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Thread: Understanding battery consumption...

  1. #1
    Pilot Hockeystud87's Avatar
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    Understanding battery consumption...

    So I have been performing some flight recently and I'm kinda confused on how to dictate your battery consumption during flight. So I have a RVOSD and it does show the total amount of mAh used, but I have been told that I don't want to have my single cell battery voltage get any lower then 3.7v. So that's a total voltage of 22.2v. DUring flight I have seen my voltage drop to a mid range point of 23.7v after a 10 min flight. But when I go to charge my battery I only apply 800mAh of juice. Shouldn't this have used up a total of 2500mAh? I have also taken into consideration when the battery is under load or not.
    Last edited by Hockeystud87; 7th May 2013 at 03:43 AM.

  2. #2
    I can't breathe Hans's Avatar
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    I think what you want to know is how long it stays at 23.7 volts until it finally drops to 22.2?

    As you can see in this example, the discharge curve stays flat for a long time before it drops off:


    I've set my voltage alarm to 3.3 and still manage to land safely. But i only fly short range and always within sight.

  3. #3
    Pilot Hockeystud87's Avatar
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    Ahhhh I see... Ok this makes sense. Cause I usually see that the discharge is rapid from the start and makes me worried about my flight times. So I should simply just push it out and the voltages will lower much slower in the middle. So this means if I wanted to keep the 20% rule I could take them down to around 3.5?
    Last edited by Hockeystud87; 7th May 2013 at 02:16 AM.

  4. #4
    I can't breathe Hans's Avatar
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    3.5v sounds reasonable safe to me

    I think i went as low as 3.3v with a 3000mah lipo when flying LOS. Charging the pack afterwards always was arround 2990 ~ 3000mah to put back in. Flying FPV i set the voltage alarm to... 3.5V (i think, i have to check when i'm at home) as you tend to fly higher and further, thus the point of no return becomes more important.

    At some point while flying when throttling up to gain altitude, the voltage drops below my alarm-level for a short period of time, indicating that i should return. After that first beep, i still have 5~6 minutes left on my quad before the alarm goes off until i land.

  5. #5
    Pilot Hockeystud87's Avatar
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    Hmm I see so you actually use your whole 3000mah when you fly? I thought that you weren't supposed to go lower then 20% of that? This is great information BTW thank you! I know now I don't need to be so worried about my battery.

  6. #6
    GO HAWKS Hucker's Avatar
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    You will get to know your batteries well if you are trying to maximize range/flight time. I suspect you will also learn to watch the voltage moreso than the current. Mine are numbered and I track how much the OSD says I use and how much the charger says they use. Some batteries don't make their ratings and others do. When I push it I land at around 10.2V on 3S but normally I don't use that much. As the chart shows there is a big flat spot in the middle that you fly, I watch the Volt numbers drain down like a hawk. I tend to fly a climb out, glide home profile which means I turn for home at 11.0V (more or less depending on wind). Gilding home only uses .4A compared to the 5.5A on the climb or 1/15th the power (radian pro not tech pod, I suspect similar numbers)

    Another thing to consider is that planes fly on Watts not Amps or Volts. If you have found on a new pack at 12V and 5A (60W) your plane will fly level you will find that at 11V you will need 5.5A (60W) to stay level. This implies that over the course of a flight where you are climbing or maintaining altitude you will slowly be adding throttle to maintain steady state. Would be nice if there was a Watt based ESC. For the longest time I was keeping my throttle constant and thinking the air density was robbing performance when in fact it was the power being reduced.

    Being the geek that I am I log every flight in excel and look at total flight distance and divide by mAh consumed to get a mAh/km metric...which gives a good estimate of efficiency and what an airframe is capable of with a given amount of battery. The radian pro was in the 110-120 range and the Nike 2 is in the 85 range. This implies the nike will easily go 22+km on the same battery the radian pro did 17km. I really hope to tell you what the techpod does soon.
    Last edited by Hucker; 8th May 2013 at 12:57 AM.
    KF7SKL,TechPod/Walrus/250mmQuad/HoneyBadger, EZUHF,SL-AAT, 1G3/2G4/5G8

  7. #7
    Sr. Keyboard Cowbow chatch's Avatar
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    voltage drops under load. crappy batteries (nanotech) usually will drop the voltage below 3.3v/cell after 60-75% capacity. better batteries, not necessarily higher C rated batteries, will hold out longer. it's good practice to land at 80% capacity discharged. so if you have 3300mah battery, land at 2700-2800mAh. 3300mAh nanotechs will hit critical voltage by 2500mAh
    Quote Originally Posted by jimmaplesong View Post
    When you're outraged at chatch, please don't quote his bullshit.

