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Device to open a circuit when a voltage is released?

Discussion in 'Electronic Basics' started by Bruce W.1, Mar 18, 2005.

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  1. Bruce W.1

    Bruce W.1 Guest

    Does anyone know of a pre-made device or simple circuit that opens a set
    of contacts when a certain voltage is met? I want to use it mostly to
    charge and discharge batteries.

    Building this would probably be easy, except the tricky part would be
    the ability to change the trigger voltage. This would need to be
    programmable, or I could settle with presets for every voltage between 1
    and 15 in half volt increments.

    Does anyone know of such a device?

    Thanks for your help.
     
  2. Active8

    Active8 Guest

    It's called a battery charger. There's a few NiCd/NiMH charger
    circuits you can find via google. You can also learn about charging
    rates that way. You'll find some stuff in the R/C modeler's sites -
    even articles on discharging. I forget some of it, but if you're
    under the impression that you have to discharge batteries often to
    prevent mythical memory effects, you'll shorten their life.

    There's some info here, but I doubt you want to deal with the PIC.

    http://www.angelfire.com/electronic/hayles/charge1.html

    A FAQ:

    http://www.repairfaq.org/ELE/F_Battery_info.html

    This one wasn't as easy as I thought to find on the net, but it
    might help:

    http://www.solorb.com/elect/tmpchrg/

    It has a temperature monitor. The thing I like is that it works like
    the charger for cordless drill batts which have built in thermistors
    to signal the charger that it's time to quit. Contrast that with an
    auto batt charger which supplies a voltage and you know it's charged
    when the charging current goes to zero or close to it.

    If you need help modifying a circuit to meet your needs, we can
    help.

    I think the enclosure for one of these projects will make it more
    expensive than an off the shelf solution, but I don't know your
    motivtion for wanting to build.
     
  3. Bruce W.1

    Bruce W.1 Guest

    =================================================

    I own a number of battery chargers as well as voltage logging software.
    Unfortunately the software does not have an alarm or alert.

    All chargers/dischargers have shortcomings, like the ability to change
    parameters like the rate of charge/discharge, discharge depth, etc.. So
    sometimes I do it manually.

    When doing it manually sometimes I forget to watch it. That's why I'm
    seeking a device to either set off an alarm or kill the process when a
    certain voltage is met.

    If I can't find one manufactured then I'll have to build it instead.
     
  4. Active8

    Active8 Guest

    I doubt you find an off the shelf unit. If you'd have read the links
    I gave you, you'd have found the Max712/Max713 fast/trickle charge
    chips. Anything wrong with those? 1 - 16 cells. No discharge feature
    mentioned, but that could be a separate circuit.

    The temperature controlled charger or at least that part of the
    circuit, is a good idea. It'll shut off if your charging rate causes
    overheating. For fast charging a comparator with voltage reference
    would do the job.

    You said programmable from 1 - 15 in .5 volt steps. What kind of
    programming? PIC based? You could use an LM317 regulator and pot for
    the reference and meter the trip point. Or use a PIC with ADC. PICs
    also come with comparitors, so there's lots of options here.
     
  5. Rich Grise

    Rich Grise Guest

    Well, there's always these:
    http://www.alarmclocksonline.com/10607.htm
    ;-)

    But seriously - what is the software, and what's it running on? It seems
    that'd be the most elegant way to do this - just fix the software, so it
    can give an indication of some kind, or if it's in a control loop it
    could do the switching.

    Failing that, just a comparator and sonalert will do what you want.
    http://www.google.com/search?hl=en&ie=ISO-8859-1&q=comparator+circuit&btnG=Google+Search

    I'm too lazy to draw an ASCII circuit, but basically it's a voltage
    divider to one input, a settable reference to the other, a little positive
    feedback for hysteresis, and a beeper.

    Good Luck!
    Rich
     
  6. Active8

    Active8 Guest

    Nice Big Ben.
    Yeah, but his exisiting chargers are lacking features.
     
