Question on adding mah capacity by adding batteries

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  • 78MaicoRider
    Senior Member
    • Jun 2013
    • 179

    #1

    Question on adding mah capacity by adding batteries

    I am currently running 2 2S1P 5000 mah 30C batteries in series for my 4S setup. I believe this to be a fairly common in a single power cat less than a meter LOA. I was considering adding a 4S 2700 mah 30C pack in parallel to increase run time and available current. I do notice a little lack of punch after 10-15 seconds of running. Doubling up the 2S packs (4 2S packs) for 10,000 mah would be too much weight.

    Will the 4S pack be depleted in 1/2 the time of the 2 2S packs? Or will they draw down equally percentage wise, keeping the voltage the same on all cells?

    I think this information will be of use to many of us.
    41" & 29" FE Aeromarine Sprint Cats, Quickdraw powered "Dollar Eater" 41" Insane Cat, 29" BL mod Graupner Cat, 24" Hydro, 29" OB Cat, BL mod NQD Tear Into Jet boat, 55" Scarab, JET SWEEP R/C pool skimmer Rescue Boat.
  • martin
    Fast Electric Addict!
    • Aug 2010
    • 2887

    #2
    You need to keep the capacity of the batteries all the same ie 5000mah, same as same c rating all the same & preferably same make batteries.

    Comment

    • 78MaicoRider
      Senior Member
      • Jun 2013
      • 179

      #3
      I took the boat to the lake and ran this configuration for less than 2 mins. then checked temps. 3740 2000kv motor was 73 degrees with 447 prop. Batts were 88 degrees. The ESC was 80s-90s except the caps were 160 degrees!!! Not sure whats going on there. I think I need to add a cap bank and maybe try the 8 khz setting on the ESC. But this is a 10 pole outrunner. So quit running that boat for the day. Got home and charged the batts to see what was used. The 2 2S 5000mah packs took about 900 mah each. the 1 4S 2700 mah pack also took about 900 mah! Can someone help me understand whats going on here? Of usable power, I had used 50% of the 4S pack but only 25% of the 2 2S packs. I see what is said about keep all the same. But I would like to understand whats really going on!
      41" & 29" FE Aeromarine Sprint Cats, Quickdraw powered "Dollar Eater" 41" Insane Cat, 29" BL mod Graupner Cat, 24" Hydro, 29" OB Cat, BL mod NQD Tear Into Jet boat, 55" Scarab, JET SWEEP R/C pool skimmer Rescue Boat.

      Comment

      • ls1fst98
        Senior Member
        • Oct 2009
        • 859

        #4
        why not just look for 4s 3000mah and run them in parallel for 6000mah instead of having 3 batteries in theboat. that many connection points im sure is bottlenecking something.
        HPR 135 redemption, HPR C5009, modded zelos 36, 32 boats and counting.
        Flier ESC dealer, pm me for details.
        https://www.facebook.com/groups/EliteRCBoats/

        Comment

        • srislash
          Not there yet
          • Mar 2011
          • 7673

          #5
          You need the mah to back up the voltage and keep it steady or it will spike all over the map and that is why your caps are getting hot. Simply put that is.
          A capacitors job is to steady the voltage to the ESC.
          I usually run 4s2p but 4000-5000mah packs which means 8000-10,000 mah on tap.
          I started with small packs too myself as I was used to our old Nicd/brush days.

          Comment

          • ReddyWatts
            Fast Electric Addict!
            • Apr 2007
            • 1711

            #6
            Does your charger read the internal resistance/impedance of your batteries?
            ReddyWatts fleet photo
            M1 Supercat - Neu 1527 1Y, 8s / Mean Machine- Feigao 580, 8s, 120 HV esc
            Mean Machine - Feigao 540 14XL, 8s, 100 amp HV esc, X537/3

            Comment

            • BHChieftain
              Fast Electric Addict
              • Nov 2009
              • 1969

              #7
              Originally posted by martin
              You need to keep the capacity of the batteries all the same ie 5000mah, same as same c rating all the same & preferably same make batteries.
              Hi,
              Just to clarify, for parallel config, the battery capacities can be different but the cell count needs to be the same-- only the Series side has to be equal capacity. Meaning, you could take 2 2S1P 5000mah packs, and 2 2S1P 2500mah packs and make a 4S2P 7500mah connection by placing 1 2S1P 5000 and 1 2S1P 2500 in parallel, and the other 2S1P 5000 and 2S1P 2500 in parallel, then putting both of these in series. You'd get a 4S2P 7500 pack.