  8. #8
    Pilot Hockeystud87's Avatar
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    Very interesting it's good to find out what other people are running there lipos down to.. Hucker IDK if your comfortable with it but would I be able to see your excel formatting? I take data down my self but it's very basic right now. It would be awesome to be able to compare notes with you at a later date over the techpod, if you decide to you can e-mail at al.creigh@yahoo.com.

    I currently only have 1 6s 5000mah that I don't think I have taken down lower then 1500mah back in on charge. So It's clear I got lots more to give, but from what I gather in your calculations is the lower the rating the more efficient your air frame is. Or simply less mAh used per kilometer traveled. This excel sheet has peeked my interest now . Also good information on having to increase throttle as voltage is depleted from your battery I did not know this.

  9. #9
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    As said, 80% is a good figure for your lipo's life... You don't want to continually drain it much beyond that.

    The voltage curve is different for different chemistry lipos so it's not a one fits all scenario. The current sensors are generally pretty accurate, so assuming your lipo was fully charged I would just go off what that tells you... As mentioned voltage will sag under load, so what is 3.7v at full throttle might be more like 4.0v resting.

    I've generally found that if my lipos have a resting voltage of 3.7v at the end then they are about 70-80% discharged... In flight on heavy current drawing machines like my 600 this can show as low as 3.3v when the current pull goes up.

    Most ESC's will have a cut off around 3-3.2v as standard, so if you go below this point you will hit LVC and the ESC will do whatever it is programmed to do at this point.

  10. #10
    GO HAWKS Hucker's Avatar
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    Hockeystud:

    Anybody reading this should note that hockeystud flys a TechPod so my input is directed knowing he flys a motor glider and probably wants to maximize duration.

    You need to take what I've posted here understanding the loads motorgliders put on batteries because it is easy to get confused by what zfly and chach are (correctly) saying for more normal applications.

    0) At launch I might hit 5C for 15 or so seconds to transition to FPV.
    1) My flight profile is climb and glide.
    2) Under climb I draw about 1.2C on a radian pro, 1.4C on the Nike and probably .7C on a techpod
    3) When I cut my throttle from cruise to glide my voltage raises by 0.1 to 0.2V.
    4) The second half of the flight is at about 0.1C
    5) My landing voltage and min voltage are almost always the same 10.5-11.0V on 3S.

    Suffice it to say my batteries have it easy. When you look at discharge curves for most lipos this current draw represents the top line...if it even has something at this low of a current draw (Hans' attached graph's top line is for 7C which is 5x more than we need to climb at 10-1). Most people don't fly this way since they want to fly fast or have inefficient airframes or quads. If you have a techpod and are planning on flying for a long time I'll bet good money you are gonna fly like what I describe. I suspect your batteries will last much longer than those used in quads and EDF's.

    One thing that might not be obvious is that time it takes for the battery to discharge (at constant current) from 3.9V to 3.7V is probably 10x longer than the time it takes to go from 3.7-3.5V. So guys flying quads see this drop off like crazy. For me when I get to 3.7V I turn home and chop the throttle and draw 1/10th the power life is pretty good for me...unless the wind changed.

    I don't trust using current because you need to know what battery you put in (I have about 6 different mAhr batteries that look the same), the charge state and if the battery can actually make its rating. Too many variables for me, I'm old and forgetful. I just watch Volts and am quite confident that you could put any battery at any charge state in my plane and I could get as much out of it without really looking at the current usage.
    KF7SKL,TechPod/Walrus/250mmQuad/HoneyBadger, EZUHF,SL-AAT, 1G3/2G4/5G8

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