  7. mike

    mike Guest

    All you need is a comparator and a relay.
    Only you know your application.

    BUT.
    Think about better than half volt resolution.
    Half a volt at 1V is BIG.
    Even on a 10-cell pack, half a volt can mean the difference between OK
    and smoked battery. Then you need to figure out the value to set.

    FWIW, I've been plotting charge curves on various kinds of batteries.
    Depending on age, history, size, type, the peak voltage of the charge
    curve varies all over the map. The only cells I've had terminate charge
    on overvoltage were those seriously worn out.

    It's much safer and less stressful on the cells if you use some kind of
    slope termination with a secondary safety based on temperature.
    mike

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  8. Bruce W.1

    Bruce W.1 Guest

    ===================================================

    A voltage comparator and a relay. I like that idea. Off the same
    regulated power supply that's running the charge or discharge circuit I
    could make a voltage divider with a rheostat.

    But what if the voltage is going up (charge) or down (discharge)? Guess
    I should read-up on voltage comparators.
     
  9. mike

    mike Guest

    swap the comparator inputs with a up/down switch.
    mike

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  10. Active8

    Active8 Guest

    Slope termination... You mean throttle back the charging rate
    (current)?
     
  11. Active8

    Active8 Guest

    When you're done, re-read my posts and see that that's one of the
    solutions I mentioned, but I suggested a regulator to set the trip
    point - might not need that. I figured you knew what a comparator
    was and that you knew you could control a relay with it.
     
  12. mike

    mike Guest

    I don't understand what you mean.

    What I meant was...
    Depending on the cell chemistry, terminate when dV/dt is appropriate
    with safety termination on temperature limit.
    mike

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  13. Active8

    Active8 Guest

    I meant that I didn't understand what you meant :)
    Oh! *That* slope.

    I'd think that when a cell of any chemistry is almost done charging,
    that a continued constant current charge would stress it and isn't
    that why it heats up? IIRC the internal resistance of the cell drops
    as it's voltage increases. I think that is what leads to the temp
    rise.
     
  14. mike

    mike Guest

    Suggest you do some more research.
    In my experience, a properly charged cell with dV/dt termination does
    NOT heat much. The only reason for the temperature cutoff is SAFETY
    when something goes wrong. With -dV/dt cutoff, the most likely
    combination of factors is 1) low charge current, 2)mismatched initial
    state of charge so they peak at different times and the total curve of
    the sum of the series string never goes negative. THEN it gets hot.

    Another problem situation is with a poorly designed charger that depends
    on none of the cells being shorted.
    mike

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  15. Active8

    Active8 Guest

    I said "I'd think...".

    I plan to whenever I get around to that deep cycle charger I'll
    need. No need to worry about NiCds now.

    I'm going to put each separate thought in it's own paragraph so you
    don't misunderstand me. Read them as if the others weren't there.
    Nothing I said was intended to suggest that it would. I opined that
    a constant current all the way up to the end might heat it up.

    I said that IIRC the internal resistance of a cell decreases as it
    reached full charge.

    I said I think that a current through a small valued resistor causes
    more heat (heat = work = energy = P*T) than the same current through
    a larger value resistor.

    I don't have any charge curves handy AFAIK but I'm guessing (hell, I
    can almost picture curves I've seen in the past) that when it's
    almost up to full charge, dV/dt is less than when it started
    charging.

    I take it you mean it's always under constant current charge for the
    dV/dt in question.
    My cordless drill batts have a thermistor inside. I thought that was
    the *only* thing that stops the charge.
    Now I have a minus sign to ponder.
    Is that the -dV/dt above? The cell or string starts uncharging?
    I'd like to know how to detect and/or deal with that situation. I'd
    guess that I'd have to determine that the string never reached it's
    expected voltage at the expected time.
     
