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Old 03-28-2011, 01:30 PM   #1
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Question 3S or 4S Battery Question(s)

I am building this Great Planes Super Sportster EP and I purchased the recommended RimFire .25 42-40-1000 Brushless Outrunner Electric Motor and the Silver Series ESC 45A Brushless with 5V/2A BEC.

Both the ESC and Motor say anything from a 2S to 4S works great, and the plane recommends a 3S 3500mAh battery.

Haven't purchased the battery yet and I have plenty of 2200mAh batteries but I was wondering if I am correct in assuming if I use a 4S I would basically increase the RPMs by 3400 RPMs since I am increasing voltage by 3.4 volts and the motor has a 1000kV rating??

Also I know Volts X Amps = Watts and this motor is rated for 666watts according the manual but at 11.1 volts I am going to be right around 500 watts and they are getting their 666Watts using a 4S battery, obviously.

So any reason I should not just use a 4S battery vs. a 3S and what this the right formula for figuring the mAh rating if I do switch?

Also a 4S 2200 mAh battery seems to be a little cheaper (not enough to make a difference to me though) than a 3S 3500 mAh battery.

Or am I out in left field and done jumped the fence thinking this way???
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Old 03-28-2011, 02:44 PM   #2
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You seem to have a very good grip on things there and your math looks correct.

Just remember that that 166 extra watts with the 4S will also drain your battery quicker. Going from a 3500mAh pack to a 2200mAh pack AND increasing the power drain will reduce your flight time.

Be SURE to keep the C rating in mind also. A 20C 3500 pack is rated at 70 amps. A 20C 2200 pack is rated at 44 amps. You will be around the limit with a 20C pack IF the C-rating is correct....and I always leave a pretty large fudge factor in there.

If you go with shorter flight times and a 2200 pack, be sure to purchase a high C rating battery.

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Old 03-28-2011, 02:57 PM   #3
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Originally Posted by RCFlyer44 View Post
I am building this Great Planes Super Sportster EP and I purchased the recommended RimFire .25 42-40-1000 Brushless Outrunner Electric Motor and the Silver Series ESC 45A Brushless with 5V/2A BEC.
Originally Posted by RCFlyer44 View Post

Both the ESC and Motor say anything from a 2S to 4S works great, and the plane recommends a 3S 3500mAh battery.

Haven't purchased the battery yet and I have plenty of 2200mAh batteries but I was wondering if I am correct in assuming if I use a 4S I would basically increase the RPMs by 3400 RPMs since I am increasing voltage by 3.4 volts and the motor has a 1000kV rating??

Also I know Volts X Amps = Watts and this motor is rated for 666watts according the manual but at 11.1 volts I am going to be right around 500 watts and they are getting their 666Watts using a 4S battery, obviously.

So any reason I should not just use a 4S battery vs. a 3S and what this the right formula for figuring the mAh rating if I do switch?

Also a 4S 2200 mAh battery seems to be a little cheaper (not enough to make a difference to me though) than a 3S 3500 mAh battery.

Or am I out in left field and done jumped the fence thinking this way???


You haven't jumped the fence, but there are a number of complicating issues to be aware of.
The KV rating indicates an unloaded ( no prop) RPM that the motor will spin at. The actual RPM you get will depend on the prop you use.
The ESC will work with a 4S battery, but for any given set up ( load and throttle setting) the 4s will result in a higher average current delivery than the 3 S.
The 11.1 v assumption you are making on the the 3S battery is the low end of the charge/ discharge voltage curve, I would use a fully charged battery voltage to think about current draw. If the top end the motor will deal with is 660 watts, at 12.5 v that will be a current draw of 52 to 53 amps, which exceeds the capacity of your ESC ( 45 amp continuous, 50 burst).
The ESC seems to be the limiting factor in your power system, so to use the 4S battery you may need to consider reducing your prop size and or pitch to maintain the same overall current draw on the system.

