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Old Dec 22, 2008, 12:47 AM
Xtreme Nut
Kuala Lumpur, Malaysia
Joined Oct 2005
2,046 Posts
Quote:
Originally Posted by jnyoun
I don't agree with the battery issue. If the battery is high dischaging rate, it should be fine with proper throttle management. The battery is rated 25C (constant) and 35C (15s burst). It can handle 55A (constant) and 77A (butst). So, the range of 72-76 amp seems ok as long as I don't do WOT for more than 15sec.

Now, only concern for me seems the 70A esc. However, if it is really 72 to 76 amps, then it should be also fine because the 70A esc is rated as 80A max for 10 sec.

Seems like 'Good to GO'... with some throttle management.

Thanks for the good information.
I believe most lipo manufacturers rate their BURST between 3-5 secs only and not 15 secs. Also if the battery is a 'true' 25C and you were to constantly push the batt to its 25C capabilities, it will not last more than say 50 cycles. Best is to always keep to a 80% rule ... if battery is good for say 60A constant dishcarge, keep it to 48A. Unless you really use batts like Kleenex then by all means, push them to their limits.

Also ... when there is tons of power so we really say that we will not go for more than 15 secs? I said that to myself one too many times and usually end up I bloating my batts or burning my motors ... heheheehee ...
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Old Dec 22, 2008, 12:50 AM
Xtreme Nut
Kuala Lumpur, Malaysia
Joined Oct 2005
2,046 Posts
Quote:
Originally Posted by hawkz
Just wanted to share something one shouldn't do.

I flew my funjet yesterday with a 6S setup (2x 3S in series). I canopy popped off and so my 500mah rx battery and one of the 3S batteries fell out. The second 3S battery was held down with velcro, so that was good. I couldn't find the two batteries that fell out But other than that, the plane's still good to go. I just gotta find a better way to keep the latch on as well as velcro ALL batteries down in the canopy.
That's funny, on all the FJ I built and flown the hatch has never come out in flight, its always been a snug fit even when the latch has not fully clipped on. Only times I lose my hatches are when the landings are slightly hard, e.g. when I abort the launch etc.
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Old Dec 22, 2008, 03:44 PM
Spritual Pilot
Silicon Valley, CA
Joined May 2001
677 Posts
Quote:
Originally Posted by tIANcI
I believe most lipo manufacturers rate their BURST between 3-5 secs only and not 15 secs. Also if the battery is a 'true' 25C and you were to constantly push the batt to its 25C capabilities, it will not last more than say 50 cycles. Best is to always keep to a 80% rule ... if battery is good for say 60A constant dishcarge, keep it to 48A. Unless you really use batts like Kleenex then by all means, push them to their limits.

Also ... when there is tons of power so we really say that we will not go for more than 15 secs? I said that to myself one too many times and usually end up I bloating my batts or burning my motors ... heheheehee ...
I did some bench test with Watt Meter. When I throttle up, it went up to 70 AMP and then at WOT, it dropped to 65AMP after a few second. And then, it stayed at this AMP. The voltage was converged to 12.3V. The battery looks very strong...

Definitely, I am pushing it to the limit.

I am trying to adjust the ATV of throttle in my Radio so that it can only draw about 50 to 60A at WOT. I haven't finished the setup yet because the sound is amazingly loud... I should do this at the field. In the garage, my neighbor may complain if I do this more..
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Old Dec 22, 2008, 05:42 PM
Ook!
skaffen's Avatar
New Zealand
Joined Aug 2007
1,496 Posts
Quote:
Originally Posted by fishboy77
Where do you get those rare earth magnets that everybody raves about?
Old hard drives, I work with computers
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Old Dec 22, 2008, 05:48 PM
Registered User
Bay Area, CA
Joined Sep 2007
1,162 Posts
Quote:
Originally Posted by jnyoun
I am trying to adjust the ATV of throttle in my Radio so that it can only draw about 50 to 60A at WOT.
That makes no difference at all. You will still damage your setup but might take little longer. This is because ESC is not linear as one might expect but rather switches ON and OFF several thousand times per second (pulses) to keep the motor running at a certain Current / RPM based on the relative position of the throttle stick .

Crucial fact to be noted : When the ESC is in "ON" state, it lets the motor draw all the current it can draw. And "OFF" state completly shuts down the power draw.

For ex, when you are 50% throttle the ESC switches on and off equal number of times.

There should be more detailed documents in the internet that would explain more in-depth as how modern ESCs work.

Happy flying !!!
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Last edited by numb_thumb; Dec 23, 2008 at 03:56 PM.
Old Dec 22, 2008, 07:45 PM
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GryphonRCU's Avatar
United States, CA, SF
Joined Mar 2008
3,880 Posts
jnyoun,

You might want to see the link on bottom of page 487.

http://www.rcgroups.com/forums/showt...74757&page=487


Read page 4 of that article.
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Old Dec 22, 2008, 09:36 PM
Spritual Pilot
Silicon Valley, CA
Joined May 2001
677 Posts
I forgot that the ESC works based on PWM (Pulse Width Modulation). Considering this, you are right. The AMP draw (peak AMP) from battery does not matter how the throttle manages.

