Voltage Sensing and Switching

Discussion in 'General Electronics Chat' started by timmfisk, Jun 5, 2015.

  1. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    So, I am working with components that are powered by batteries. However, the batteries can drop power at inconvenient times. My hope is to find a circuit that can sense the voltage of a battery. This circuit would be able to sense when when a threshold is met, then to switch to another battery without the loss of power.

    The input voltage would be at a maximum of 12v and the threshold would be 2volts. But it would be really nice to adjust the threshold. It is necessary to keep power to the components. I am guessing a large enough Capacitor to help in the switching?

    Can someone suggest a schematic as a resource or possibly a commercial product?
     
  2. Phil_84

    New Member

    May 21, 2015
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    If you are able to switch to another battery then the back-up batteries must be in the same place as the first battery. Can't you just wire them all in parallel to start with.

    Voltage sensing is pretty easy. Do you have any more info about the components / circuits you are powering, otherwise there is no way to suggest a good method for you to use.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    The success of having a reserve battery to switch in when the primary runs low would depend on the shelf life of the reserve.

    You could use diodes to "OR" the two batteries and use voltage monitors to tell you when one of them went below your threshold.
     
  4. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    What I am working with is an RC Quadcopter. Its payload is up to 8lbs. The Quad, itself, is around 2lbs total... which gives me plenty of room to have a spare battery onboard... or more, which would be nice.

    The batteries that power the Quad are 12v @ 25A. Ya, lots-O-juice. But flight time is limited to about 8 minutes.

    Long story short, I would like to have a spare battery onboard (two would be really nice -- but not absolutely necessary) just in case there is not enough time to bring the Quad to a safe landing should the primary battery be too low.

    So, I would like to build a Voltage Sensing Circuit that can switch to the spare battery -- without loss of power. If power is lost, then the Quad's Reciever wont have enough time to link-up with the Transmitter which means "kaboom" when the Quad hits the ground from a height of about 100 feet (because we all know that Gravity and sensitive electronic components do not get along). ;)

    I am guessing that if I use an MC34161, then I would need a Relay due to the High Voltages and Amps?

    Anyway... I am pretty good at reading schematics but an engineer I am not.

    Hopefully that helps answer your questions and gives you a better idea of what my needs are.

    Thanks for your questions and your quick responses.
     
  5. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    BTW... I am considering incorporating an Arduino on this Quad.
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    Why make it so complicated? Just connect the batteries in parallel.
     
  7. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    dl324: There are a variety of safety factors involved.
    • I do not want to risk overheating the motors which would immediately cause a catastrophic failure.
    • If someone or something was in the vicinity of an 8lb Quad falling from the sky, it would be a disaster.
    • Certainly, with both batteries running in parallel, this would effectively produce twice the run-time on the motors
    • and with such an extended run-time, I do not want to risk burning-out $120.00 motors.
    Why not answer the question instead of considering the problem?
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Running batteries down from 12V to 2V would almost certainly kill them, regardless of their chemistry. LiPo batteries in particular need careful discharge control. Check out www.batteryuniversity.com.
    Regarding battery change-over and monitoring, there are ICs available specially for that (e.g. LTC4110).
    I'm not sure I understand your point about extended run-time and motor burn-out. Having an automatic battery change-over would surely run the same risk of burn-out as simply paralleling two batteries (and thus doubling the potential run-time)?
     
    Last edited: Jun 5, 2015
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  9. BR-549

    Well-Known Member

    Sep 22, 2013
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    The only advantage to multiple batteries is, being able to eject spent battery from craft.
    Why not a higher capacity battery?
     
  10. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    Thanks for the info on the LiPo's. My point with the battery levels is that I was wanting a variable control for adjusting thresholds with multiple batteries. This way I could monitor other components on other circuits that use other power sources.

    I have Telemetry systems on-board the Quad which I can use to send vital information about performance which includes
    • Battery status
    • Flight status
    • Motor temperatures
    • etc...
    The only purpose for the Voltage Sensing Switch (if that is its technical term) is so that in an event of battery failure, the back-up battery could be put into service.

    This is not an issue of convenience, unless on thinks that Safety and Cost is a convenience.

    Seriously people, I think too much effort is being put into everything but the simple question that I asked which is:

    Can someone suggest a schematic as a resource or possibly a commercial product?

