Exceeded 6A firing current on SLCF

Discussion in 'Rocketry Electronics and Software' started by Buckeye, Dec 24, 2019.

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  1. Dec 28, 2019 #31

    manixFan

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    Ok, I was off by nearly a factor 10 then. IIRC, the RDAS draws about 100mAh on standby, so no wonder it's been hard for me to get used to these new units that sip rather than gulp. And while an e-match does draw a fire amount of current, it does so for such a brief time that it does not have a huge impact on battery life either.

    It would be interesting to figure out how many e-matches a 130 mAh battery can fire.


    Tony
     
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  2. Dec 28, 2019 #32

    Voyager1

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    Although this is a little off topic for this thread, it is generally not a good idea to let a LiPo discharge to less than about 3.5V per cell. Some people actually use a higher lower limit of, say, 3.7V. While the typical lower limit before damage is about 3.0V per cell, its advisable to leave a buffer. This means that you might need to limit your expected battery capacity to 70-80% of the claimed value. So, your 130 mAh might only be about 90 mAh. In some cases, depending on battery health, the realised capacity might be significantly lower.
     
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  3. Dec 28, 2019 #33

    jderimig

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    Duracell copper tops have a mah rating of >310 mah. Using a 180mah Lipo to simulate a 9V alkaline makes no sense unless you are after the modest size and weight reduction.

    However the cost reduction is significant, which is why I use Lipos.
     
  4. Dec 28, 2019 #34

    mpitfield

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    And you want a rechargable battery that you can top up before each launch, as well as take advantage of the benefits that lithium battery chemistry provides.
     
  5. Dec 28, 2019 #35

    jderimig

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    I agree! But if you are going to use a Lipo don't emasculate it and add 2 or more possible failure points with current limiting resistors. That's my position and I cede it may not be the most popular one.
     
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  6. Dec 28, 2019 #36

    manixFan

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    I'm on the other side - altimeter battery cost is relatively insignificant in the scheme of high power rockets, but size and weight are not. I fly a lot of small - 29mm-38mm MD rockets (but not on small motors, think Loki K627). I can fit a Featherweight Raven and Perfectflite SLCF with a 130mAh and a 180mAh Lipo battery in about the same space as two 9 volt batteries, and the total wight is below that of the 9 volt batteries. If I was using 9 volt batteries, I would need an E-bay that is twice as long as what I currently use, not a minor nit on these smaller rockets. For my rockets that use long burn motors (8-10 secs), weight is key, and again, the LiPos are far lighter. Plus the smaller, lighter batteries are less likely to break loose and do damage in case something bad happens.

    By the time I add up everything I've spent on LiPo chargers, special charging bags, voltage testers, new wiring harnesses, etc., I doubt I'm saving any money. I'm a very reluctant fan of LiPos, and only then when they are the right tool for the job.


    Tony
     
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  7. Dec 29, 2019 #37

    mpitfield

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    John, I am feeling a pull to the darkside on this one. The resistor idea sounds like a prudent measure to mitigate failure, but I now see your point that it is adding a point of failure.
     
  8. Dec 29, 2019 #38

    Reinhard

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    The e-matches I use can be fired by a 470µF capacitor at 5V. That's 5.9mJ of stored energy. The energy content of a 130mAh battery at nominally 3.7V is 1730J. That 130mAh battery can fire roughly 300,000 e-matches, at least in theory.

    Reinhard
     
  9. Jan 1, 2020 #39

    Handeman

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    This whole thread is why I don't bother with LiPo batteries on altimeters. They are just way too high maintenance and too much trouble. They seem to introduce more problems than they solve.

    I don't understand what LiPos are supposed to get you. I use 9V Duracell and change them when the altimeter reports battery voltage below 8.9V. That's about every three years or about 20 flights.
     
  10. Jan 1, 2020 #40

    dhbarr

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    Into smaller tubes, for one.
     
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  11. Jan 1, 2020 #41

    manixFan

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    As I mentioned in my last post (#36), I can get two altimeters and batteries in the same space as two 9 volt batteries, at less weight than the 9 volt batteries along. That's a huge difference in a minimum diameter 38mm rocket. More importantly, I can easily do (ok, not so easy) dual deploy in a 29mm MD rocket where a 9 volt battery would not even fit. Plus LiPos are far better under cold conditions that Alkalines.

    If you don't fly small rockets, then 9 volts are great, I still use them in many of my larger rockets. But for smaller and lighter, LiPos are the way to go.