- The throttle only controls the rate of ON-OFF (in other words, duty-cycle).
- The magnitude (peak) of pulse is determined by the motor and the load.
- Bigger prop produces higher peak.
- This means the peak of the pulse can be INFINITE (theoretically) if I hold the prop (preventing rotating) while the throttle stick is even at 10% position., because the load is infinite. In this case, four things can be happened.

o Burning motor
o Burning ESC
o Destroy Battery
o Any combination of the above.


So, if Watt Meter says 65A at WOT, the peak of the pulse is close to or equal to 65A. In this case, regardless of the throttle position, the motor will draw 65A when the pulse is ON.

It seems I understand now…

However,

How about this battery discharging specification? I assume the discharge rate is based ‘linear’ or ‘continuous’, not ‘switching’ or ‘PWM’. So, when it says 20C (2200mah), it can draw 44A continuous. My next question is:

Will the battery be damaged if the motor tries to take 88A at 50% duty-cycle (which is 44A on average)? I am curious about this because there is a burst current rate, for example 30C (for 5 sec). It seems the battery can handle a short burst of high AMP.

I am not sure whether I can interpret this as 60C (for 2.5sec), 120C (for (1.25s), 240C (0.625s)… or something like this. However, if the ON time is shorter, it should handle higher AMP (according to the logic of 'burst' AMP).

In other words, if the ON period is very short, higher AMP draw will still damage the battery that badly? If there is a possibility that the chance of battery damage is lower if the ON period is short, does it mean that the throttle management does still make sense?

Thanks
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Last edited by jnyoun; Dec 23, 2008 at 01:42 AM.
Old Dec 23, 2008, 10:33 AM
The sky is falling!
ChickenStick's Avatar
Cleveland, Ohio
Joined Nov 2005
640 Posts
My understanding is that limiting your throttle end point will help your battery, but will not help your ESC.

This is because the switch rate is thousands of times per second, and the battery will see the average (like a human watching TV). The ESC still suffers the peak voltages + switching inefficencies (which may not be a problem in your case).

Example, set your end point to 50% and fly at full throttle. Your battery will be used at about a 50% rate. The ESC will be the same temperature (or perhaps a little hotter!) as if you were at 100% throttle.

LMR guys use full throttle on a switch (full on only) to minimize the switching losses in the ESC. The battery does not care about the switching losses, as these are just a few more watts of heat (adsorbed by the ESC) vs. the thousands of watts through the ESC. But when you are running a CC125 at 180+ amps, switching losses become more important to your ESC. Eliminating them completely (on/off switch only), is one of the ways to "safely" overdrive your ESC.

- Cliff
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Old Dec 23, 2008, 11:40 AM
Suspended Account
Joined Jan 2005
1,752 Posts
What temperatures should I be concened about the foam and adhesives of the funjet getting brittle and failing?
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Old Dec 23, 2008, 01:58 PM
Crash - Rebuild - Repeat
efether's Avatar
Atlanta, GA
Joined Aug 2007
500 Posts
Quote:
Originally Posted by lammy1000
What temperatures should I be concened about the foam and adhesives of the funjet getting brittle and failing?
Your batteries will fail before the glue or foam does. Ask a northerner here (Dennis...you here?)....I'm sure they'll tell you about packs failing when temps get below freezing. I live in the south...temps don't stay below freezing very long

-Eric
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Old Dec 23, 2008, 02:19 PM
Registered User
Bay Area, CA
Joined Sep 2007
1,162 Posts
Quote:
How about this battery discharging specification? I assume the discharge rate is based ‘linear’ or ‘continuous’, not ‘switching’ or ‘PWM’. So, when it says 20C (2200mah), it can draw 44A continuous. My next question is:

Will the battery be damaged if the motor tries to take 88A at 50% duty-cycle (which is 44A on average)? I am curious about this because there is a burst current rate, for example 30C (for 5 sec). It seems the battery can handle a short burst of high AMP.

I am not sure whether I can interpret this as 60C (for 2.5sec), 120C (for (1.25s), 240C (0.625s)… or something like this. However, if the ON time is shorter, it should handle higher AMP (according to the logic of 'burst' AMP).

In other words, if the ON period is very short, higher AMP draw will still damage the battery that badly? If there is a possibility that the chance of battery damage is lower if the ON period is short, does it mean that the throttle management does still make sense?

Thanks
Throttle 100%

ESC pulse is ON all the time. This is same as having no ESC at all. In other words the gates of the ESC is wide open. It will let the motor hog all the current it can from the battery. This is why ESC gets burned when more than rated current pass thru it.

Throttle 0%

Simple. The ESC shuts down the gates completely. No current passes thru from battery to motor.

