|Dec 03, 2009, 04:09 PM|
How do I know I won't puff a lipo?
I apologize in advance if there's a thread covering this. I searched and couldn't find exactly what I'm looking for.
I am relatively new to RC. I've got several EDF jets and a Wingtiger prop plane. They all came with stock 15C batteries. I upgraded the prop plane to a bigger motor and ESC, however I puffed every darn lipo. I was told the motor I chose was too strong for my system. I purchased a Watt Meter, however I don't know how to use it to know what C rating I need on my batteries so I won't be puffing them anymore.
I've read that getting a higher C rating battery for a stock EDFs will boost performance slightly and that the stock system will not overdraw more than what it can handle... Is that true?
I hooked up my Wattsup to an Art-Tech EDF F18 Hornet and got a max of 28amps on the stock 15C at WOT, and then tried a 25C and got 31Amps. The ESC this plane comes with is only rated for 30Amps max, and seeing a reading of 31Amps leads me to believe a higher C rating will allow for more amps than what's safe. Either that or the stock ESC on this plane is not capable of handling what this plane's stock motor can take. I don't want to burn up my ESC or motor in flight and crash.
Is there a threat that talks about what and how to calculate to be sure one is in safe operating limits?
|Dec 03, 2009, 04:36 PM|
Here is how it works, I am sure someone else will add on though.
The larger motor sucks more amps than the battery can provide. ESC amp rating has nothing to do with the battery, just the motor. ESC wont suck 30 amps from a battery, but your motor will suck that 30 amps from the ESC which then sucks it from your battery. If your motor is drawing 30 amps from a 15 amp ESC, your ESC will insta-fry!
If your motor/esc is drawing 30 amps from a battery that can put out 20amps, the battery will puff. If your motor draw 30 amps from an ESC that can deliver 40 amps from a battery that can deliver 45-50 amps, you wont have a problem. But a motor sucking 30a from an ESC that can put out 35amp from a 40amp battery is also safe.
If your ESC is hot to the touch, use a larger ESC. If your batteries are puffing, its because your motor/esc are drawing too much, use a higher C or higher mah battery.
The 15c means the battery is able to discharge "15 x mah/1000" amps. (I say its able to discharge 15 times its Capacity, but that may raise a lot of eyebrows if taken too literally)
If you have a 900mah pack at 11.1 volts (900mah, 3sp1 lipo) with a 15c discharge rate, it is capable of supplying 15c x 900mah/1000 = 13.5 amps of power. You divide you mah by 1000 because there are 1000 miliamp hour in 1 amp-hour.
So your battery can supply at best 15amps of power. Just say your motor draws 10amps of power and your ESC is rated 15amps (but only sucks 10amps cause thats what the motor is taking) at full throttle. You will be fine with this combination as long as you dont fly wide open all the time. (You could fly wide open actually if you have enough cooling on the motor but I am trying to set you up to be conservative because being conservative wont puff lipos!)
I dont think there is a formula for what you are asking, its a matter of you making the decision based on what the plane tells ya.
Please Post the following information
# of cells
C rating (15c right?)
Motor model number (like AXi 2208/34 ect)
Prop (9x5 ect)
ESC model (like thunder bird 18 or something)
I honestly use the "tower" approach. Since the battery supplies everything, it must be able to supply more amps than the ESC can take from it, and the ESC must be larger than what the Motor can take.
I noticed your ESC provides 30 amps but your motor is drawing 31 amps, assuming wide open throttle. Knowing art tech and EDF's, I assume you fly in the upper 3/4rds of throttle. Thats ok as long as your ESC is over NOT over heating. If you use a higher C rated battery, the battery will last longer and thats pretty much all noticeble that will happen on the Wattsup meter.
Most ARFs , especially EDFs stretch the ratings a long way. I take a little off what every they say the ratings are.
I am sure some one will try to correct me though.
|Dec 03, 2009, 05:09 PM|
I still might upgrade the Art-Tech F18's ESC since I have a few spare 40AMP ESCs sitting around.
As far as wingtiger prop job, here are the specs:
Motor: 2814-08 from Grayson Hobby (no longer on their website)
ESC 70AMP (overkill I'm pretty sure, but I had a 40AMP fail, possibly due to a bad servo though)
Battery: 11.1v 3s, 1600mah, 15C (capable of 22.5 Amps at best?)
