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Purpose of CUVC?

Ken Cowan
Prodigy 130 points

I'm trying to figure out what the purpose of the compensated undervoltage protection is for, it just doesn't make sense.

Let's say for example your battery load periodically (maybe once a day) goes from very light to 2A for 20 seconds or so.  With the default settings (CUV=2.5V and CUVC=2.4V) if the battery went nearly a day without a load pulse, it would hit the CUV threshold of 2.5V and shut down with essentially an open circuit voltage of 2.5V.  However, let's say the open circuit voltage was at 2.8V and a pulse came along.  Let's also assume that the internal cell resistance is 0.1 ohms and the fuel gauge is tracking it accurately, what will happen is that when the 2A pulse hits, the cell voltage will drop to 2.6V, the fuel gauge will read that 2.6V and subtract another 0.2V and trip the CUVC because it's set to the default 2.4V.  Then it shuts of the discharge FET and the cell voltage jumps back up to 2.8V.  So now I have some batteries tripping at 2.5V open circuit and some at 2.8V open circuit.  How does this help me.

What would help me is if the fuel gauge actually let the cell voltage fall below the CUV by whatever the current * impedance is, and the CUVC would be used as a lower limit for the loaded cell voltage.  In this way I could set my CUVC to 2.3V and the battery would trip at the same capacity whether it was being loaded or not.

Can someone please help me understand how this is supposed to work.

  • 0
    TI__Expert 3385 points
    Hi Ken,

    CUVC is another protection on top of CUV to ensure that the battery does not get over discharged. The values of CUV=2.5V and CUVC=2.4V are just defaults used for a typical lithium ion profile. In many cases, for Li-Ion, the remaining capacity between these points will be very small or even negligible. However, if you are using a different chemistry for your cell, you are free to adjust these parameters to match your cell. In some cases, it might make a difference in amount of usable capacity.

    Best,
    Justin
  • 0 in reply to Justin Erfan
    Prodigy 130 points

    Justin, thanks for your response.  I understand that these values are just defaults, however, what I don't understand is how CUVC will do anything useful working as it is described in the reference manual.

    The operation is described as: Min cell voltage1..4 – Current() × Cell Resistance ≤ CUVC: Threshold for CUVC:Delay duration.

    It seems to me that it should add, not subtract the Current() × Cell Resistance.  In this way it would actually be "compensating" for the difference between the open circuit voltage and the loaded cell voltage.  In my example above, this would allow my loaded cell voltage to get down to 2.2V during the pulse and when the battery shut down the open circuit voltage would bounce back to 2.4V = CUVC.  This would be a useful function, allowing a lower trip point under a short term heavy load.

    If you subtract the internal resistance * current from the loaded cell voltage you aren't compensating for anything, you are simply projecting what the loaded cell voltage would be if you doubled the load current.  If you add the internal resistance * current then you are compensating by projecting what the open circuit cell voltage would be if the load were removed.

    Can you please verify what the intended behavior of this function is.  Is it to actually "compensate" and project the open circuit voltage?  Is it possible that the reference manual doesn't describe this function accurately?

    Thank you for your help.

    Ken

  • 0 in reply to Ken Cowan
    TI__Guru 73830 points
    Ken,
    The gauge offers two under voltage protection features - CUV and CUVC. The intent is to use one or the other, but not both in operation. If the cell should not be discharged below 2.5V to prevent damage to the cell, then we set the CUV threshold to 2500mV. The CUVC protections feature offers the same protection, but it removes the drop across the series resistance. You still do not want the internal "storage voltage" of the battery to drop below 2.5V, so the CUVC threshold is set to this minimum voltage minus to voltage drop across the series resistance. So, assuming that the drop across the series resistance is 100mV and CUVC is set to 2400mV, the cell voltage can drop to 2400mV. A fault will not occur until the internal "storage voltage" drops below 2.5V. (2400mV CUVC threshold) The CUVC protection feature may be more useful with pulsed loads or high current applications.

    Tom
  • 0 in reply to ThomasCosby
    Prodigy 130 points
    Tom, thank you for your reply. I still don't agree that the way you just described how CUVC works and how the reference manual describe it are the same. According to the reference manual, in your example, with the current*resistance = 100mV and CUVC set to 2400mV, when the fuel gauge measures a minimum cell voltage of 2500mV and then subtracts 100mV and compares this to CUVC which is 2400mV it will trip. With the load removed, the voltage will then return to the internal "storage voltage" of 2600mV.
  • 0 in reply to Ken Cowan
    TI__Guru 73830 points

    Ken,

    In our example, we have these thresholds.

    CUVC: Fault detection threshold

    "Storage voltage": Voltage across the voltage storage material inside the cell.

    Cell voltage: Voltage at the cell terminals ("storage voltage" +/- the voltage across the series resistance)

    If the voltage across the series resistance is 100mV, then the cell voltage can drop to 2400mV before the CUVC fault is triggered. The "storage voltage" (no load voltage) will be at 2500mV.

    Tom

  • 0 in reply to ThomasCosby
    Prodigy 130 points
    But the reference manual, on page 12, states that it will trip when Min cell voltage1..4 – Current() × Cell Resistance ≤ CUVC. I interpret this to mean that it will trip when the measured cell voltage (not the internal storage) minus the 100mV becomes less than or equal to CUVC. So that would be 2500mV (measured cell voltage) - 100mV (current*resistance) = 2400mV =(CUVC).