Throttle 50%

ESC pulse is ON for approximately 6500 times and is OFF for approximately 6500 times per second. It alternates between ON and OFF equally. The number of pulses may vary on the price or quality of the ESC and hence sometimes some ESCs are pricey than others.

You may find more "interesting" theoritical details elsewhere but the following is based on my "expensive" experience and diligent record keeping with my batteries.

Example:

Lets says you put a larger prop than the motor is rated for (Most of us do...some just a tad bit and some all the way to burn the motor).

Lets say the motor now hogs 100 amps max.

We have a 125 amps esc and a battery rated at 80 amps continuous.

At 100% throttle: The motor hogs 100 amps or as much as it can from the battery. Battery is rated for 80 amps.

Result: Battery gets hot and depending on how long you continue to fly like this you may puff or increase the voltage drop dramatically in one or few flights.

At 50% throttle: The motor hogs 100 amps for approximately 6500 times in a second. Its far beyond human and chemical reaction can comprehend. The chemical reaction that releases current cannot recognize 6500 hits for 100 amps in a second but overall it feels someone is sucking more current than the battery is designed to supply.

Result: The battery will do its best to supply the current resulting in a combination of voltage drop, puffness or simple irreversible break down of chemicals. Its more like a controled damage to the battery. Battery gets hot and depending on how long you continue to fly like this you may puff or increase the voltage drop dramatically in few flights. Ever heard of LVC kicking in for no reason after 10 or 20 flights with the battery?

Burst Myth
Someone can correct me if I'm wrong here but burst rate is total nonsense. Its more like a catch phrase giving that extra edge of false confidence to the flyer. I guess it may have been of significance with brushed motors but not with brushless technology.

If you want long battery life:

Among other precautions, prop your motor in such a way that it hogs 70-80% of the rated continous amps of the battery. works well for good as well as cheap batteries.

If you want moderate battery life:

Among other precautions, prop your motor in such a way that it hogs close to 100% of the rated continous amps of the battery. Use throttle control moderately to gain a tad bit more life out of the battery.

Here is where the difference between good and cheap battery plays a significant role. The good batteries (usually expensive) last longer within what I call " the moderate battery life". The cheap ones die relatively faster.

Fun way to destroy a battery: or you are looking to break any record or want smoke trail inflight

prop your motor in such a way that it hogs close to 100% of the rated burst amps of the battery. Use throttle control rigorously to squeeze out a bit more life out of the already short battery life. Again, cheap ones dies sooner and the good ones.

Happy flying !!!
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Last edited by numb_thumb; Dec 23, 2008 at 04:16 PM.
Old Dec 23, 2008, 02:42 PM
Spritual Pilot
Silicon Valley, CA
Joined May 2001
677 Posts
Whew~~~

Ok... due to tremendous concerns on my battery from you guys, I changed my mind... In fact, I have two of 4s-2100mah-25C rated. I will use two of them in parallel. So, it will be a 4s-4200mah-25C. With this, it is now 105A continuous. 84A (80% of 105A) should be enough for my setup.

Now, I came back to my very original question.... whether a 70A rated ESC will work?...I don't wanna spend another extra money for 100A ESC... I don't think I am going to fly any other plane (that needs 100A esc) any soon. So, if I burn the 70A ESC, I will seriously consider to retire my funjet.....
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Last edited by jnyoun; Dec 23, 2008 at 04:21 PM.
Old Dec 23, 2008, 03:26 PM
zoom zoom
gotboostedvr6's Avatar
USA, NJ, Mt Laurel
Joined Oct 2007
3,691 Posts
A 100 amp esc runs around 25-32 on ebay. i have found 200 amp esc's for $ 38 they are made by suppo.
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Old Dec 23, 2008, 03:32 PM
Registered User
CFisher1421's Avatar
Geneva, IL
Joined Jul 2007
1,506 Posts
Numb Thumb - Great description of the PWM action on our ESCs. It's the first and possibly easiest to understand verbiage I've seen. Still see LOTS of folks thinking erroneously that half-throttle means half-current. Just means half the time.
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Old Dec 23, 2008, 03:40 PM
The Kid
Thekid3418's Avatar
Detroit, MI
Joined Dec 2007
2,587 Posts
I don't think you have to be that worried about your batteries. If you were to temporarily draw 30C, 40C, maybe even 50C from your pack, there wouldn't be a significant amount of harm done, in my experience at least. As long as you keep the bursts at that level below 5 secs it should be fine. Just give it plenty of cruising time to relax. If you abuse a pack for 30 seconds well above its rated discharge then it will go bad though

Look at the LMR guys, I have read of someone drawing 120C (400 amps from a 2600 mah pack i believe)!! And this battery lasted a season at levels like that. They just keep their motor runs nice and short, and they don't discharge the battery all the way.

Another key if you're going to abuse lipos is to not discharge them as much, a lipo will be much happier only discharging to 50% or 40% of its capacity. If you push every flight down to 20% (the recommended level), your batteries will start wearing out sooner. The further your batteries are drained the more likely they are to have differenent voltages in the cells, this will cause wear and tear
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