Because the ESC deliver current in a time gated fashion ( full current on then off with the interval determined by your thottle setting) you cannot depend on the just assuming you can fly at less than full throttle to keep your current delivery in a safe range for your ESC.

Another vital factor to be aware of with the Silver series ESC is that they use a % of initial voltage to determine the LVC, as opposed to the fixed voltage that most other manufactures use. As stated in their instructions, never use a partially discharged battery on thier ESC as it may set the LVC at a point that will result in battery damage if flown to the LVC.
I typically do not fly to the LVC, but especially with this unit I would be very careful in that regard.
This has been sort a rambling post on my part, but like I said there are a lot of issues that complicate going to a 4s from 3 s. As many others have noted if you are going to try this you absolutely should have a wattmeter to get direct measurments and see what effect the changes you are making result in. It is a good practice to do that in any case even if using the stock setups to verify that what you are seeing is in line with what the manufacture says it should be.
Good luck with it, I am sure that there are parts of my reply which are clear to me but clear as mud to you so hopefully others will chime in here as well.
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Old 03-28-2011, 03:14 PM   #4
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Originally Posted by cliffh View Post
You seem to have a very good grip on things there and your math looks correct.

Just remember that that 166 extra watts with the 4S will also drain your battery quicker. Going from a 3500mAh pack to a 2200mAh pack AND increasing the power drain will reduce your flight time.

Be SURE to keep the C rating in mind also. A 20C 3500 pack is rated at 70 amps. A 20C 2200 pack is rated at 44 amps. You will be around the limit with a 20C pack IF the C-rating is correct....and I always leave a pretty large fudge factor in there.

If you go with shorter flight times and a 2200 pack, be sure to purchase a high C rating battery.

Cliff
Thanks Cliff, the 4S I was looking at was a 35C, but that would be too much for my ESC now that you mention it since it is rated for 45 amp continuous run with a 50 amp surge.

So wouldn't a 20C 3000 -3500 mAh battery be too much for the ESC since that could potentially be pulling 60 to 70 amps? Do I need to get an ESC rated for 60 amps or more? Seems to me that could cause the ESC to burn up but then I am not sure that is the case.

Sorry for all the questions guys but being new to electric and pretty much just so sticking with recommended batteries and pre-installed ESCs I want learn as much as I can about this and start experimenting with my own set ups but don't want to burn up equipment and be paying through the nose.
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Old 03-28-2011, 03:21 PM   #5
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Originally Posted by dumo01 View Post
Good luck with it, I am sure that there are parts of my reply which are clear to me but clear as mud to you so hopefully others will chime in here as well.
Thanks Dumo I was responding to Cliff when your response came in. I do understand what you are saying and I think you answered part of my question in my second post, I basically need to up the ESC if I want to use the 4S battery.

It brings up another question though why would they recommend a 20C 3200-3500 mAh batter if they know it exceeds the limits of the ESC that they also recommend. I assume voltage is not really relevant to the amp draw here and both 3S and 4S in the same set up would generate 64 to 70amps, correct?
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Old 03-28-2011, 03:47 PM   #6
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Well, things get a little complicated when changing voltages and thinking current. I'll try to make it a little simpler.

First, yes with a 1000kv motor, increasing the voltage increases motor speed, but the actual amount will vary. The motor will WANT to increase speed by 1000 rpm (per volt), but will lose some to heat and what have you. I wouldn't get too bogged down on that part right now.

Watts = volts X amps

A higher VOLTAGE setup is often used to REDUCE the AMPS required. If you look at the formula above, if you increase the voltage, you reduce the amps.

The esc amp rating has NOTHING to do with the amount of power your setup will draw. It is a MAXIMUM rating for the esc. Think of an electrical fuse. You can flow anything from zero up to the fuse limit with no problems. Exceed the maximum rating and bad things happen. Smaller esc's are usually used to save weight, room, and money.

Similar with the C rating of the battery pack. C rating x the battery capacity (in amp hours) = the number of amps the battery CAN provide at its maximum level. You can certainly use a higher C rating battery, but not a LOWER C rating.

