Understanding Batteries

Discussion in 'Batteries & Power' started by Tim Sessler, May 6, 2014.

  1. Tim Sessler

    Tim Sessler Active Member

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    Hi guys - I have am trying to understand something about my lipo batteries that has given me some headache for a while now.

    While charging my set of Freefly 6S 9000mAh lipos, I realized the other day that my charger only put about 8000mAh back into a set that I completely emptied (meaning I went down to 3.6V/Cell) - so less than 50% of the total 18,000mAh capacity.
    That happened with every set, which makes me wonder why I would only use about half the Ah of what I am actually flying around.

    Long story short, yesterday I flew my copter down to 3.6V/Cell and got a total flight time of roughly 6 minutes. Now a friend of mine had his copter out (same X8 set-up, same electronics, same settings, similar AUW, only difference is that he is using CF props and I am using Xoars).
    To our surprise he got 11 minutes of flight time with another set of my Freefly 6S lipos.

    We both ended the flights at 21.6V - now without being under load my set was back up to 22.7V, his at 22.4.
    The interesting thing is: I was able to add back 10,400mAh in my two batteries, but 15,200mAh in the set that he had used.

    My question is: how could that be? Why am I reaching the cut-off voltage so much earlier, yet only use 2/3 of the Ah. Whats the average A that should be drawn during a full flight? 50% seems very low.
    Or differently asked: ( and that brings us to the very basics understanding of batteries) whats the relationship between V and A?

    Thanks guys!
     
  2. Gary Haynes

    Gary Haynes Administrator
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    Tim did you both charge them on the same charger? What are you using to check the voltage on both sets? In other words are you using all of the same tools? How old are your batteries (cycles and time) compared to his? Can you swap batteries with him? If your charger does it have you tried doing a storage charge and then a recharge at say 4.5a/.5C nice and slow charge? What are the individual cell voltages when discharged and when fully charged? What it the IR (internal resistance)?

    I may be that the batteries are starting to wear out. I keep a log for all my batteries and record flight time, recharge mAh, time to charge, cell gap and IR so that I can hopefully see when batteries are beginning their end of life sequence. Other benefit is I have a flight time log for the day when the FAA finally says we can fly legally but need some type of recorded flight log time for a commercial license.
     
  3. Tim Sessler

    Tim Sessler Active Member

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    Thanks for your response Gary!

    Both sets are my batteries - I got them all at the same time about a month ago and they both have less than 20 cycles. Both charged on the same charger, even at the same time. The IR is 4 milliohm on the ones that I used, 5 on the ones he used.
    photo.JPG

    I don't think the issue here is the charger: its the fact that one copter flew for 11 min using 15,000mAh before it hit the 21.6V low-voltage alarm and the other flew only 6 min using 10,000mAh, yet also hit the 21.6V mark.

    Does that make any sense?
     
  4. Gary Haynes

    Gary Haynes Administrator
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    What are you using for a charge rate? What else is different between the two ships? If you fly the battery set that he was using do you see a difference? Maybe Andy will have some thoughts.
     
  5. Tim Sessler

    Tim Sessler Active Member

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    Charge rate is 1C. Besides the props everything is the same - same frame, same motors, same electronics, same software etc.

    The interesting thing is that his copter always used around 75-80% of the capacity, while mine always only uses around 50-55%.
    I'd understand if I'd be drawing a higher amperage (for whatever reason) and therefor hit the low voltage earlier. But I don't understand why it would only draw half of the capacity and still be empty.

    Would love to hear Andy's thoughts!

    Thanks.
     
  6. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I'd love to hear my thoughts too, Tim. :) But some days, nothing rational comes out....

    Can you tell us how you have stored the batteries between flying days?

    Based on what you say, it's hard to figure out why, given the same chargers, you're seeing such different performance.

    Andy.
     
  7. Tim Sessler

    Tim Sessler Active Member

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    Sorry if I wasn't clear before:

    I was using two sets of batteries (same charge, same age, same cycles, same IR) - on copter Nr.1 they only use about 50-55% of the mAh before they hit the low voltage alarm (21.V6) - on copter Nr.2 (that is pretty much identical to copter Nr.1) its closer to 80% of the mAh that are being used.
    Copter 1 uses 10,400mAh and flies for 6 minutes.
    Copter 2 uses 15,000mAh and flies for nearly 11 minutes.

    Now copter Nr.1 unfortunately is always only using around 50-55% of the mAh of all of my batteries. All I am wondering is what the relationship between the used Ah and the Voltage is. How could copter Nr.1 use 1/3 less Ah but still hit the same voltage?
     
  8. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Let me take a stab at answering your question:

    As a battery ages, its voltage drops under load more rapidly. The problem is that the batteries storage capacity probably has not diminished -- only its ability to deliver all of its storage charge and maintain a high voltage. (Although it's capacity may well have diminished too - I'm not excluding that possibility.)

    What really matters, at least to a copter, is the batteries ability to deliver wattage (which is a measure of electrical power) under load.

    Watts = volts times amps. (Definition. Thanks to James Watt for that definition.)

    So if the voltage drops under load, even though the battery could still theoretically deliver relatively high current -- it cannot deliver the same amount of power (because the voltage is dropping). Nice little Catch-22.

