Low battery LiPo protection circuit for Eggfinder

Discussion in 'Rocketry Electronics and Software' started by JordanT, Nov 26, 2018.

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  1. Nov 26, 2018 #1

    JordanT

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    What are people using for battery protection with their Eggfinder. I forgot to unhook mine after the last launch and the LiPo I used went dead flat. Zero. Kaput. 1.0V and toes in the air. I tried a second run today to get a better handle on my time with another 350mA 2S cell and turned my back (had dinner) at the wrong moment and it went from 7.4V to sad trombone while I was away. That test puts my total time in the 4-5 hour range, though it takes close to an hour to get a GPS fix sometimes, so that's a drag on total time on battery since any cold boot seems to re-acquire from scratch.

    Anybody have an easy circuit to add? If not I'll look to replace these lipos with ones that have a built-in low voltage protection, though that seems exceedingly rare for small packs.
     
  2. Nov 26, 2018 #2

    Tobor

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    To date I haven't used any LV cutoff device, but I think it will become a necessity soon.

    Anyhoot a quick Google search turned up a few things

    Cutoff switches
    Servo City: Low Voltage Cutoff Switch - $14.99 + Shipping (Link)
    Dimension Engineering LLC: LipoShield - $19.99 + Shipping (Link)
    BED ELECTRONICS (UK based): LiPo BATTERY PROTECTOR - €34.00 (Link)

    LiPo Battery Monitors
    Amazon Search Listing (Link)

    DIY Projects
    Stack Exchange: Low voltage protection for a LiPo (Link)
    Digi-Key eeWiki: Simple Undervoltage and Overcurrent Protection (Link)
     
  3. Nov 26, 2018 #3

    JordanT

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    I've seen the commercial products, and they're mostly enormous (for my rockets, at least - I'm barely fitting the EggfinderMini into some of them). I was hoping to solder up something I could shrink wrap to the battery or tracker. I found an auto-cutoff circuit on the internet but...well, I found it on the internet and I don't know enough about MOSFET circuits to check it.
     
  4. Nov 26, 2018 #4

    roytyson

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    The things they have I would have never considered. HMM, has the creative juices flowing. Google how to revive dead lipo batteries. I have done it a time or two.
     
  5. Nov 26, 2018 #5

    BradMilkomeda

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    Consider the disadvantage of using such a device... If your rocket is on the ground and you have not yet found it do you want it to shut down to save the battery at the potential cost of not finding your rocket?
     
  6. Nov 26, 2018 #6

    SpaceManMat

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    1. Pretty sure a low cutoff device will still drain battery just a lot more slowly.
    2. Keep it simple, just use a switch
    3. Don’t try reviving a dead LiPo fairly likely it will let you down. Even if it does appear to work it will most likely not hold charge or fail under high current demand (such as firing ematches).
    4. Should never take an hour to get a lock. Sounds like either it does not have good access to the sky (ie indoors) or perhaps low voltage issues.
     
  7. Nov 26, 2018 #7

    timbucktoo

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    Big red Bee 900 has a switch that allows user to select low voltage shutoff through software. Maybe contact Greg to see how he does it.
     
  8. Nov 26, 2018 #7

    timbucktoo

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    Duplicate
     
    Last edited: Nov 26, 2018
  9. Nov 26, 2018 #8

    OverTheTop

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    Personally I consider the batteries, even LiPos, consumable. If I am silly enough to let it drain past the limit I will take the hit and replace it. I would rather have it keep transmitting if it is laying in a field somewhere, as someone else stated. LV cutoff introduces a whole lot of new failure modes. YMMV.
     
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  10. Nov 26, 2018 #9

    cerving

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    This is exactly why we don't put a battery cutoff circuit on Eggfinders. Also, what voltage would you want it to cut out at? 3.2V/cell would be maximum, 3.4V would be better, but that's assuming that you are using a 2S LiPo. If you're using something else (LiFe, for example), that cutoff would not be correct.
     
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  11. Nov 26, 2018 #10

    dmo

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  12. Nov 26, 2018 #11

    rharshberger

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    That looks like a PTC or some sort of circuit breaker to prevent an amp spike that will damage the device powered by a LiPo, what is being discussed here is a battery saver that prevents a LiPo from discharging to too low a voltage to prevent the battery from being damaged, most RC vehicles and LiPo powered tools have a device that kills power as soon as the voltage drops to minimum for whatever the LiPo's min voltage is. I have considered one of the devices you linked (or similar) for use with my Missleworks RRC3 to limit the amperage that a LiPo can dish out, unlike the 9v batteries they were designed for.
     
  13. Nov 26, 2018 #12

    dmo

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    No, it is a full battery protection circuit. It is a control IC driving a pair of MOSFETs. It can disconnect the battery in case of overcharge, undercharge, or over current. According to the specs, it stops charging if when either battery's voltage goes over 4.3V, or drops below 2.4V. It will also cut off at 5.5A, but that shouldn't be a concern with the Eggfinder, unless you get a short circuit.
     
  14. Nov 26, 2018 #13

    rharshberger

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    Sorry mis-interpreted the spec sheet, might not be a bad unit for my RRC3 if I ever decide to use LiPo's with it.
     
  15. Nov 28, 2018 #14

    scsager

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    Hi Jordan,

    I have some inexpensive 'cut-off' PCB's on order... (slow boat from China) - I'm trying to save some 3s 2000mha lipos from low voltage death.

    I agree with cerving, when it comes to the Eggfinder lipo. Keep the Eggfinder powered and kill the battery -is a better option than save the battery and lose everything.

    Here's a link to what I ordered... if it ever gets here, I'll let you know what I think of this solution.

    Low voltage cutoff
     
  16. Nov 28, 2018 #15

    dshmel

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    I assume you adjust the cutoffs using a DMM.
     