A lot to learn for electrics!

Hope this helps you.

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Old 03-28-2011, 03:51 PM   #7
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Oh, one more thing, since the motor will want to spin at a faster rpm, there is often a small diameter or lower pitch prop used with a 4S setup. This would reduce the load somewhat keeping the current draw within limits of the supplied esc.

The only way to know for sure if you have the correct prop and setup is to use a wattmeter. That way you will know your amps, volts, and watts.

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Old 03-28-2011, 04:04 PM   #8
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One more "one more thing"

The esc operates as a "high speed switch" if you will. It requlates the motor speed by turning on and off very quickly (at very precise times)....several thousand times a second. The longer the "on" times are, the faster you motor spins and the more power it consumes......BUT, it is always either ON or OFF.

What that means really to you is that if your motor draws a MAXIMUM of say 50 amps, then EVERY time it is switched on it draws 50 amps. Reducing your throttle will reduce the AVERAGE power used and slow things down, but the esc will see FULL power during every "on" period.

Your esc must be rated higher than your MAXIMUM amp draw for your setup.

You can make adjustments with your propeller size and pitch to adjust maximum amp draw.

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Old 03-28-2011, 04:29 PM   #9
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what Cliff said
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Old 03-28-2011, 05:22 PM   #10
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OK thanks guys, so if the ESC is the limiting factor here does that mean increasing my voltage and/or the C rating will NOT burn up the ESC but that the ESC will limit what current is pulled??
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Old 03-28-2011, 06:44 PM   #11
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Originally Posted by RCFlyer44 View Post
OK thanks guys, so if the ESC is the limiting factor here does that mean increasing my voltage and/or the C rating will NOT burn up the ESC but that the ESC will limit what current is pulled??
Well I see that I have you confused. This stuff takes a while to understand.

Let me break your question down into a few parts.

"so if the ESC is the limiting factor" ---Well, the ESC may or may not be a limiting factor here. The ESC has a limit, in your case 45 amps. Voltage does not matter as long as you do not exceed the 4 cell limit. 500 watts at 11 volts would be 45 amps roughly. (11v x 45a = (almost)500 watts.

BUT, if you use a 4 cell (and maybe prop down a little bit) 500 watts at 14 volts would be 36 amps roughly. (14v x 36a = 500 watts (roughly)

The C-rating ONLY determines how much power the battery CAN supply, not WILL supply. It is a limit to NOT EXCEED only. Too low a C-rating can damage the battery, too high a C-rating...well, higher C rating batteries cost more and are a bit heavier.

Your ESC will NOT limit the current pulled. Your ESC is simply a very precisely timed switch that turns off/on the full power the system uses at full power. In other words, if "full throttle" is 43.35 amps...it switches that full 43.35 amps in pulses, closer together for high power, farther apart for lower power. Your ESC needs to be sized correctly for the FULL power requirements of your system. In your case, as long as full throttle is less than 45 amps (and you have reasonable cooling) you will be fine.

Your battery needs to be able to supply at least (preferably a bit more for a safety margin) the maximum amperage required at full throttle. In your case, at LEAST 45 amps continuous.

Your motor must also be rated for that 45 amps!

Your propeller will do the MOST to determine the power the system will attempt to draw. Nothing will determine the power (or amps drawn) than the propeller.

And last but not least, without a Wattmeter, you cannot tell how many amps your system is using. It is an important investment, along with a small selection of propellers.

In the mean time, maybe someone with a tested setup like you want to use can tell you what they are using.