              But you are right that you can't do what the OP asked-- take a 2 x 2S1P 5000 setup and add to it a single 4S1P 2700mah setup as the Series side of the connection would not be equal. If his 4S pack were made from 2 x 2S1P packs, then he could rig it up right.

              To the Orig poster, how big is the boat? I have a 31 inch mono that runs 4S2P 10,000 mah (made from four 2S1P 5000 packs) that carries the weight just fine.

              Chief

              EDIT: I need to retract these statements on using mixed capacities in //-- from what I've seen the data supports // CHARGING with different mah capacities (and I personally do this between 5000 and 5500 packs, same brand, same C rating, same condition, 1C charging rates), but as T.S. Davis points out the battery behavior is different under load (voltage changes as packs discharge). I would need to see or run some experiments for me to be confident...
              Last edited by BHChieftain; 02-07-2014, 11:04 AM.

              Comment

              • 78MaicoRider
                Senior Member
                • Jun 2013
                • 179

                #8
                If my charger reads the impedance of the batts, it doesn't display that reading.

                "Meaning, you could take 2 2S1P 5000mah packs, and 2 2S1P 2500mah packs and make a 4S2P 7500mah connection by placing 1 2S1P 5000 and 1 2S1P 2500 in parallel, and the other 2S1P 5000 and 2S1P 2500 in parallel, then putting both of these in series. You'd get a 4S2P 7500 pack."

                Iv'e thought this over and over. On one hand I see what you are saying. However, Mr. Ohm (the man that wrote the law) says it shouldn't matter if they are paralleled at 2S or 4S. They are same C rating (internal impedance equivalent).

                The boat is what I believe is commonly called a Apparition Cat. Not a lot of room inside this boat. With the three packs in it, the waterline is on the deck at the transom! CG is a little forward causing it to run a little wet. Prop angle is 0. Going 4 2S 5000 mah packs may put the transom under.

                I just picked up a new Aeromarine .21 Sprint Cat. It is a 12 year old brand new nitro layup. Although it's shorter in length, there is so much more room inside! I'm thinking a 4082/92 1700-1800kv motor on 6S will be the ticket for it. 10000 mah at 6S will not be a problem for it.
                41" & 29" FE Aeromarine Sprint Cats, Quickdraw powered "Dollar Eater" 41" Insane Cat, 29" BL mod Graupner Cat, 24" Hydro, 29" OB Cat, BL mod NQD Tear Into Jet boat, 55" Scarab, JET SWEEP R/C pool skimmer Rescue Boat.

                Comment

                • ReddyWatts
                  Fast Electric Addict!
                  • Apr 2007
                  • 1711

                  #9
                  Interesting questions.

                  Will the 4S pack be depleted in 1/2 the time of the 2 2S packs? Yes a 2700 mah pack will not last as long as the 5000 mah packs. The 2700 mah battery used 900 / 2700 = 33% and both 5000mah used 900 / 5000 = 18%. The smaller 2700 battery is the limiting factor for your run time.

                  Or will they draw down equally percentage wise, keeping the voltage the same on all cells? No you are using different mah batteries.

