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Balancing Battery Packs with Motor size

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  • Balancing Battery Packs with Motor size

    Ok, so I'm a nu-bee at this and the learning curve seems to be kicking my ass.... Would appreciate any help available.
    I've been trying to pick a couple of batteries to run in series for my Rascal 72'' weighing in around 8-9 Lbs flyable weight. (inc. batteries) I was given a somewhat large Himax outrunner motor (HC5030-390) which has a max power of 1500W, 12000 max RPM, kv=530, Rm= .031, Lo= 1.8 @ 10v. I'm using an 80 Amp burshless speed controller everything running through 12 gage wire and banks fittings. I'm looking at placing two 4000mAh 4S 40c batteries in series to run the show.... I know the prop makes a difference bu tset that aside for the moment,

    My question is: calculating 3.3 volts per cell (suggested by the motor's manual) X 8 cells (two in series) = 26.4 total volts which is right up this motors alley. But when I calculate the 4 amps X 40c I think I get the potential of a 160A burst discharge which is way over a motor saying it has an efficient operating current of 30-50A, 65A max for 15 sec.. So it seems to me if I open the throttle full on takeoff I'd be cramming (potentially) 160A into the motor... which I believe should turn things red hot in seconds. What am I missing here? Do I need to limit the throttle in my radio down to... what 35% to keep the Amps in the 65-70A range? Do I try to compensate with an oversize prop? I don't want to fry things but I don't have any experience with this and need a bit of advise.
    Thanks in advance for your help.
    drd

  • #2
    The motor draws what it needs to turn the prop; the only time the current rating of the battery comes in is if it is too small, e.g. a 10C 4000mAh battery would get hot trying to provide 40A or more, and probably limit the power. A battery rated for a much higher current will stay cooler, and may be able to give a burst of power near the end of flight, if needed.

    BTW, at full charge, a LiPo battery is 4.2V per cell; depending on the load, it will probably start about 4.0V per cell at takeoff. If the cell Voltage drops to 3.3V, you should have landed already, unless the model is hanging by the prop; I land when the loaded Voltage is about 3.5V-3.6V in level flight, so the battery will recover to about 3.7V per cell when the load is removed. If you don't have battery monitor TM, then use a timer, and check the cell Voltage after landing; if it is more than 3.8V, you can increase the time; less than 3.7V, decrease the time. Of course, more aggressive flying will shorten the time some. Also, get a Watt meter - it will save you money when trying out new power systems: prop, battery, motor, ESC.

    Oh, and if you are using LiFe batteries, then, yes, cell Voltage is 3.3V under most conditions, and will drop really fast when it gets below about 3.0V.

    Comment


    • #3
      Thanks for the quick come back. Ok, then if I understand you correctly I don't need to be concerned because the motor will not take more than it needs and that will top out (for this motor) around 65-70 amps or so. correct? I don't have a remote battery monitor but can check them once I land. Looks like some experimentation is in order.
      Thanks again. Just couldn't get my head around that one.

      Comment


      • #4
        Originally posted by Drd View Post
        Thanks for the quick come back. Ok, then if I understand you correctly I don't need to be concerned because the motor will not take more than it needs and that will top out (for this motor) around 65-70 amps or so. correct? I don't have a remote battery monitor but can check them once I land. Looks like some experimentation is in order.
        Thanks again. Just couldn't get my head around that one.
        The motor takes what it needs based upon the load put upon it which could be more amperage than what is rated for so you do need to be concerned.
        This is why it is SO VERY important to use a watt meter as Wintrsol indicated when messing around the factors he mentioned.
        The prop is the load that determines what power requirements are needed for motor and ESC. The diameter, pitch and blade count increases the load exponentially as RPM's go up.
        I looked up the specs on this bird and the motor you are using is a big overkill.
        You can run this Rascal on one 4S 4000mAh 40C you mention in your first post.
        Recommended motor is highlighted red
        Here's a cut-n-paste from Sig's product page.
        http://www.sigplanes.com/SIG-Rascal-...SABEgIKCvD_BwE
        SPECIFICATIONS:Wing Span: 72 inches (1829 mm)
        Wing Area: 720 sq. inches (46.5 dm2)
        Length: 51.75 inches (1315 mm)
        Flying Weight: 5 - 5.5 oz.. (2268 - 2495 g)
        Wing Loading: 16 - 17.6 oz./sq. ft. (49 - 54 g/dm2)
        Radio Req.: 4-Channel with 5 Standard Servos
        Glow Power: 2-Stroke .40 - .46 cu. in. (6.5 - 7.5 cc)
        4-Stroke .40 - .54 cu. in. (6.5 - 8.8 cc)
        Electric Power: 500 watt (3528 - 1000kv) Brushless Motor;
        50 - 60A ESC; Lipo Battery Pack
        The product page even recommends a Himax HC4220-770 Brushless Electric Motor using a APC 13x6.5E Propeller ;)
        This is a 4S set up and will easily yield 6-8 minutes of flight time and I believe is a much better choice than the spec page.

