Electric Stunt > Gettin all AMP'ed up!
5s vs 6s Battery on a Twin
Ken Culbertson:
I have a question about battery size on a twin. I am building a full size (700sq) twin. It is time to order the esc's and motors. I have fixed on the BadAss 2320-820 motors. I am knee deep in Jeti stuff so I want to use their ESC's. If I use a 5s battery I can use the Spin 33 ESC's but I will not be able to use a 6s battery which the motor supports. I see very little difference in the 5s vs 6s in my other planes other than a better boost with the 6s which can be adjusted to be about the same using a 5s by the active timer. Obviously I could use the Spin 44 but that is $40 bucks and 2 1/2 ounces heavier.
I have been told that 2 23xx motors will actually use less mAh than a single 35xx. If that is true than the 5s 2700 with a few more RPM's and a stronger boost may be the right answer. Ironically my 5s batteries are 4grams lighter than my 6s so there would be no weight savings.
ken
Mark wood:
Higher voltage means lower current for a given power requirement. Lower current means less power loss to heat. The power required at the propeller is what is left over after the power lost in transmission is removed. Another way of saying this is that he power the battery must supply in what the propeller requires plus the power lost in transmission. In an ideal world where we could make batteries exactly to fit the application, which we can't, the higher voltage battery suitable for the mission profile will always be lighter.. The trouble we find is that all manufacturers of battery pack play games with C rating and MAH derating certain pack for greater C rating visa versa. What we truly need to know and understand is the total energy capacity of the pack or cell. in general the 18650 cells have a greater energy density than the poly cells. Trouble with them is they are often bigger than necessary for a specific mission. For the mathematically inclined:
Power supplied by the battery = power required by the propeller + the power lost to heat
Power supplied by the battery = (Torque x RPM/ 5252) + (Current x Total resistance (Wires, motor, back EMF...) )
Voltage x MAH = ((Torque x RPM/ 5252) + (Current x Total resistance)) x Time
Note, the MAH on the pack doesn't mean a whole lot. Some manufacturers put the Watt Hour rating on the pack. That's the value we need to compare. Battery A Whr = Battery B Whr. You can get that roughly by the product of the pack voltage time the MAH rating. However it's the chemistry that makes the big difference. The 18650 cells win here as well as they also have the best energy per volume.
Here's how this impacts our model application. The parasitic power, the energy lost to heat that does us no good, is a function of the square of the current. Simply put is is very hurtful to our cause because of the square term. Write the first equation:
Pbattery = Pprop + Plost
Power in an electrical system:
Power = Current x Voltage, where Voltage = Current x Resistance
Then:
Plost = I x V = I x (I x Rttl ) = I^2 x Rttl
Now to understand how this impacts us and drives us to always want to use the highest voltage for a specific size and weight we solve for the current:
I = Square root ( Plost / V )
This is kind of a circular definition but you can see that the current is vary much driven inversely to the voltage. The higher the voltage to lower the current by a very significant amount. Keep in mind this current thing is what drives the heat up and down. Batteries get hotter, wires get hotter, MOTORS get hotter causing issues with plastic mounts....
So, short version. Always use the highest voltage you can. To compare two packs, multiply their voltage by their MAH. IF you know what one configuration is that works, use that V x MAH and then use the comparing packs V x MAH. If the latter is very mush greater than the former, look to see if there is smaller cell pack.
My $0.02 worth what paid for.
Frank Imbriaco:
Being as you are using Badass motors, I mostly follow Lucien's charts for battery and prop size and go from there. Simple.
Some will make a point that his charts are intended for R/C.
Mark wood:
The Badass 2320 is capable of running with 6s batteries. The trouble is finding a 20 or 30 Amp ESC to run them that is capable of using a 6s battery. For a given flight a higher voltage battery will consume less power over the flight due to the lower I^2R power loss. The trouble is finding proportionally smaller cells. It's difficult combination to sort out effectively. In terms of responsiveness the extra overhead is good too.
I really like that BA 2320 specs and am hunting similarly for the package. I'm about to just give in and go for the bigger ESCs. The BA 2320 "fits" within the original mount rails for a 35 size ICE requiring only minor amount of grinding. The Cobra 2221 does a good job of making the Ringmaster / Flight Streaks fly. We see some mount issues when the Cobra is loaded up and gets hot. The BA 2320 on a 5s or 6s should make more output power and run cooler. That configuration is a few shop hours away from going to trial. The test bed will be a Banshee rescued from some garage rafters after hanging for 25 years.
My mission there is bolt on conversion of these old planes hanging around. I'm very close. There has been three iterations since the first Flight Streak I posted last summer.
If'n it were me doing what you're doing Ken, I'd go 6s and use whatever ESC fit. But then that's just me.
My $0.02, worth the price paid.
Ken Culbertson:
Just for the record, I had a set of 6s Li-Ion's made up in my favorite configuration for the BS2320's and Spin 33 esc's From what I am being told this combo with 2 10-5 BadAss wood 3 blade props will out power a single BA 3515 and do it using less battery. I am, as always skeptical but hopeful!
Ken
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