    Tony
     
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  12. Jan 1, 2020 #42

    jderimig

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    Advantages of a Lipo:
    1. Cost. They will outlive the average rocket easily.
    2. More robust electrical connection.
    3. Fast charge, can top off quickly off car battery with 12vdc chargers.
    4. High capacity allows worry free launch pad sitting time.

    Disadvantages:
    1. Many if you are using them with an altimeter not designed for them.
     
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  13. Jan 1, 2020 #43

    Buckeye

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    I agree with you, Handeman. In this case, I am retrofitting a small ebay and wanted small batteries. I also wanted to see what all the fuss was about over these LiPos. Now, I have a frickin' electrical engineering problem to deal with. So, yeah, I am not impressed.

    Bingo. This is the root of all the problems.

    More disadvantages:
    • LiPos are not standardized in physical size/shape
    • Lipos are not standardized in capacity/rating
    • Lipos are not standardized in connector style
    • If you find one you like, wait a month and it becomes obsolete
    • You need a charger, power supply, and various plugs/adapters, ie more $$$$ to operate Lipos
    • The instructions for the charger are likely written in translated Chinese jibberish
    • You constantly have to worry about voltage, amperage, resistance, and frying your altimeter or the battery itself.
     
    Last edited: Jan 2, 2020
  14. Jan 3, 2020 #44

    Handeman

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    I understand where you're coming from, but for me using 3" or larger rockets, those advantages don't work.
    1. Each 9V lasts about 3 years and I don't have to buy a charger. Not sure cost is an advantage
    2. I don't believe the LiPo battery connectors are any better than the snap on 9V connectors. If you're putting the 9V into a battery holder, you are probably right.
    3. I never have to charge my 9V, ever, so I don't have to open the av-bay or mess with the connections and wires.
    4. My 9V usually last 3 years, unless the rocket hangs in a tree for a week with the altimeter beeping. Pad time, even hours, is not an issue.

    I don't know of any altimeters that are not designed to use 9V batteries.

    I will concede that small 29 & 38 mm Minimum diameter rocket can probably use LiPo where 9V won't fit. In that case, they are about the only option and you need to make them work. Personally, with 2.6" and above, I find 9V to be much easier, less trouble, just as reliable, and a lot less effort and cost. YMMV.
     
  15. Jan 3, 2020 #45

    manixFan

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    You started by posting you don't bother with and don't understand why folks use LiPo batteries. I don't understand how you can trust possibly $100's of dollars worth of rocket to a 2 or 3 year old 9 volt battery. To me, that seems like a senseless risk and a penny-wise, pound-foolish approach. But it's worked for you, so there you go.

    To each our own, no concessions needed.


    Tony
     
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  16. Jan 4, 2020 #46

    Buckeye

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    It is not just cost, but convenience as well. I do as Handeman and get multiple flights on a 9V without ever having to open the avbay to "top off" or "storage charge" the battery. My personal best is 12 flights over couple years. This was with a Perfectflight MAWD and its large capacitor for firing events.

    The MAWD also allowed me to use the tiny A10 9V alkaline, which is smaller than any 2S LiPo. It easily runs for hours. Less than a buck apiece, they are good for maybe 2 flights. Not sure if the modern altimeters could use the A10 these days.

    The A10 has no natural battery clip, so I had to fashion my own, and it was tricky. I got tired of this fiddlyness, so I am trying to retrofit an SLCF and LiPo on to my sled. That is the origin of this post!

    Anyway, I got my resistors and stuff as suggested. Once I put things together and test it out on the SLCF, I will report back.
     
    Last edited: Jan 4, 2020
  17. Jan 4, 2020 #47

    cerving

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    LiPo's have much more capacity and current-sourcing capability for their size and weight than a 9V battery, which is particularly important for high-drain applications such as GPS trackers. In a very low drain application they may not have the same advantages, although in the case of a non-wireless altimeter such as a SLCF or an Eggtimer Classic or Quark you can use a very small LiPo and save some size/weight over a 9V battery; I've used batteries as small as 80 mAH in a Quark. I would be more concerned with the mechanical characteristics of the 9V battery, they're made up of several cells welded (or pressed) together, and they are not designed for high G/shock applications. LiPo batteries are basically made in one piece and are inherently G/shock resistant... personally, I think that's reason enough to be using them over 9V batteries.
     
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  18. Jan 12, 2020 #48

    Handeman

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    Yes, I didn't understand the need for LiPos because I don't have small minimum diameter where a 9V doesn't fit so I didn't consider them. That was pointed out and I completely agree that for those situations a LiPo is probably the only way to go.

