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keeping guard on battery voltage during discharge — Parallax Forums

keeping guard on battery voltage during discharge

MatthewMatthew Posts: 200
edited 2005-03-10 04:24 in General Discussion
Hello. I'm building a circuit in which a battery must not go below 2.5v while operating. The nominal voltage for the battery is about 7.4v. If I wanted to keep watch of the battery level, would it be a mere case of just adding a second wire to each terminal on the battery, and having the wires lead to an ADC?

Thanks,
Matthew

Comments

  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-03-09 05:46
    thats one way of doing it, but there are other means which would require much less current. Since its late, I'll just give you the one off the top of my head, Get a really dinky microcontroller like the ATtiny11, write a small program that enables the brown out detector then enters an infinite loop where it keeps entering sleep mode. In the interrupt routine for the brown out detector shift the level of an output pin. The current consumption of the system should be in the microamps.

    Heres another since it is simple, use a micropower voltage reference of 2.5V feed it into the reference input of an analog comparator and the battery into the second input, problem with this is it would likely require another power source because it may act weird when trying to analyse a voltage that is the same as the supply voltage, but an ADC solution would also have the same potential problem.

    Post Edited (Paul Baker) : 3/9/2005 5:50:38 AM GMT
  • KenMKenM Posts: 657
    edited 2005-03-09 06:01
    Will there be more than one voltage source in this circuit?

    Or is the 7.4 volt battery the only voltage source?

    If there is a second voltage source, what is the voltage, and can the ground for the two voltage sources be tied together?
  • MatthewMatthew Posts: 200
    edited 2005-03-09 06:36
    Ken,

    The 7.4v battery is connected to an ESC (electronic speed controller). In fact, three sets of wire are coming out of the ESC, one goes to the BASIC STAMP to read pulses, one goes to the 7.4v battery, and the other goes to the motor. The motor will draw around 30 amps most of the time.

    So no, there's only one power source powering the motor. But there are a total of two sources (the other being a 9v battery for the microcontroller) in the entire setup.

    I'm not sure if I answered your question though...

    Thanks,
    Matthew
  • KenMKenM Posts: 657
    edited 2005-03-09 07:17
    With seperate power supplies there is an easy solution. See attached diagram.

    When the voltage from the battery drops less than 2.5* volts the output to the stamp will be at 5 volts. When the battery is greater than 2.5 volts, the output to the stamp will be 0 volts.

    *Depending on the resistor tolerance and the actual value of the stamp 5 volts supply, the trip point might be 2.6 v 2.55 v, etc.undefinedundefinedundefined

    You also may consider putting an optional 220 ohm resistor to protect the stamp in case you mistakenly make the stamp an output high, with the comparitor low.

    Post Edited (KenM) : 3/9/2005 7:22:22 AM GMT
    2.5 volt battery monitor.JPG
    593 x 476 - 19K
  • The Dead BugThe Dead Bug Posts: 73
    edited 2005-03-09 16:00
    Why not just use the same device that the Stamp uses to detect brownouts?

    The 1381 is a simple 3-terminal device that gives a logic level on a pin when the voltage it is monitoring drops below a given threshold.

    http://rocky.digikey.com/WebLib/Panasonic/Web%20data/MN1380%20Series.pdf

    You will want a 1381-J I think. It has a 2.6 volt threshold. 1381-G is 2.4 volts.

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  • Robert SchwartzRobert Schwartz Posts: 141
    edited 2005-03-09 16:23
    I would also recommend the 1831. The only draw about 1uA of current and are accurate. The 1831G will detect ranges from 2.4 to 2.6V. If you want an earlier warning, the 1831H will detect 2.5 to 2.7V.

    Dead bug, I'm pretty sure that they dont check to see if the voltage is below a certain level, but rather in a predefined threshold.
  • KenMKenM Posts: 657
    edited 2005-03-09 16:23
    Cool,

    Didn't know a device such as that existed. Certainly less to connect being a 3 terminal device.
  • BobNBobN Posts: 11
    edited 2005-03-09 17:44
    Matthew,

    I'm making some assumptions here.

    If I understand your application, you are trying to monitor the voltage of a Li-poly (2 cell = 7.4V nominal) pack going to ESC and you don't want the pack to go below 2.5 volts.· Is this correct?

    IF so, then you really don't want the pack to go below 5.0 volts (2.5 volts per cell).· Taking a two cell pack down to 2.5 volts (total) will kill it.· Some people suggest stopping the drain at 3.0 volts per cell.

    I am looking at doing the same thing with my setup.

    - Bob -
  • MatthewMatthew Posts: 200
    edited 2005-03-10 04:23
    Bob! Wow, you're good! Thanks for catching my mistake and saving me almost $80 worth of batteries!!
  • MatthewMatthew Posts: 200
    edited 2005-03-10 04:24
    Thanks also to Paul, Ken, Bruce, and Robert!
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