I've since bought:
3s 2200mah 20C
3s 2200mah 25C, AND
3s 1500 25C battery
I don't want to puff those, but since Grayson Hobby no longer has info on their page about this motor I have no idea what it's specs are. I just remember seeing that it could handle a model weight well over what this plane is.
|Dec 03, 2009, 05:31 PM|
Im sure Dr. Kiwi is firing up his keyboard now.
1600/1000 = 1.6 so 1.6 x 15 = 24amps. 24 amps is the max, but some lipos allow you to go a little over but then it really starts to get damaged. 22.5? how did you get that? But thats safe too assuming the MFG isnt lying like they usually do.
9x6 sounds about right if thats the motor I thinking of. I am guestimating if it draws 16ish amps on a 9x5 gws and 20ish on a 10x6, you are probably pulling somewhere inbetween, probably around 18amps. I hope someone has the data on the motor though, cant do to much with the wrong information.
You choose the motor on the flying you want to do, not the weight. (Well sometimes the weight, other times price, availability... it depends on the pilot) My GWS formosa 2 accepts pretty much any power system under 600 watts and over 150watts. At 600 watts, sure you have nearly 1hp under the hood of a 1 1/2 - 2lb piece of foam. At 150 watts you are making lazy circles. I went with around 250watts which is enough to have a lot of fun without pouring too much cash in. Thats how I picked the motor. They were also out of stock for the one I wanted (Eflight power 10) and sold me another motor for only $18!
|Dec 03, 2009, 05:51 PM|
Thank you for your help.
Yeah, I sure learned the hard way that it wasn't the best way to figure out what kind of motor I wanted. LOL
I got 22.5 max Amps on that 15C battery by adding in a little wiggle room as I've read that mfg's are overly optimistic on C ratings.
Your link looks like my motor. I wish I knew for sure the max amp rating. I believe the recommended prop was a max of a 10x5.
I originally had a 40Amp ESC in the plane. While flying, the thing just lost all control and luckily glided down without much damage. I thought I fried the ESC, however I later learned (after adding the 70Amp ESC and it didn't work) that a Corona Digitral metal gear servo I added was fried too. Having the fried servo connected prevented the ESC from working. No idea why the servo would have blown other than maybe since it was digital and the others were analog that my 40Amp ESC couldn't handle it? At the time I didn't know digital servos require more voltage than analog.
|Dec 03, 2009, 07:37 PM|
Joined Jan 2004
still to complicated answers,
Try this, using your watt meter to know if your battery is up to the challenge you need to veiw what the volts are showing at full load.
A decent battery can be loaded down to 3.45volts per cell, so the math is 3s X3.45 under load, your watt meter should be showing 10.35 volts.
under full load.
This is how I know if I'm hauling around to much battery or not loading down the motor enough. 3.35volts per cell would be lowest I use for great wingloading but will be hard on battery.
Batteries PUFF from heat not watt ratings, any battery even the cheap ones can withstand 100deg without puffing. ThunderPowers puff at 137 deg, longMax around 140deg, Turnigy 140deg and some korean cells wont puff until 145deg. The good ones puff round so you cant even get them out until cool, then perfectly flat. I say this because if you cram your battery into a confined space without cooling AND its within all the specs stated above its still gonna puff........
To me batteries are as expendable as a gallon of fuel, I like to have the battery as the weakest link in my e-power setup. this provides for best wing loading and max performance.
In your setup as 3s and a 30amp ESC I would be running a 1300-1500 MAH pack with a weight range of 128-138 grams for battery penalty.
Now thats OFF da Hook...............
Not knowing a motors capability is no big deal either as a temp gun and keeping it under 120 deg is going to let you know when its maxing out.
|Dec 04, 2009, 02:24 AM|
Heading to work, Ill write more when I get back.
|Dec 04, 2009, 11:50 PM|
Joined Jan 2004
Problem as I see it is, your calculations are using the battery makers label to apply the math. Quite frankly its all pretty much a game as there is no standard of measurement applied, and you will get caught puffed with your pants down
I still contend if you know your cell count and you use a wattmeter, and you insure that the under full load volts are holding 3.45 VDC per cell, your battery is up to the task and will not puff.
|Dec 05, 2009, 02:56 AM|
I know the cheaper lipo manufactures are, er, optimistic in their ratings. He showed to understand this when he said 22.5amps, not the 24 I calculated.
Its true though, it seems all of the mfgs tend to bump up their ratings a little to sell stuff better. He asked for maths, I gave him maths to put him in the ball park, as he was using a 70amp ESC on a bird in the 20-40 amp range.
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