        Best regards,
        Warbird Charlie
        HSD Skyraider FlightLine OV-10 FMS 1400: P-40B, P-51, F4U, F6F, T-28, P-40E, Pitts, 1700 F4U & F7F, FOX glider Freewing A-6, T-33, P-51 Dynam ME-262, Waco TF Giant P-47; ESM F7F-3 LX PBJ-1 EFL CZ T-28, C-150, 1500 P-51 & FW-190

        Comment


        • #5
          If he was given the motor, and wants to use it, why not? Since his motor is 530kv, and the recommended motor is 1000kv, running at 8S is viable, as the rpm will be similar. As a 770kv motor will safely turn a 13x6.5 prop on 4S, I'd start with a 12x7 with the set up he has on 6S or 8S, and test with a Watt meter. Sure, that motor he has is very heavy, and balancing the model will add even more weight if he can't move the battery and ESC far enough aft, but it CAN be made to work; it will be kind of a heavy model to land, though.

          Comment


          • #6
            Wintr.........the motor he was given is actually a 390kV and not 530kV as he claimed;)
            Warbird Charlie
            HSD Skyraider FlightLine OV-10 FMS 1400: P-40B, P-51, F4U, F6F, T-28, P-40E, Pitts, 1700 F4U & F7F, FOX glider Freewing A-6, T-33, P-51 Dynam ME-262, Waco TF Giant P-47; ESM F7F-3 LX PBJ-1 EFL CZ T-28, C-150, 1500 P-51 & FW-190

            Comment


            • #7
              Originally posted by OV10 View Post
              Wintr.........the motor he was given is actually a 390kV and not 530kV as he claimed;)
              In which case, 8S could work. Just tune the power with prop selection, and a Watt meter. I turn a 14x8 prop with a 4S battery and 810kV motor, which peaks about 76A, so a similar size prop with 8S and 390kV should yield similar results, but with ~38A. He's paying a penalty in weight from the batteries and motor, but it should still be able to do verticals.

              Comment


              • #8
                Ok, thank you all for the great information. Since last post I have completed and flown the Rascal with the set up stated above using a 13X6.5 prop. I powered it with two 4 cell 4000 mAh, 40C batteries in series. Balance was obtained by standing the two batteries up and moving them back as far as possible. The plane flew very well... balanced and reacted to all commands great. Landings and take offs were amazingly well behaved. Since plane weighed in about 2.5 pounds over what they suggested the "flight weight" should be, I credit the large wing and floatty design of the Rascal. Plane did accelerate straight up and it was the opinion of the club it would have continued to do so until out of sight if we hadn't reduce throttle. Think I was lucky with this one... but I need to add batteries for additional flights...

                So my question now is: Can you run in series a three cell and a four cell battery. I would prefer to purchase a couple of three cells because they would be useful also in other smaller planes. I'm thinking I can run a 3 & 4 cell in series in the Rascal making a 7 cell and staying in the lower range of what this motor needs. If this is possible, do I need to match the mAh and C rating to do so?
                Thanks again for all your great impute.
                /d

                Comment


                • #9
                  P/s- I am purchasing a bench tester to see what is really going on under load. Should be interesting.

                  Comment


                  • #10
                    Originally posted by Drd View Post
                    So my question now is: Can you run in series a three cell and a four cell battery. I would prefer to purchase a couple of three cells because they would be useful also in other smaller planes. I'm thinking I can run a 3 & 4 cell in series in the Rascal making a 7 cell and staying in the lower range of what this motor needs. If this is possible, do I need to match the mAh and C rating to do so?
                    Thanks again for all your great impute.
                    /d
                    Yes you can run a 3S and 4S in series for a 7S application. Do match the mAh and C rating. Use the same Manuf also because C ratings(actual versus printed) are not the same from manufactures.
                    Warbird Charlie
                    HSD Skyraider FlightLine OV-10 FMS 1400: P-40B, P-51, F4U, F6F, T-28, P-40E, Pitts, 1700 F4U & F7F, FOX glider Freewing A-6, T-33, P-51 Dynam ME-262, Waco TF Giant P-47; ESM F7F-3 LX PBJ-1 EFL CZ T-28, C-150, 1500 P-51 & FW-190

                    Comment


                    • #11
                      Thanks again for the impute... learning every day!

                      Comment


                      • #12
                        Need an opinion... I am looking to run a 4s and 3s LiPo in series. I have a 4000mAh, 4s, 40C Admiral battery but can only find a 3700mAh, 3s, 40C in the same brand. Will that work? I was advised to stay with same brand for best matching...