    Why is the voltage dropping? Usually because the internal resistance of the battery is increasing with "age" (or wear and tear). That internal resistance is part of the electrical circuit and the more power that flows through a resistance (even one that's inside the battery) the larger the voltage drop across that resistance (you can thank Mr. Ohm for Ohm's Law), and thus the current is reduced too. The voltage, current and resistance are all deeply intertwingled. (You can thank Ted Nelson for the concept of Intertwingularity.)

    What ages a LiPo?
    Things like:

    1. Time passing. (You knew that, right? ;) )
    2. Keeping it stored at fully charged voltage.
    3. Over-disharging it.
    4. Cheap LiPos.
    5. When it knows that, in your heart, you really don't trust it.

    (OK, OK, I made 5. up. :) )

    Hope this helps. Sorry to be the bearer of possibly unhelpful news....
    Andy.
     
  9. cody white

    cody white New Member

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    Hi guys, I'm the guy with the other copter. Great explanations, but I think the problem is not a battery one. Here is the scenario:

    Copter a: ~6min flight time, down to 22.6v under load
    Copter b: ~11.5min flight time, down to 22.6v under load

    Both copters use the same electronics, same motors, and are within .5lb of each other. The same batteries were used so it is not a battery issue. When the batteries were charged, it was determined that copter A used approx 5000mah less than copter B. This rules out the explanation that copter A is using higher current, therefore causing the batteries voltage to sag more under load, and therefore reaching the cut off faster. Also, being the same batteries, it's not an issue of IR or battery age.

    As you can see this is quite puzzling....
     
  10. Gary Haynes

    Gary Haynes Administrator
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    Props? Copies of the GPX flight logs if you are using MK electronics.
     
  11. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Can your chargers measure the internal resistance of the different sets of batteries?
    And, as Gary says, the GPX will reveal all if you can post them.

    Thanks
    Andy.
     
  12. Tim Sessler

    Tim Sessler Active Member

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    I just had my first flight with KDE 4014 and 17" props. Same issue though - I only used 9,948mAh for flight one and 9,853mAh with a second set of Freefly 6S 9,000mAh lipos (according to the Hyperion Duo).

    The GPX files are attached - though not too helpful as there isn't any data regarding to the current due to the i2c converter.
    I can try to get some more info form the Castle ESC logs.

    As far as IR - both sets are at 2 milliohm, which should be pretty good if I am not mistaken.

    Another thought was that the MK FC possibly miscalculates the voltage - though the MK numbers are matching up with the lipo meter.
     

    Attached Files:

  13. Steve Maller

    Steve Maller UAV Grief Counselor

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    I am seeing a similar issue using Zippy batteries. I assume both copters have a parallel battery splitter. That’s what I’m seeing, too.

    I wonder if there is a subtle difference in the wiring that could cause more resistance in the LiPo->PDB path? But the results are quite dramatic. I have about 7-8” of 8AWG wire from the PDB to an EC5, then about 7-8” of 8AWG wire from another EC5 to each end of the splitter. Then, of course, there’s the actual LiPo leads.

    Could I have something like the wrong solder that’s causing a cascade of lost potential (my parents once said that about me :oops:)? I’m using good quality EC5 connectors from ProgressiveRC and good quality silicone wire, too.
     
  14. Steve Maller

    Steve Maller UAV Grief Counselor

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    Here’s a photo of the layout on my copter. Could this be an issue with the amount of wire between the LiPos and the PDB?

    IMG_9698.jpg
     
  15. Tim Sessler

    Tim Sessler Active Member

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    Actually one of the differences is that my parallel adapter (https://www.quadrocopter.com/EC5-Parallel-Adapter_p_617.html) connects via EC5 adapter to the power distribution cable (https://www.quadrocopter.com/ESC-Power-Distribution-Cable-1-to-8_p_759.html) - Cody has removed his EC5 adapter and soldered them together.
    But that by itself should definitely not make such a huge difference in the resistance.

    Is there any way to measure the resistance of all the connections and solder points? Whats would be the best way to test the overall resistance?
     
  16. Steve Maller

    Steve Maller UAV Grief Counselor

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    I’m going to try an experiment today if I have time. I’m going to fly with one battery connected directly upstream of the splitter. While not 100% scientific, it might shed some light on whether my splitter is causing an issue. My guess was that the problem has more to do with the batteries in my case, but it’s perplexing that you’re having the issue with different performance with the same batteries.
     
  17. Tristan Twisselman

    Tristan Twisselman Active Member

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    I am noticing the same issue with one of my copters as you Tim. I figured it was the low 25c rating of the batteries and maybe the props you two are using is making the big difference in flight time. But, my power leads are also somewhat long with a parallel splitter so that could be the thing we all have in common.
     
  18. Tim Sessler

    Tim Sessler Active Member

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    Thats very odd! Its definitely not the props. I tested the Tmotor 15" and only got a slightly improved flighttime. A few seconds - definitely not 5 minutes though.

    I'll have to test what happens if I only fly with one 6S lipo, without using the parallel splitter. It will probably only draw around 4,500mAh - but it would still be worth a try.
     
  19. Steve Maller

    Steve Maller UAV Grief Counselor

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    So I charged up a pair of my Zippy 6S LiPos, and I believe their internal resistance is 12-13 milliohms. The individual cells vary a bit, but they're all pretty close. And from what I can tell out there on the inter webs, lower IR is better, and lower than 20 or so is good. Am I right?
    IMG_9904.jpg
     
  20. Gary Haynes

    Gary Haynes Administrator
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