  16. mike

    mike Guest

    Thanks, I need all the help I can get.
    Mr. Ohm and those starting from his equations are all turning over in
    their graves about now.
    I'm not guessing, my battery charger plots the curves every time it charges.
    For NiCds dV/dt peaks before it levels off then turns negative.
    That's a reasonable assumption.
    It is. There are many cheap chargers that do just that. They charge
    the thing until it gets hot enough to turn off. Great way to cook
    batteries to death. I've rebuilt about a dozen drill packs. Usually,
    all the cells have vented.
    FWIW, this technique works reasonably if you
    fully discharge the pack...but not too fully to reverse a cell...before
    charging. During the full recharge time, there's enough heat built up
    to bring the thermal mass up to cutoff without serious damage. Problem
    is when you put a 70% charged pack on charge. The internal temperature
    comes up too fast for the external sensor to trip. Cell vents.

    A key phrase seems to have been snipped from my earlier post"
    "In my experience, a properly charged cell..."
    Temperature only cutoff is not a properly designed charger.
    Today, you'd use that only if you cared about the lowest possible
    product sales price, and you made a bundle off replacement batteries.
    Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
    Nope, still charging. There's lots of stuff on the web on proper
    charging of NiCd cells.
    Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
    Or start at the cadex website.
    No problem. Make the charge current independent of pack voltage.
    Charge at the proper current relative to cell capacity. (there's
    argument over exactly what this is. I usually use C"
    Terminate on voltage slope rather than voltage.
    Have three safety cutoffs
    1)over temperature
    2)over voltage
    3)over charge I x T > 1.2 x pack capacity
    And apply a trickle charge if the pack voltage is below some minimum
    voltage. Only start fast charge if the pack voltage is within some
    min/max range and the temperature is within some min/max range.

    The only reason my charger needs to know the voltage is so it can
    properly normalize the graph so every pack looks the same on screen
    and I can immediately recognize a problem.

    mike
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  17. Active8

    Active8 Guest

    I respect your modesty. Commo's a bitch sometimes.
    And yet again my heads up my ass[-]backwards! :( Caffeine, sugar and
    a long day or two? Day/night - whatever it was. "A V across..." That
    kills that hypothesis.
    Got some returns on dV/dt and dT/dt. It sounds similar to a slight
    peak at the cutoff of a LP filter. From the recombination. I
    probably saw that in the curves from the enerhell (sic) quidbook,
    wherever that is at the moment. Last I checked, the online thing
    sucked.

    "With the NiMH battery the voltage depression is smaller, and harder
    to detect than with the NiCad battery"
    "with dV/dt termination does
    NOT heat much."

    It's up there below my first comment. We'd have lost nothing if it
    were snipped, I got it the first time. Didn't mean to suggest
    otherwise.
    I read only a bit of what I've stumbled across years ago re: R/C.
    The quote is not a typo?
    If the day comes that I need to really get into it, I can add that
    logging into one of my progs. How many uses for a DSP/FIR/IIR with
    time domain/freq domain and plotting prog?

    What is this? :

    http://www.repairfaq.org/ELE/F_Battery_info.html

    seach find: "Type ANSI" (no quotes)

    What's this C5maH ? 5*C in maH ?

    Hey! I think we did well on commo this time. And with me wired on
    caffeine.
     
  18. mike

    mike Guest

    Active8 wrote:
    snip
    It's right under the chart:
    Note that this is the capacity when the battery is discharged over 5
    hours time period.

    Remember that cell capacities are 3-4X what they were when that was written.
    mike


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  19. Active8

    Active8 Guest

    Oh. I read the lines below, duh. At any discharge rate? Is it a
    permanent loss of capacity?
    About that dV/dt thing:

    "Another reason memory effect is a myth since all the consumer
    charger's I've seen actually overcharge until there is a slight
    voltage drop (due to an increase in resistance from the formation of
    larger cadmium hydroxide particules that cause contact loss). It's
    because consumer chargers actually overcharge that you have to give
    the battery a deep discharge from time to time. It has nothing to do
    with memory."

    Does this mean that they go further into voltage depression than you
    would allow them using the technique you recommend?
     
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