  17. Nov 28, 2018 #16

    ksaves2

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    Lipos are cheap. Live with it. Once they are dorked, they are dorked. That said, I had some NOS lipos sent from Hobby King that arrived DOA.
    They sent replacements without expecting the dorked ones back. 1S cells between 2 to 2.8V. Tired a different setting on the charger (not all can do this)
    and carefully walked the battery up to 3.3V. Then switched to lipo and took it up the rest of the way. Now how long it will last I know not.
    With 2S and up packs, forget it. It is not worth the trouble. I'd rather launch the rocket knowing if there is a difficult recovery, the pack is going to run
    to its death to give me as much time as possible for recovery. If there is any question about a pack longevity, ditch it or use it for ground testing only.
    Lipos eventually die anyways so man up and buy new. Or watch your rocket come in ballistic or have your tracker die on that
    totally sight unseen flight. Kurt Savegnago
     
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  18. Nov 28, 2018 #17

    OverTheTop

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    I just treat LiPos as consumables. Not worth risking a rocket for a cheap battery IMHO. If they are somehow mistreated (over-discharged is my main problem) they just get replaced. I have had a couple puff up after flights, particularly flights where altitude was gained quickly, and they have been junked too.

    Don't forget to cut the wires and connectors off as they can be useful in the future.

    The more I uses LiPos the more I appreciate NiMH and NiCad batteries. The LiPos have the power density but are finicky to deal with and seem to have a finite life regardless of how well you treat them.
     
  19. Nov 28, 2018 #18

    Banzai88

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    Lipos are cheap and easy enough to come by, charge, and monitor. To me, they're seasonal disposables in that after the flying season they become ground test and set up batteries ONLY. New ones are procured at the start of flying season, charged, evaluated, and put into service.

    I'm not gonna lose a $500+ rocket and make a missile hazard for a $5 battery. Ever.
     
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  20. Nov 29, 2018 #19

    JordanT

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    The gap between 2.8 and 3v on a LiPo, which is recoverable through charging, and <2v which is basically unrecoverable (or merely untrustworthy) is very small. My "oops" when I was testing was 4.5h = good (7v on a 2S), 5.0h = dead (<3v). And somewhere in there the circuit shut down so I would have "lost" it anyway. In reality, though, for the eggtimer parts I already know pretty much where the rocket is via telemetry; I'm not foxhunting because I have a coordinate that's within 20-30m of when I lost radio contact. I suppose it might be an issue if it went sideways and out of range, or hit the river and started floating away, but we're still talking about 10-20 minutes out of a 4 hour window. Personally, I'd rather have the option to go into life-support mode after voltage drops below a settable nominal (7v on a 2s, for example) where I get a 30 second burst of data every 5-10 minutes than to have it nosedive. Wishes, fishes and all that.
     
  21. Nov 29, 2018 #20

    cbrarick

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    I dorked some lipo's (2 cell)and found a procedure where you charge them as though they were nicads to a certain voltage and then stop and charge them as normal. They've held up well in the year and half since then,but full disclosure is that i replaced one on the board (2 independent circuts) just to be certain I didn't wipe out the rocket....the one i took out holds voltage and the one that's in works as well as the new one...
     
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  22. Nov 29, 2018 #21

    JordanT

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    I suppose I should explicitly state here (For Cris as well as forum members) that I'm not accusing Eggtimer of any shortcoming or flaw. If my other recent thread is to be believed I'm now a chronic complainer and poor all-around sport. Quite the contrary - I'm trying to protect myself from my own laziness in forgetting to unplug the trackers after my flights are complete. (In fact, I just convinced half a dozen people in my club to buy Eggtimer hardware this week and I'm holding a "build party" so we can help one another get more hardware in the air)
     
  23. Nov 29, 2018 #22

    Charles_McG

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    I asked Cris about this a couple of years ago. I fully understand his answer. And given the battery options out there, it makes sense to put it on the battery (and thus the user) side of the equation, rather than the altimeter side.

    Since I fly at Bong, with a modest waiver, I’m much more likely to know exactly where the rocket is while the battery drains away. Hanging right from that tree limb. Even a Quark will eat a battery if left overnight. <sigh>

    As far as Egg-side options- the eggfinder is a simple pipe, routing GPS packets to the radio. Changing the ‘chirp’ interval wouldn’t save much power, I think, since the GPS receiver is running the whole time. And would take more power to drive a processor that decides when to let the data packet through to the radio. The TRS is much the same - the processor doesn’t read the GPS data packet and send it to the radio, it watches it go by, then tacks baro data on the end.
     
  24. Nov 29, 2018 #23

    BayouRat

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    I’ve gone back to using good old Duracell 9v Batteries. I found to many inconsistencies with LiPo batteries. One being that they have a tendency to die suddenly and without warning. ⚠️
     
  25. Nov 30, 2018 #24

    Wayco

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    Most of my Eggfinders use a 950 Mah 2S battery for power. They cost $6.18 on Hobby King. I have forgotten to unplug them in the past, but don't do that so much now. I think that making mistakes is a good way to learn.
     
  26. Nov 30, 2018 #25

    ksaves2

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    One should not limit transmission of NMEA sentences with 100mW, 900mhz trackers. You will not
    receive 100% of the positions for a variety of reasons and to maximize what you get, you want the tracker to transmit on this band as much as possible. Again, that is for the tracker type above. Change the band or power output? That is a different story. Kurt
     
  27. Dec 4, 2018 #26

    JordanT

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    I agree, that's why I was curious about what people are using. Seems most go with either throw-away alkalines every flight (9v) or throw-away LiPos every time they can't disconnect before the voltage drops below 3v/cell.
     

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