Hope I didn't confuse you more

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Old 03-28-2011, 07:07 PM   #12
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Originally Posted by RCFlyer44 View Post
OK thanks guys, so if the ESC is the limiting factor here does that mean increasing my voltage and/or the C rating will NOT burn up the ESC but that the ESC will limit what current is pulled??
The current that is pulled will be determined by load you put on the motor.
The C rating indicates what the battery is capable of supplying without damaging the battery, it does not indicate that the battery will try to force more current though the system.
If you keep the power delivered by your system unchanged regardless of the input voltage, then the ESC will be OK using the P=IV principle. The motor speed will increase with the higher input voltage, so the only way to keep your power the same is to either prop down, in pitch or size, or limit the throttle you use. As noted before, the throttle down idea is limited by the way the ESC delivers energy.
Without knowing how much current the recommended 3s setup draws vs the 45 amp limit your ESC will support, it is tough to say if the 4s setup will over stress the ESC. The only way to know for sure is put a wattmeter on it, run it up gently and see what the numbers tell you as you run it up. I think the prevailing thought is to have at least a 10% buffer on ESC capacity vs actual current draw, so if you are at half throttle and drawing 40 a, then back off and get a bigger ESC ( overkill on the numbers but you get the idea). The alternative to a bigger ESC is to use a smaller prop, which, by turning at the 4S rate, whatever that is, may generate the same thrust as the bigger prop you used at 3S, and the plane may fly fine, but with a lower current draw so your ESC is OK.

Like Cliff said, you can kind of think of the ESC as a fuse, but not a fuse you can reset. When it pops it is dead, and if you are in the air, maybe the plane as well

Looks like I took long enough that Cliff beat me to it, but I think we are both saying the same thing.
They really are great toys, it just takes awhile to wrap your head around all the "buts" and "ifs"
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Old 03-28-2011, 07:17 PM   #13
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Originally Posted by dumo01 View Post
snip)
Looks like I took long enough that Cliff beat me to it, but I think we are both saying the same thing.
They really are great toys, it just takes awhile to wrap your head around all the "buts" and "ifs"
Yep, we are in agreement

This electric stuff is a lot to learn. Don't feel bad about not getting it completely for a while. In the end though, you will probably find it easier than I find setting the idle mixture on a glow engine

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Old 03-28-2011, 07:24 PM   #14
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OK, I think I have a better handle on it now, thanks guys! I will definitely have to pick up a watt meter and if I don't do that before I am finished building it I will stick to the recommended battery and then later test to see if it flies any better or worse.
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Old 03-28-2011, 07:34 PM   #15
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Hi RCFlyer with a 4 cell lipo, always use a UBEC, and with a 2 amp or less bec on the ESC, use a UBEC, and if your using a 2.4 radio system, regardless of cell count, use a UBEC, its Cheap insurance, and will help to prevent a brown out using 2.4, Hope that helps, Chellie

http://www.wattflyer.com/forums/show...t=ubec+chellie

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Old 03-28-2011, 10:38 PM   #16
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Originally Posted by CHELLIE View Post
Hi RCFlyer with a 4 cell lipo, always use a UBEC, and with a 2 amp or less bec on the ESC, use a UBEC, and if your using a 2.4 radio system, regardless of cell count, use a UBEC, its Cheap insurance, and will help to prevent a brown out using 2.4, Hope that helps, Chellie

http://www.wattflyer.com/forums/show...t=ubec+chellie
Thanks Chellie, so even if the ESC goes out the built in BEC will not work? And if I understand your thread on this I must disable the built in BEC before installing the BEC, correct? Wow this is getting more complicated...lol but I love learning about it and it is still 10 times better than carrying glow fuel around, cleaning the mess off the plane after every flight and finding proper storage for all the mess.
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Old 03-29-2011, 01:14 AM   #17
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Originally Posted by RCFlyer44 View Post
Thanks Chellie, so even if the ESC goes out the built in BEC will not work? And if I understand your thread on this I must disable the built in BEC before installing the BEC, correct? Wow this is getting more complicated...lol but I love learning about it and it is still 10 times better than carrying glow fuel around, cleaning the mess off the plane after every flight and finding proper storage for all the mess.
Its very easy Even a Cave Man can do it all you do is remove the red wire from the ESC control plug that goes to the Receiver, use a needle, lift up the locking tab, and pull the wire out, then tape it up so it does not short out anything, that eliminates the built in ESC-BEC, the BEC in most ESC is very very weak, even though it says its a 2 amp, (most better ESC are 3 amp), its not, the BEC in less expensive ESC is a Linear (Resistor Type) voltage regulator, and do not work very well, and get weaker with heat and higher cell counts, A UBEC, Universal Batteryis Eliminator Circuit, a Switching voltage regulator, its uses electronics to reduce the voltage and is stronger, also it not affected by heat from the ESC, because its seperate from it, Even if your ESC goes bad while flying, you still have electricity to everything, because the UBEC is run off the battery.

