

To me it sounds like the parameters are referring to the same thing if not extremely close, and I want to be able to differentiate when the TRM mentions them. Could you clarify or correct me if I am mistaken on what these refer to?


Thank you for your time.

Blake

  • 0
    TI__Mastermind 26225 points

    Hi Blake,

    Your understanding of Qmax and FCC is correct. They are different because the battery impedance will produce a voltage drop when discharging. This means you will always reach your termination voltage before using all of your chemical capacity. For a battery in optimal conditions, the difference between Qmax and FCC is fairly small. But if you are discharging at a heavy load or if you are discharging at low temperatures (where battery impedance is higher), you will reach the termination voltage much faster and the difference between Qmax and FCC is much greater.

    Take a look at slides 28 and 29 for reference:2015_BMS_DD_GAUGE_Fundamentals_of_Gauging_4x3_Rev2.pptx

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Intellectual 720 points

    Ben,

    In other words, the graph seems to indicate that the FCC is akin to the "usable capacity" of the cell, which may be initially close to Qmax, but will reduce over the life of the cell. Thank you for the fuel gauge guide. I found it quite helpful.

    Blake