    I have heard that argument against using 9V batteries for long periods of time or for using a new one every flight. I think doing that has higher risk then using a proven one for a long time. Since I can fire an ematch with a single AAA battery and the 9V has six AAAs internally, I believe the highest risk isn't a reduction of the voltage or current capacity over time, but a faulty internal connection that causes a loss of power to the altimeter. Because of that, I feel a flight proven 9V battery is less risk than a brand new one. Every time you put a new battery in, you risk a bad/loose/manufacturer defective internal connection that passes production tests but may fail under flight stresses. I feel the successful first flight reduces risk on the following flights because the battery internals have been tested and proven.

    As for the three years, it is about voltage and current capacity. Since I can fire my ematches with a single AAA battery and the 9V has six of these internal to it, I see a significant excess capacity there. I change them about 3 years not because of the time, but because at about that time the voltage has dropped to 8.8V as read by the altimeter under load, 8.9V on the meter with no load. That is again, a risk reduction decision. I suspect the battery would work just fine with a voltage as low as 8.5 or less, but as you say, I won't risk $100s testing that.

    So for me, I think the risk of using a LiPo that requires the av-bay be opened, battery charged, the connections removed and reattached, generally fiddled with the wiring in the av-bay before every flight is much riskier than using an av-bay with proven success and that remains sealed for multiple flights. Then again, I don't download flight data either so I will leave the av-bay sealed until the battery needs to be replaced.
     
  19. Jan 13, 2020 #49

    cerving

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    I recommend opening up the AV bay and checking the battery voltage and, just as importantly, any screw-terminal connections before every flight. Just because everything worked fine on your last flight doesn't mean that it will on the next flight, unless you make it so.
     
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  20. Jan 13, 2020 #50

    mpitfield

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    I agree. I follow a mitigated approach, which is drilled into me from my cyber security background. Establish protocols to validate and mitigate all reasonable points of failure. It's just takes a bit more effort. I measure all voltages and inspect all connections and electronics, with a 10x jeweler's loupe, the night before, or day of the launch.
     
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  21. Jan 13, 2020 #51

    manixFan

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    (Rather than quote all of post 48, I’ll just leave it to the reader to refer to it.)

    I respect your well reasoned answer from your perspective. But it implies you aren’t downloading the flight data from the altimeter after each flight. For me, that’s kinda the whole point of the altimeter, to provide descriptive and diagnostic info after every flight. (Other than the somewhat important duty of firing the deployment charges). So do you not ever download data from your altimeters? Or am I missing something in how you have everything set up?


    Tony
     
  22. Jan 13, 2020 #52

    beeblebrox

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  23. Jan 13, 2020 #53

    Handeman

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    You are correct. I don't download the data at all.
     
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  24. Jan 14, 2020 #54

    StanO

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    Ditto
     
  25. Jan 20, 2020 #55

    jd2cylman

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    I have a bunch of RRC3's and several PF SL's of various sizes. The most I've ever done is hooked up the MW LCD occasionally to read peak altitude or change main deployment. I never take a laptop to the field. I hate trying to connect the altimeters to the computer, even at home. I'm not a data junkie at all. I want the rocket to go up, come down, and tell me how high it went. I'm probably in the minority, but hey, that's what some people like.
     
  26. Jan 21, 2020 #56

    Buckeye

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    I rigged up a 1.2 Ohm, 3W resistor on each deployment output. The SLCF fires the ematches no problem in test mode on the computer. However, the firing current is not reported in test mode. I will have to simulate a flight with vacuum or actually fly it.

    Please don't laugh at my wiring and soldering skills! I assume it is OK for the resistors to touch each other. This is a sled for a 38mm avbay.

    20200120_192321.jpg 20200120_192257.jpg
     
  27. Jan 21, 2020 #57

    g.pitts

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    Looks pretty good to me. No issues if they are in contact with one another. What's your output activation time on the SLCF? I suspect it is around one second. Wirewound resistors are pretty robust, so I'd expect they would hold up to the instantaneous current and power levels you'd be experiencing. I can't tell if the resistors are secured to the sled, but it may not be a bad idea to do so with (for example) some silicone caulk just to keep things from moving in flight.
     
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  28. Jan 26, 2020 #58

    Buckeye

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    I ran a simulated flight on my resistor rig in a vacuum chamber. Both events fired as expected and pulled only 3A each through the SLCF. No more warnings. Yea! :)

    I learned something, so this was a good discussion and exercise. However, the easiest thing to do is to just use 2 ematches in series!!!
     

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