http://www.headsuprc.com/servlet/the...iversal/Detail

If you have a 4 channel receiver, you will need to use a Y Harness, plug the UBEC and the Plug from the ESC into the y harness, then into the Throttle Port on the Receiver. there your done
If you have a 6 channel Receiver, you can normally plug it into any unused receiver port.


You cant use the ESC BEC and a seperate UBEC at the same time, they will fight each other. the ESC-BEC has to be eliminated by disconnecting the red wire, then the UBEC Takes over. Hope that helps to clear things up a little, Take care, Chellie

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Old 03-29-2011, 01:22 AM   #18
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The Battery Wires on the UBEC are soldered onto the Dean connector on the ESC, the bottom pics shows how to remove the BEC Wire from the receiver plug


http://www.fatlion.com/sailplanes/servos.html


Servos and How They Are Wired

Some of the in's and out's of Servo Wiring...
About 1991 or soon after, most of the major brands of analog servos became compatible with each other. When this happened, you can use any of these brands of servos with any brand of receiver, but you must be careful about the polarity of the wiring. Some brands of servos are really great for a particular use in a particular plane, and there are other companies that sell their servos with each different manufacturer's connector installed (you just have to ask for what you want).
You can mix Futaba servos with an Airtronics receiver, mix Hitec & JR servos with a Futaba receiver, etc. as long as you are careful about polarity. In Futaba, Hitec, and JR Radio servos, the servo and battery connections have the same polarity (+/-) and signal wiring, although the connectors are slightly different.
One thing to remember: You can't reverse the direction of a servo (reversed servo) by just swapping (+) and (-) wires. If you do, you'll burn out either the servo or the receiver or both. If you're really good at soldering very small wires, you can reverse the normal direction of servo by swapping the wires that connect directly to the servo motor inside the servo case as well as the little servo wiper that moves as the servo moves. However, it's a lot easier to buy any of the newer radios; even the cheaper, standard radios these days have servo reversing as a built-in feature of the transmitter.
Because Airtronics received so much bad press about the fact that their servos had reversed polarity (+/-), in December 1997, Airtronics started shipping servos with the optional "Z" connector that mostly matches the connector found on JR Radio or Hitec servos (or other brands). This means that if you have a Airtronics servo with one of the "Z" connectors, you won't have to switch (+) and (-) wires to use that servo with Futaba, Hitec, or JR, or other receivers.
If you want to switch the wires around, almost all the connectors have little tabs or pins that you can push with a small pin to remove the wires and push them back into the connector in the right place.
For Airtronics servos with the older, non-"Z" connector, follow the diagram:


For Futaba, JR, & Hitec servos (or servos WITH the Airtronics "Z" connector), use this diagram:

