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garyp1029 04-19-2014 04:18 AM

C Discharge????
 
I have read "Everything you Want To Know . . ." as well as several other posts concerning C discharge and I still don't get it. I understand that C is a discharge rate of drawing a certain quantity in a specified time period, BUT EXACTLY HOW does it affect real life choices? Using a plane I have ordered (Electrifly Super Sportster rxr) as a specific example---It has a motor of unknown details, an ESC of 18 amps, and a recommended 3-cell battery of 1300A and 30C discharge rate. QUESTION----Assuming weight is NOT a factor, can I safely use a 1300A 20C??? A 1500A 20C??? How about a 1800A 20C??? What would be the UNQUESTIONABLE result of reducing the C from 30 to 20? Are these figures cast in stone or are they wild estimates?
Is there a simple method of converting one of these to the other?
Your input would be greatly appreciated. Thanks. Gary

fhhuber 04-19-2014 04:52 AM

"C" rating has to do with internal resistance of the battery and other factors and is a somewhat non-standard "quantity"

It (supposedly) means you can "continuously" disharge at X * capacity

But "continuous" can mean for up to 1 minute or for the whole pack's charge or just about anything the person who slapped the label on wants it to mean...

There's an infamous battery maker's claim of 150C (drain the pack flat in 24 seconds) where the pack had wires which would melt at that current in under 5 seconds.

C rating meanings are pretty consistent within any given brand and as long as you are comparing within one brand they are useful.
Don't try to equate the 20C of Sky LiPo vs Turnigy vs Thunder Power vs Polyquest vs Max Amps packs. If they have any relationship its accidental.

20C will have higher internal resistance than 30C and will get hotter under the same load. You'll also waste a little more of the pack energy creating that heat.

Within the same brand packs you can expect higher C to give SLIGHTLY better peak performance at the cost of a bit of duration (due to higher current flow). Unless you are demanding high current you may not be able to notice it.

You can't even really compare the capacity of different brands well... what one labels 2000 ma another might label 1800 or 2200. I have a 3S 2200 ma 20C "Dynam" labeled pack that gives less duration than a 3S 1800 ma 20C Sky LiPo

pizzano 04-19-2014 05:04 AM

Well.....if you read everything, then this link should have helped:

http://www.wattflyer.com/forums/showthread.php?t=65869

It's a little techie....but the best explanation I've seen yet....Denny knows his stuff!

You stated that "weight is not a factor".....but keep in mind weight is an issue related to both mAh, "C" value and C/G........very few RC air craft are not affected by C/G or the weight distribution factor going up or down dramatically in mAh & "C" value.

Also, many here and at other forums have found the "C" value stated on most low to mid quality batteries can be very mis-leading, especially when the "C" is over the 30 value.

Also...."C" factor is often referred to as the "burst" rate related in amps the battery can safely deliver within a given time cycle (say 10 sec.).....it's quite common to need higher "burst" rates in muti-prop craft or extremely acrobatic planes.......for the obvious reasons!

kyleservicetech 04-19-2014 05:17 AM

Quote:

Originally Posted by garyp1029 (Post 945823)
I have read "Everything you Want To Know . . ." as well as several other posts concerning C discharge and I still don't get it. I understand that C is a discharge rate of drawing a certain quantity in a specified time period, BUT EXACTLY HOW does it affect real life choices? Using a plane I have ordered (Electrifly Super Sportster rxr) as a specific example---It has a motor of unknown details, an ESC of 18 amps, and a recommended 3-cell battery of 1300A and 30C discharge rate. QUESTION----Assuming weight is NOT a factor, can I safely use a 1300A 20C??? A 1500A 20C??? How about a 1800A 20C??? What would be the UNQUESTIONABLE result of reducing the C from 30 to 20? Are these figures cast in stone or are they wild estimates?
Is there a simple method of converting one of these to the other?
Your input would be greatly appreciated. Thanks. Gary

That "C" term is a general term that describes just how fast your battery can be discharged. The same thing applies to how fast you can recharge your battery.

If the battery mfg is honest in their specs, the higher the "C" rating, the better the battery will put out high currents without the battery voltage sagging to much. Kind of like an old lead acid car battery that can barely turn over your engine, as compared to a brand new battery.

IMHO, any C rating above perhaps 60 or so leads to diminishing returns. If you actually discharged a battery at 60C for the entire flight, your flight time would be less than one minute.