    As an aside from the primary question: Are there any Arduino people in this forum that have experience with this kind of component?
    I have been looking into using an AnalogRead Function to poll each battery at regular intervals to monitor the voltages and include that into the Telemetry packet.
     
  11. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    ALECT: Thank you for pointing me into the right direction. This looks like a good thing to pursue. Your insights are greatly appreciated and to the point.
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
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    Paralleling 2 batteries would increase your runtime. There would be no additional overheating risk unless that depends on how long you run the motors and you control that.
    This is irrelevant. Don't even think about asking me how I feel about those things...
    You control running time.
    Sometimes people don't know enough to ask the right question and I'm just offering my opinion. I like to over engineer as much as the next guy. The only difference being that I can do it with my own knowledge. I offered my suggestion; you can take it or leave it.
     
  13. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    DL324:
    Thanks for explaining yourself and your input.

    I had stated that "If someone or something was in the vicinity of an 8lb Quad falling from the sky, it would be a disaster."

    Your response was:
    ... that is confusing and stunning.

    Safety is just good 'ol common sense when considering an 8lb QuadCopter anywhere from 300ft to 500ft in the air (it's not just a toy but a tool).

    In terms of a Cost Effective platform and ROI, it is reasonable and practical to include features like redundancy to avoid complete and total loss of an investment combined with regular maintenance costs.

    <Soap_Box>
    This kind of approach is what made American products so successful years ago. But in recent years it seems like people are more interested in band-aids and half-baked ideas (like M$ and Motor City products today that cheat consumers through false advertising) instead of quality products that last for years.
    </Soap_Box>

    Those who have commissioned this project would not think of such an approach as Over Engineering but instead would view it as a point of integrity, responsibility and quality engineering.

    Like I said earlier, thanks for your input. I can see that you have people who appreciate your advice. Maybe you could gain a larger following if these things were incorporated in your responses.
     
  14. dl324

    Distinguished Member

    Mar 30, 2015
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    Irrelevant wrt to connecting 2 batteries in parallel...
    If they're so unsafe, don't fly them. And they're restricted to line of sight under 400' in the USA. My property is registered as a drone no-fly zone and I'll shoot at any flying over my property.
    I'm not motivated by who likes me or who doesn't. I try to help people and if someone thinks I helped them, I'm satisfied. If someone thinks I didn't, I don't care. Free advice is free, you get what you pay for. If you want to play semantic games with me, play all you want. I'm used to debating a dozen PhD's every day and I enjoyed the intellectual sparring...
     
  15. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    That's funny.

    <snip>

    Later.
     
    Last edited by a moderator: Jun 6, 2015
  16. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    You can see the circuit as below, if you found any questions then point out.

    Two12V25ABatDetectorAndSwitcherRelay-01_timmfisk_ScottWang.gif
     
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  17. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    Great Stuff, Scott. I really appreciate the time spent in getting this specifically designed to my question. Good Stuff! Is Bat3 the power source for this circuit?
     
  18. timmfisk

    Thread Starter New Member

    Jun 5, 2015
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    For those interested:
    Here is a link to using an Arduino as a Voltage Meter. Really good stuff.
    http://www.allaboutcircuits.com/projects/make-a-digital-voltmeter-using-the-arduino/
     
  19. dl324

    Distinguished Member

    Mar 30, 2015
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    You should consider the fact that reliability can have an inverse relationship with complexity. It is possible for this circuit to fail in a manner that will cause a weak/dead battery to be selected... Wiring the batteries in parallel, possibly using diodes, is a simpler, more robust design.

    I remember reading a disclaimer in a databook stating, to the effect, that that particular companies parts were not certified for use in applications where lives could be placed at risk and that any application of that type required written approval from the president of the company...
     
  20. ScottWang

    Moderator

    Aug 23, 2012
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    In the beginning that I want to using the Bat1(Backup Battery) to be the power source, but considering the voltage of Bat1 will be going down when it working through a period of time then the circuit can't keep in a stable situation to works, so I decided to use a Bat3, the circuit is you want, if you have better idea then we can discuss that. and the OP1 has comparing function, so the voltage must be fixed and stable, if there is no battery way then the zener diode is needed and the inputs of OP1 could be using voltage divider to reducing the voltage for some ratio.

    If that two batteries using two high current rectifier diodes and adding to the motor, is that will be a problem to you?
     
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