If you are handy with a razor knife, you can get all the brands of connectors to fit each other. I usually cut off the little flat tab on the Futaba connector to get it to fit in a Hitec or Airtronics receiver. You can also cut off the three little 'teeth' on the Airtronics connector to get it to fit in another brand's receiver.
Some of the Airtronics servos have white colored wires for the 'Signal' wire, so it's easier to determine which black wire is (+). I can't emphasize this enough, just be sure that if you connect Airtronics with the older (non-"Z" connector) to any other brand, that you reverse the (+) and (-) wires as you will burn out either the servo, receiver, or both.
I usually wrap all my servos that I've converted to Airtronics polarities with a little 'band' of bright blue tape to remind myself that the servo is setup as an Airtronics servo.
Lots of people are now asking, "What is the difference between analog and digital servos?"
The motor of an analog servo receives a signal from the servo amplifier (inside the servo) at 30 times a second. This pulsing signal tells the servo motor when to start rotating and which way to rotate. Since it only happens 30 times a second, that's the minimum reaction time. Digital servos use a higher frequency amplifier that sends a signal to the servo motor 300 times a second (or sometimes more on very fast servos, such as those used for helicopter tail rotors). Since this signal is received by a digital servo's motor more often, it is able to react much faster and hold its position better. This means the servo has better centering and considerably higher holding power. This power comes at a cost, however, as digital servos tend to draw a lot more power from the on-board battery which means your battery just won't last as long.
There are also different types of servo motors available: cored, coreless, and now brushless. You can read definitions of brushless, coreless, and cored servo motors in the glossary.
Below are the four standard connectors that are used by the 'big' four radio manufacturers (other manufacturers or third-party servo makers, such as Cirrus, use the Hitec standard):




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Old 03-29-2011, 01:34 AM   #19
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Originally Posted by CHELLIE View Post
... Hope that helps to clear things up a little, Take care, Chellie
Sure does, thank you.
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Old 03-29-2011, 01:35 AM   #20
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Originally Posted by RCFlyer44 View Post
I am building this Great Planes Super Sportster EP and I purchased the recommended RimFire .25 42-40-1000 Brushless Outrunner Electric Motor and the Silver Series ESC 45A Brushless with 5V/2A BEC.

Both the ESC and Motor say anything from a 2S to 4S works great, and the plane recommends a 3S 3500mAh battery.

Haven't purchased the battery yet and I have plenty of 2200mAh batteries but I was wondering if I am correct in assuming if I use a 4S I would basically increase the RPMs by 3400 RPMs since I am increasing voltage by 3.4 volts and the motor has a 1000kV rating??

Also I know Volts X Amps = Watts and this motor is rated for 666watts according the manual but at 11.1 volts I am going to be right around 500 watts and they are getting their 666Watts using a 4S battery, obviously.

So any reason I should not just use a 4S battery vs. a 3S and what this the right formula for figuring the mAh rating if I do switch?

Also a 4S 2200 mAh battery seems to be a little cheaper (not enough to make a difference to me though) than a 3S 3500 mAh battery.

Or am I out in left field and done jumped the fence thinking this way???
Just a quick note:
Going from a 3 cell battery to a 4 cell battery, and not changing the motor or prop will result in the current input to your motor increasing by perhaps 50%, and your power input to the motor about doubling! (It's that RPM ratio raised to the third power factor)

As others in this thread have indicated, be absolutely certain to monitor everything with a wattmeter while checking out different batteries.

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Old 03-29-2011, 02:33 AM   #21
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Default UBEC and Opto ESC

Great info from Chellie on UBEC. The only thin I would add is that if you use an OPTO ESC such as this:
http://www.headsuprc.com/servlet/the...ite--70/Detail

you do not pull the red wire on the ESC lead to the receiver. An OPTO ESC does not have a BEC built into it so you have to use a UBEC or separate receiver battery pack, but if you pull the red wire on the ESC lead the ESC will not function. I found that out on my first OPTO, as luck would have it I just pulled the lead rather than cutting the wire so no harm no foul.
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Old 03-29-2011, 03:51 AM   #22
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Originally Posted by kyleservicetech View Post
Just a quick note:
Going from a 3 cell battery to a 4 cell battery, and not changing the motor or prop will result in the current input to your motor increasing by perhaps 50%, and your power input to the motor about doubling! (It's that RPM ratio raised to the third power factor)

As others in this thread have indicated, be absolutely certain to monitor everything with a wattmeter while checking out different batteries.
The single most important bit of advice you've been given so far

I think I need a signature.
Larry
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Old 03-29-2011, 04:55 AM   #23
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I was gonna add something, but I'm so overwhelmed by all the information in this thread that I forgot what it was.





Oh, yeah. When in doubt, use a bigger ESC.

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