1 battery vs 2 batteries that equal the same mah?

G'day DIY Drones,

I have a question about batteries, yes, I know this has been done to death, but I would like some real world experiences rather than the math.  

I have used the search bar but I guess I am not using the right keywords, hence the reason for this post.

I am considering using 1 10000mah 4s instead of 2 5000mah 4s batteries to get more flight time from my 690mm photography rig.

A bigger battery of the same C rating or close to it.

Here is the math (feel free to correct me if I'm wrong)

2 x 5000mah 4s at 25 C = 125 max amp draw and has 10000mah. Weight, approx 1300g

1 x 10000mah 4s at 25 C = 250 max amp draw and has 10000mah. Weight, approx 800-850g

I only need about 115 max amps so both will do the job.

My question is about flight time and battery voltage sag.

I don't want theory but real world experience please.

Will the 10A, lighter battery give me more flight time because of the weight saving or will the voltage drop off sooner because it is only a single battery?

Another way to put it would be, would the 2 5A batteries hold the voltage longer than the 1 10A battery given the same flight situations?

Another way to put it is, are two batteries that equal the one in MAh, better?

I hope I am making sense here 

Cheers

Aussie

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Replies

  • As the OP, I would like to thank everyone here who has taken the time to give me their wisdom and I have read every post, however, I think we are now getting off track from my original questions and are getting into lukewarm/heated debate/arguments about who is right and who is no quite as right as the next person.

    Someone answered 1 battery is better because it is cheaper and takes less time to charge.  Valid to a degree for some, but not what I was after.

    Although very interesting (and way over my head in some cases) I am more confused now than I was before.

    -------------------------------------------------------------------------------------------------------------------------------------------------------------------

    My setup

    2 x 5000mah 4s 30C batteries, new and from the same manufacturer (not mismatched) AUW with batteries and camera is 2748 grams and she hovers at about 35-40% throttle (a little over powered I know and should probably be closer to 45-50%)

    I get about 13-14 minutes of lazy to medium flying until I it 13.7 volts and bring her in. (it's a stable hexa camera rig, not set up for speed)

    Opinion and reason

    I have a larger surface area for keeping batteries cool.

    I have less voltage sag at the toward end? (correct me if I am wrong)

    On my rig, I have better weight distribution due to design constraints 

    Question 1

    In one sentence, please state your most basic answer and why.  (ie. 1 is better because of less failure points......or, 2 is better because you have more amps......or,  x is better because.......)

    Question 2

    Does 13 to 14 minutes of flight time sound like a good amount of time?  I have nothing but a cheep park flyer that gets 20 minutes of vigorous flying to compare it to.

    Question 3

    The batteries usually get about 6500ma put back into them while on the charger after a flight. Do I have my "reserve light" come on to early at 13.7v 

    Cheers

    Aussie

    • Moderator
      Hi Aussie,

      I have re read the your opening posts and can see that you asked for a non theory discussion. Please accept my sincere apologies for my part in the thread drift. I have deleted all of my theory related posts in order to get the thread back on track. So to your questions;



      1. I use one battery as it's easier to keep track ang log it's performance during it's life, the aircraft was designed around a single pack to maximise efficiency/ flight times.

      2. That sounds like a normal flight time for a hex of that weight, so many variables to comment further without a detailed description of your setup.

      3. That failsafe is about right, absolute minimum is 3v per cell but there is a rapid drop off around 3.7 to 3.3 so it is best not to go too low. If you see the post by Brad Hughey in the thread below you will see it exlplained nicely.

      http://diydrones.com/forum/topics/lowest-safe-voltage-per-cell-on-lipo
      • G'day Martin,

        I like your passion so no worries that you straying into water that was way over my head :)

        After reading all your posts, and others who replied to you, I was leaning towards one larger battery, so between you and Joe's comments (above) I think the largest Mah battery with the highest C rating in 4s would be a better way to go.

        I am planning on rebuilding the frame on my current hex to replace the round tube arms with square tubes as I am having issues with trims as the Tarot arms can twist in the frame causing drifting.  I will most likely incorporate a single battery set up at that time.

        Thanks again for all of your wisdom and the wisdom of others who posted here.

        I hope this thread can help others with the same question in the future.

        Cheers

        Aussie

    • I have less voltage sag at the toward end? (correct me if I am wrong)

      For same bat type and total capacity should not be different on voltage SAG.

      Question 1

      In one sentence, please state your most basic answer and why.  (ie. 1 is better because of less failure points......or, 2 is better because you have more amps......or,  x is better because.......)

                No major difference, minor increase in wiring and for same type battery and overall cumulative capacity voltage sag and total flight time should be the same.

      Question 2

      Does 13 to 14 minutes of flight time sound like a good amount of time?  I have nothing but a cheep park flyer that gets 20 minutes of vigorous flying to compare it to.

      This is OK but you should use something like ecalc to figure out if you can get significantly more flight time with larger batteries or higher energy density batteries.  

      It is impossible to tell if our system can do better without much more information such as motor type, frame and payload weight, props size and pitch etc.  Multicopters geenerally get shorter flight times than planes due to efficient issues.  With payload 15 to 30 minutes is typical with may coming in around 15 to 20 minutes.

