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BQ35100: How to calculate remaining battery percentage using BQ35100 for SAFT Li-SOCL2 batteries?

yash
Prodigy 10 points
Part Number: BQ35100

Hi,

Can you explain, how to calculate the remaining battery percentage using BQ35100? I'm using a SAFT Li-Socl2 battery.

Thanks,

YASH

  • 0
    TI__Guru** 102395 points

    Hello Yash,

    Please review the section in the TRM: 5.3.3 End-Of-Service Smoothing

    The gauge cannot reliable give SOH% information in EOS mode, only in certain circumstances and near the end of discharge is the SOH% reported in EOS mode possibly valid. Each case requires some system evaluation. The main focus of the EOS mode is to provide an alert when the battery is nearing end of life.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 10 points

    Thanks for your reply,

    As per accumulator mode, we can find passed discharge using AccumulatedCapacity() so based on this passed discharge value, can we find the remaining battery percentage?

    Thanks,

    Yash

  • 0 in reply to yash
    TI__Genius 11770 points

    Hello Yash,

    Accumulator mode is essentially just a coulomb counter. So you can roughly estimate the remaining capacity based on the specified capacity and the accumulated charge. However, it can't account for self-discharge, temperature, or resistance. EOS mode does work quite well for your chemistry. Ultimately, gauging primary batteries is very difficult and all of the methods have their advantages/drawbacks. 

    Thanks,

    Alex M.

  • 0 in reply to Alexander Mazany
    Prodigy 10 points

    Hi,

    So how can I calculate the remaining capacity based on the specified capacity and the accumulated charge? Can you suggest to me any algorithm?

    Thanks,

    Yash

  • 0 in reply to yash
    TI__Genius 11770 points

    Hi Yash,

    What I mean is something as simple as:

    full capacity (from cell spec) - accumulated charge = remaining charge

    Then you could divide remaining by full if you want a percentage. The only downside is that this method has some inaccuracies, for example the full capacity may vary from the spec somewhat. So the SOH could be off (over or under) and it would be impossible to tell. Also, near the end of life, the cell resistance increases dramatically. If you have large loads on the battery, its voltage drop will increase and could fall below the system's required voltage level before you anticipated.

    Thanks,

    Alex M.

  • 0 in reply to Alexander Mazany
    Prodigy 10 points

    Thanks for your response,

    Can we calculate impedance continuously, and how?

    Thanks,

    Yash

  • 0 in reply to yash
    TI__Guru** 102395 points

    Hello Yash,

    This is the purpose of EOS mode, in EOS mode when you send the gauge_start() command to measure a pulse we take the OCV before the pulse and compare it with the current and the voltage under load. This is how we estimate the internal resistance, which is reported out as measured Z by the gauge.

    It would be difficult to do this method in ACC mode since it is designed for accumulating charge. If your discharge is longer than 1 second (the sampling rate of our ADC) then you could crudely calculate the resistance with a similar method. I would not recommend this, I would recommend using the EOS mode to get an alert when the battery life is very low.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 10 points

    Thanks for your response,

    Hi,

    So can you please explain to me the method to continuously measure impedance in EOS mode?

    Thanks,

    Yash

  • 0 in reply to yash
    TI__Guru** 102395 points

    Hello Yash,

    It is not possible to continuously measure impedance of the cell, the gauge is not designed for this. The gauge relies on taking a measurement of a short current burst in order to get a impedance measurement. There must be relaxation time between the pulses.

    Sincerely,

    Wyatt Keller