                        Comment


                        • #13
                          Originally posted by Drd View Post
                          Need an opinion... I am looking to run a 4s and 3s LiPo in series. I have a 4000mAh, 4s, 40C Admiral battery but can only find a 3700mAh, 3s, 40C in the same brand. Will that work? I was advised to stay with same brand for best matching...
                          What I said in post #10 is imperative.
                          You will have permanently damaged batteries if you put batteries of unequal capacity in series!
                          It is bad practice to connect batteries in series when they don't have the same capacity. The battery with the smaller capacity will be empty before the larger one, resulting in a lower voltage for the smaller battery. At that point things will start to get interesting as the larger battery will start to charge the smaller one through the connected circuit and with reversed voltage. The cell is not designed for being reversed and charged and bad things will happen.
                          Warbird Charlie
                          HSD Skyraider FlightLine OV-10 FMS 1400: P-40B, P-51, F4U, F6F, T-28, P-40E, Pitts, 1700 F4U & F7F, FOX glider Freewing A-6, T-33, P-51 Dynam ME-262, Waco TF Giant P-47; ESM F7F-3 LX PBJ-1 EFL CZ T-28, C-150, 1500 P-51 & FW-190

                          Comment


                          • #14
                            Yep, I took what you said in that earlier post to heart... But I was just hoping that a 3700mAh and 4000mAh were close enough... Given everything else the same. Hope springs eternal.
                            thanks again.

                            Comment


                            • #15
                              Originally posted by Drd View Post
                              Yep, I took what you said in that earlier post to heart... But I was just hoping that a 3700mAh and 4000mAh were close enough... Given everything else the same. Hope springs eternal.
                              thanks again.
                              Hard question to answer without test data. All else equal, the effective series resistance gets smaller as the mAh go up, so the first assumption is, no, probably not close enough. If, however, the smaller battery actually has nearly the same ESR, then it would work, as long as long as you always assume the smaller battery determines the available capacity.
                              For example, the smaller battery may have a measured ESR of 10milliOhms, and the larger 9. That means that, at a discharge rate of 25A, each cell of the smaller battery would have an internal Voltage drop of 0.25V, while the larger cells would drop 0.225V. While that doesn't seem like much, it becomes significant when you get to 80% discharge, or ~3000mAh used, which would take about 7 minutes at a steady draw of 25A. Around that point, the smaller battery will begin losing Voltage faster than the larger one. The ESC would have calculated a cut-off Voltage based on the cell count, but the larger battery will fool it, having a smaller drop from ESR, and an overall higher state of charge. You can easily fly until the cells in the smaller battery start to get hot because they are nearing empty before the ESC shuts down; even if you are very careful to time your flights, average power use can change with conditions and use up the battery faster; without remote sensing on the cells, you can easily reach LVC in the ESC and damage the smaller battery. Note that it is also possible the smaller battery actually has the same, or even lower, ESR than the larger one, in which case problems are a lot less likely.

                              BTW, the same can happen with an old and new pack of the same capacity, as the ESR starts to rise with use, and can create the same Voltage drop conditions, which is why most will recommend use of packs of similar age, too. If you really want to care for the batteries in a way an engineer would, you would get an ESR meter to track them, and only combine those that are close for series operation. This is being very picky, I know, but I'm one of those engineers who is data-driven, and I like doing it. I have a couple batteries marked for bench only use, because the ESR started to rise too much, and flight time and power started to suffer..

                              Comment


                              • #16
                                You CAN get away with it if you are VERY careful.
                                Its still not a great idea because its pretty easy to over-discharge and/or overheat a pack and possibly have a flaming airplane come down out of control.

                                I might try it (one 4000 mah and one 3700 mah in series) one or two SHORT ground test runs while waiting for a matched pair of packs to arrive.

                                I wouldn't fly it. You can't watch the packs well enough and its too tempting to fly to LVC, which may not protect the smaller pack from over-discharge.
                                FF gliders and rubber power since 1966, CL 1970-1990, RC since 1975.

                                current planes from 1/2 oz to 22 lbs

                                Comment


                                • #17
                                  Originally posted by fhhuber View Post
                                  You CAN get away with it if you are VERY careful.
                                  Its still not a great idea because its pretty easy to over-discharge and/or overheat a pack and possibly have a flaming airplane come down out of control.

                                  I might try it (one 4000 mah and one 3700 mah in series) one or two SHORT ground test runs while waiting for a matched pair of packs to arrive.

                                  I wouldn't fly it. You can't watch the packs well enough and its too tempting to fly to LVC, which may not protect the smaller pack from over-discharge.
                                  :Scared:That's like telling someone you CAN get away with crossing the street in downtown NYC against the traffic if you are VERY careful.
                                  WHY even bother when there are so many risks involved against doing so.:Confused:.
                                  Safety with LiPo's should be paramount and going against known standards when informed of such is:Silly:and irresponsible.
                                  Just putting that out there for those that don't know of or understand the technology. Better to be safe than sorry;)
                                  Warbird Charlie
                                  HSD Skyraider FlightLine OV-10 FMS 1400: P-40B, P-51, F4U, F6F, T-28, P-40E, Pitts, 1700 F4U & F7F, FOX glider Freewing A-6, T-33, P-51 Dynam ME-262, Waco TF Giant P-47; ESM F7F-3 LX PBJ-1 EFL CZ T-28, C-150, 1500 P-51 & FW-190

                                  Comment

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