      Question 3

      The batteries usually get about 6500ma put back into them while on the charger after a flight. Do I have my "reserve light" come on to early at 13.7v 

      I would suspect you are coming in a bit early.  If the batteries are truely 5000 and total capacity therefore 10000 then you are using 65% of capacity.   I typically aim for 80% to not stress the batteries and give a little for safe landing.  If you might be further away when the low voltage might trigger I would increase this a bit.  I normally plan to be back in close proximity when bat life begins to get low.  Be careful and make minor increases in flight time/ lower voltage and then check batteries as you have been doing. While current capability is often waay overstated total energy capacity is usually a bit close to specs.  but.......

      • G'day Joe,

        Thanks for your input and thanks for answering each of my questions.  All put into my memory bank and processed to the best of my ability. 

        I'm now leaning more toward the use of one larger battery for a future build.

        Cheers

        Aussie

  • Martin,

    why do you promote prank science.

    In case of Li-Ion cells connected in parallel

    you need to employ balancer to support safe charging, since individual cells never match.

    ---

    Now to the summary, you have for the most part quite correctly stated that for matched cells in parallel "the issue with unequal loading are relatively insignificant". The performance issues I'm discussing are only of note for systems that have high drain requirements towards the end of their actual discharge capability. This is where these other factors will take you beyond the limit. Most users will probably never notice the affects

    • In my situation, I have three parallel packs (Venom 5s, 5000mah, 25c.  I ahve been in the air for three months now.  Ecalc predicts max battery current load of 20c, for a max system current of over 300 amps.  In practice, I am seeing a max of around 130 to 150 amps, and at those times the voltage sag is significant and will trigger the battery fail safe.  The gentlemen at Atomikrc made the original statement to me that parallel lipo packs will not necessarily result in additive current capacity.  Now it may be true that my battery monitor setup is not accurate, but still, I am seeing between 1/2 and 1/3 of the current that would be expected from a parallel setup with truly additive current capacity.

      Therefor, two possibilities exist.  First, the gentlemen at Atomikrc are making excuses to cover for the Venom batterys that really provide a 10C, but are sold as 25C. or, Martins explanation above is accurate.  I suspect each explanation is valid.

      My current conclusions,  (for those who require high current)

      1.  Only use 1/2 of any C rating, this should reduce the bad voltage lag at higher current.

      I have ordered a set of 60C batteries, as my load is predicted to be 20C.

      2.  Always parallel the balance leads with parallel packs.  I suspect the DJI battery that uses parallel cells might work a bit better than separate packs because they are charged and balanced together as a single unit.

      3.  Use real test data to verify and calibrate the battery monitor.  Bad data provides bad results.   I have ordered an in line load analyzer. 

  • Martin:

    I made the comment in reference to the hobby shop's opinion that parallel lipo packs should not be viewed to have multiple the current capacity.   IMHO, Your explanation is the most reasonable I have read in this thread.  (BTW it was AtomikRC that provided this opinion with my Venom lipo packs.)  I was very hesitant to just bash these guys, as I am certainly not the expert, and just looking to understand the concepts.  The others on this thread that jumped in to with the rock solid opinion that the current is additive just like a resistance load should take notice, as this issue is clearly not commonly understood.  Your explanation also clarifys why my solution of  monitoring only one of the parallel packs (and compensating with the amps per volt parameter) would provide an inconsistent reading.

    • Harry and martin,

      I'll jump in one more time.  battery capacity in parallel is additive.  This is very basic principle of electronics.  

      Electric car battery packs are made this way with many sub units collected in a array of series and parallel packs to reach both current and voltage desired. I have a battery system for my solar array.  Here  I use batteries sets in parallel to be able to meet the current requirements.  

      Take the DJI (sorry) Phantom 2 pack.  It is a 5200mah 11.1 battery.  Internally it has 6 cells 3 series  sets of 2 cells in parallel.  Each cell is 2600mah giving a total capacity of 5200mah. It makes no difference whether this is done by wiring the sets internally or externally as separate batteries.  

      While I agree that manufacturers blow up their claims and this can lead to sets not meeting specifications this does not change the basic rules that apply.

      I also agree that more connections wire etc increases chances of failure but in the case of one or two batteries I think the chances are extremely low compared to other failure modes.  I have never had a battery failure in the air but have had many other failure mode cause a ground intersect. Personally I am not arguing 1 versus 2.  I see this as primarily a packaging issue space size COG etc.

      One should  use batteries that are matched and in good shape .  Same chemistry and discharge curves. In this case issue with unequal loading are relatively insignificant.  If batteries of different type are used Martin note and not producing a pure additive current capacity would be true.  But not with same chemistry / construction. The variation between cells is very low in good batteries.  Mixing a old used battery and new one can cause problems as well.  Whether using a single battery with cells in series or batteries in parallel monitoring each battery cell and total pack health is a good idea to prevent in air failure.  I do pack charge discharge curve on new packs looking at overall capacity and cell matching.  I do this periodically  ensuring the life of the battery to help ensure I do not have in use failure.  Typically with lipos I see single cell failures which is bad however wired up.

      As Martin notes below don't trust specification but test the system.  But whatever the individual battery capacity and discharge capability they should obey the laws of electronics.

  • Moderator
    Good stuff, you'll still get lock in about zero, but don't forget-when it is effectively derivative, factored more around the dead time in the dither than overall accumulation, you'll have to take the variable as a potential error, which can be compensated within normal limits.
    Back to the batteries, I can see why some find the two packs helpful when using an existing aircraft with space constraints to maintain optimal c of g. But if designing your own machine, good planning and layout would often make the single pack a better option.
This reply was deleted.

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