                  Formula: Battery Mah used (times) 0.06 (divided by) run time = average motor load in amps

                  Example: You stated all batteries used 900 mah, so (900 mah X .06 / 2 minutes run time) = 27 amps for a total of 54 amps. The 2700 mah 4S pack was supplying 27 amps. The 5000 mah 2S packs were supplying 27 amps each since they are in series.
                  ReddyWatts fleet photo
                  M1 Supercat - Neu 1527 1Y, 8s / Mean Machine- Feigao 580, 8s, 120 HV esc
                  Mean Machine - Feigao 540 14XL, 8s, 100 amp HV esc, X537/3

                  Comment

                  • BHChieftain
                    Fast Electric Addict
                    • Nov 2009
                    • 1969

                    #10
                    Originally posted by 78MaicoRider

                    "Meaning, you could take 2 2S1P 5000mah packs, and 2 2S1P 2500mah packs and make a 4S2P 7500mah connection by placing 1 2S1P 5000 and 1 2S1P 2500 in parallel, and the other 2S1P 5000 and 2S1P 2500 in parallel, then putting both of these in series. You'd get a 4S2P 7500 pack."

                    Iv'e thought this over and over. On one hand I see what you are saying. However, Mr. Ohm (the man that wrote the law) says it shouldn't matter if they are paralleled at 2S or 4S. They are same C rating (internal impedance equivalent).
                    My comment wasn't about one side being 2x 2S packs and the other side being a 4S pack. That can be ok. My comment was that one side of the Serial connection has to be the same total capacity of the other side of the Serial connection. You cannot put a 5000mah pack on one side (either 2 x 2S1P 5000 or 4S1P 5000) and 4S 2500 (however you make it- either 2X2S1p 2700 or 4S1P 2700) on the other side of the serial connection. If you do that, the smaller mah capacity side will be overdrained and will kill the cells and could start a fire.

                    But you can add different capacity packs in // as long as the serial sides are equivalent:

                    -2 x 2S1P 5000mah + 4S1P 2500 in // + the same config in serial on the other side = 8S2P 7500mah. Safe to run
                    or
                    - 4S1P 5000mah + 4S1P 2500 in // + same config in serial on the other side= 8S2P 7500mah. Safe to run

                    -4S1P 5000mah on one side + 4S1P 5000 in // on the other side = 4S2P 10,000 mah. Safe to run

                    4S1P 5000mah on one side + 4S1P 5000 in serial = 8S1P 5000mah. Safe to run


                    BUT

                    -4S1P 5000mah on one side + 4S1P 2500 in serial on the other side = Fire hazard!

                    Does that make sense?


                    Chief

                    EDIT: I need to retract these statements on using mixed capacities in //-- from what I've seen the data supports // CHARGING with different mah capacities (and I personally do this between 5000 and 5500 packs, same brand, same C rating, same condition, 1C charging rates), but as T.S. Davis points out the battery behavior is different under load (voltage changes as packs discharge). I would need to see or run some experiments for me to be confident...
                    Last edited by BHChieftain; 02-07-2014, 11:06 AM.

                    Comment

                    • 78MaicoRider
                      Senior Member
                      • Jun 2013
                      • 179

                      #11
                      First I want to thank you all for spending so much of your time and effort discussing this!!!

                      Chief, If I may call you by that? In your examples above, you are losing me when you state " 4S1P 5000mah + 4S1P 2500 in // + same config in serial on the other side= 4S2P 7500mah. Safe to run" "IN SERIAL" this would yield 8S2P 7500 mah total. If in parallel It would be 4S 15000 mah. As the power is used the voltage drops. The decline in voltage will be slower on the higher capacity cells. And they will be supplying most of the power to the system. The system cannot pull power from the lower capacity cells if the voltage of those cells is lower than the higher capacity cells. This is what keeps me thinking it should be ok if connected in parallel. Can't pull the 2500 mah cells down to 3.4 volts if the 5000 mah cells are at 3.8 volts.
                      41" & 29" FE Aeromarine Sprint Cats, Quickdraw powered "Dollar Eater" 41" Insane Cat, 29" BL mod Graupner Cat, 24" Hydro, 29" OB Cat, BL mod NQD Tear Into Jet boat, 55" Scarab, JET SWEEP R/C pool skimmer Rescue Boat.

                      Comment

                      • BHChieftain
                        Fast Electric Addict
                        • Nov 2009
                        • 1969

                        #12
                        Originally posted by 78MaicoRider
                        First I want to thank you all for spending so much of your time and effort discussing this!!!

