Should I have a load to monitor battery voltage?

Discussion in 'General Electronics Chat' started by spinnaker, Nov 17, 2017.

  1. spinnaker

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    I didn't know to post this here or in the embedded forum. I figured it is more general so posted it here.

    My microcrontroller project has a backup battery to power the RTCC in the event of a power failure. . I want to monitor battery level. I was thinking that I will need a load to get a true reading of battery status. The battery is going to be a 3V coin cell battery.

    Should I measure across a load? If so what value resistor should I use?

    upload_2017-11-17_18-40-49.png
     
  2. AlbertHall

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    Yes a load will change the battery voltage. Just measure the voltage while it is supplying the RTC as that is the load it has in normal operation.
     
  3. spinnaker

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    But under normal conditions the RTCC won't be powered by the battery. It is on board the Pic. Blocking diode will not be forward biased during normal operation so basically the battery will have no load other than the analog input.
     
  4. nsaspook

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  5. AlbertHall

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    In that case it might be a good idea to connect a load equivalent to the RTC while measuring the voltage.
     
  6. spinnaker

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    I don't understand. What do you mean by a "pulse test"?

    I only have one remaining pin to work with and that is my analog input. It can be made to be a digital out if that works.
     
  7. spinnaker

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    Can't only have one pin left. Unless I can figure out how to do everything with the same pin.
     
  8. nsaspook

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    I updated my post. You could config it as a output to sink current for a time and then quickly reprogram as an analog input to measure a voltage drop before the cell recovers completely.
     
    Last edited: Nov 17, 2017
  9. spinnaker

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    I take that back. I have a couple of extra pins on that side of the chip but it might be a challenge to get to them on my single sided board. I might need to redesign that side of the pic.
     
  10. spinnaker

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    That would be ideal if I can do it with one pin but what do I output to? What kind of circuit to provide a temporary load?
     
  11. AlbertHall

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    A resistor between the pin and the battery will do the job.
     
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  12. nsaspook

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    The output pin transistor to ground is the load. You should put a small load resistor in series from the cell to pin to limit current without affecting the ADC level too much in analog mode.
     
    Last edited: Nov 17, 2017
  13. spinnaker

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    OK stepped away for a few minutes and I was thinking just that. You mean something like this? I think I will just go ahead and us that other pin. I already have the PCB designed so I have some fixing to do but I'd rather know for sure my load is switched in before measuring the voltage.

    upload_2017-11-17_19-47-17.png
     
  14. spinnaker

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    I also have a 1K om the analog input.
     
  15. nsaspook

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    Sure you won't see the voltage under actual load but the battery voltage won't recover instantly when the load is off due to the time it takes for the chemistry to work. That should be plenty of time to make a relative voltage measurement of good and bad cell using the same pin as load with serial resistor and measurement ADC.
     
  16. spinnaker

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    But if I use a separate pin to switch in the load I will read the actual voltage?
     
  17. nsaspook

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    Sure, if reading the actual voltage during the load is an important requirement then it's worth the effort.
     
  18. GopherT

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    It is not a question of recovery. Current must be flowing to get the voltage under load. Internal resistance follows ohms law just like the resistance of a resistor.

    Your voltage measurement method won't work any better than disconnecting one of the resistors of a voltage divider and then measuring quickly. You'll get a voltage very close to one not underload if you disconnect and measure immediately after. The I*R voltage drop inside the battery still needs "I" value to make the voltage drop.
     
  19. nsaspook

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    As you well know batteries are chemical devices and don't recover instantaneously due to the rates of diffusion to recreate the charge separation needed for normal cell voltage after a sizable (for the type of battery) load. It takes under 100 microseconds to switch modes and do a full ADC measurement on a slow controller and much less than that for the switch and sample-hold function.

    http://batteryuniversity.com/learn/article/testing_lithium_based_batteries

    [​IMG]
     
    Last edited: Nov 17, 2017
  20. spinnaker

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    It is moot. I redesigned the board so I have a pin that turns on the load as well as a pin to read the voltage.
     
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