                        Chief, If I may call you by that? In your examples above, you are losing me when you state " 4S1P 5000mah + 4S1P 2500 in // + same config in serial on the other side= 4S2P 7500mah. Safe to run" "IN SERIAL" this would yield 8S2P 7500 mah total. If in parallel It would be 4S 15000 mah. As the power is used the voltage drops. The decline in voltage will be slower on the higher capacity cells. And they will be supplying most of the power to the system. The system cannot pull power from the lower capacity cells if the voltage of those cells is lower than the higher capacity cells. This is what keeps me thinking it should be ok if connected in parallel. Can't pull the 2500 mah cells down to 3.4 volts if the 5000 mah cells are at 3.8 volts.
                        Whoops, my bad on the cell counts-- I was typing in a hurry-- you are right with the configs I listed they would be 8S due to the serial connection. I was trying to demonstrate that the // side of the connections would be ok if different capacities. It is just that if you want to add a serial connection, the total capacity on both "sides" need to be the same. (I edited my earlier post to correct it).

                        To simplify, if you only had 2 packs, a 21PS 5000mah pack and a 2S1P 2500 pack then you can put them in // to make a 2S2P 7500mah pack no problem, but you should not put them in serial to make a 4S pack, as the smaller pack will be killed in that type of connection. Does that sound right to you?

                        Chief

                        EDIT: I need to retract these statements on using mixed capacities in //-- from what I've seen the data supports // CHARGING with different mah capacities (and I personally do this between 5000 and 5500 packs, same brand, same C rating, same condition, 1C charging rates), but as T.S. Davis points out the battery behavior is different under load (voltage changes as packs discharge). I would need to see or run some experiments for me to be confident...
                        Last edited by BHChieftain; 02-07-2014, 11:04 AM.

                        Comment

                        • T.S.Davis
                          Fast Electric Addict!
                          • Oct 2009
                          • 6221

                          #13
                          Never though about this.

                          Series for sure it's a problem. The two packs would deliver different voltage under load.

                          Have to think about this a minute.

                          Under load again, wouldn't the voltage coming from each cell be lower from the 4s2700's than the voltage delivered by the 2s5000' in series? I suppose it would depend on how hard you were pushing the cells.

                          I think Hill answered this question on the red board 10 years ago.
                          Noisy person

                          Comment

                          • ReddyWatts
                            Fast Electric Addict!
                            • Apr 2007
                            • 1711

                            #14
                            If the mah used was the same for both sides of the circuit, doesn't that mean the voltage was the same? Luckily!
                            ReddyWatts fleet photo
                            M1 Supercat - Neu 1527 1Y, 8s / Mean Machine- Feigao 580, 8s, 120 HV esc
                            Mean Machine - Feigao 540 14XL, 8s, 100 amp HV esc, X537/3

                            Comment

                            • T.S.Davis
                              Fast Electric Addict!
                              • Oct 2009
                              • 6221

                              #15
                              What I'm saying is that:

                              The 2700's at 30c are supposed to deliver 3.7 volts at up to 81amps continuous. Over that amperage the voltage drops off. Less than those 81 amps and the voltage is higher. Somewhere between the 3.7 rated and the 4.2 static.

                              The 5000's at 30c are supposed to deliver 3.7 volts at 150amps. Then if you are drawing less than 150 amps the voltage will again be somewhere between the 3.7 rated and the 4.2 static.

                              So if you are drawing 80 amps on your setup your 2700's are delivering 3.7 volts per cell and your 5000's are delivering maybe 4 volts per cell.

                              This is like replacing 2 of 4 double A's in your radio. You have two good cells trying to support the 2 weeks cells. The good cells die trying to carry the dead weight and the week cells are over voltage. Sounds like a ripple current nite mere.

                              I don't know if there will be damage or what the prognosis long term is but it's a bad idea IMO. It's not like fuel where you put high octane and low octane in a tank and swirl it around resulting in 7500mah octane.

                              Surely there's someone smarter than me with an opinion on this.
                              Noisy person

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