If you discharge your battery at a rate of 30C, that indicates a flight time of two minutes assuming full power for the entire flight. And, 20C would be three minutes.

Now, you've got the batteries MAXIMUM current capability, defined as per example, 30C. With a 1300 Mah battery, that is 1.3 Amp Hours. So, 30C would be 30C times 1.3 Amp Hours, or a maximum current of 39 Amps.

And, you've got "Flight" "C" which is how hard you are actually operating your battery. If you're pulling 22 Amps on a 1.3 Amp Hour battery, the value of C would be 22/1.3 or C= 16.92. Flight time would be 60 minutes/16.92 or about 3 1/2 minutes at full throttle for the entire flight.

Problem is, most battery mfg's don't indicate how long your battery will last running it at maximum C. We've had some guys in my club that only got 5 flights before the battery was junk.

Flight time is equal to 60 minutes divided by the "C" value actually being used while in flight.

And as Pizzano indicated, I've got a thread on this subject:

"C" and what it is
http://www.wattflyer.com/forums/showthread.php?t=65869

Hope that helps :D

pizzano 04-19-2014 05:20 AM

Beat ya to it Denny.......LOL.......that link (thread) you put together is the best one I've ever seen on any forum......that's why I refer to it still today......;)

kyleservicetech 04-19-2014 05:33 AM

Quote:

Originally Posted by pizzano (Post 945832)
Beat ya to it Denny.......LOL.......that link (thread) you put together is the best one I've ever seen on any forum......that's why I refer to it still today......;)


Thanks!

This old geezer has had some 50 years experience in this electric stuff, so some of it does sink in after awhile. :D :D

kyleservicetech 04-19-2014 05:42 AM

Quote:

Originally Posted by fhhuber (Post 945827)

There's an infamous battery maker's claim of 150C (drain the pack flat in 24 seconds) where the pack had wires which would melt at that current in under 5 seconds.

Yeah, here you go on that batteries 150C claim.

http://www.wattflyer.com/forums/show...highlight=150c

IMHO, that 150C rating isn't worth the paper its written on. I'm sure it is a good battery. But 150C ????

This company is still claiming 150C on their batteries:
http://www.maxamps.com/Lipo-8000-185-DC-Pack.htm

xmech2k 04-19-2014 05:51 AM

Gary, the most important thing is that your battery can supply the power demanded by your esc and motor. That's measured in amps. To determine how many amps your battery is rated to safely run at, you just use the equation you probably already learned:

(mAh/1000)xC

So in your examples above, your 1300 mah 20 C battery can safely provide 26 amps. The 1500 mah 20 C can provide 30 amps.

Your motor/prop determines how many amps the system needs. The esc must be rated to handle it. The battery must also be rated to supply it. If you get too close to a batteries limits, or is old and/or abused, it can suffer from voltage sag when under load, causing poor performance.

I hope this is along the lines of the info you're looking for.

kyleservicetech 04-19-2014 05:58 AM

Quote:

Originally Posted by xmech2k (Post 945838)

(mAh/100)xC

So in your examples above, your 1300 mah 20 C battery can safely provide 26 amps. The 1500 mah 20 C can provide 30 amps.

Your motor/prop determines how many amps the system needs. The esc must be rated to handle it. The battery must also be rated to supply it. If you get too close to a batteries limits, or is old and/or abused, it can suffer from voltage sag when under load, causing poor performance.

I hope this is along the lines of the info you're looking for.

Oops, you left a zero off :eek: :D

Maximum current equals (mah/1000)xC

Good point on the motor and prop specs. Just changing the prop by several sizes can have a drastic effect on the "Effective C" being used during flight. From not being able to get off the ground with to small of a prop, to having wild power with to big of a prop, with smoke soon to follow.

solentlife 04-19-2014 05:59 AM

To the OP .... Denny's knowledge is far exceeding most ... so go with him !

Taking your post ... you ask if it's OK to use a lower C rated in a model. Yeh go ahead - BUT don't expect to have same performance ... Don't expect battery to last the punishment you will subject it to.

If a model is spec'd for 30C - then using a 20C is definitely NOT recommended. The only time you could get away with that is if you increased the size of the pack to get back to AMP capability ...

Example :

1500mAh pack of 30C = amp capability of 1.5 x 30 = 45A

you would need a pack of 45 / 20 = 2250mAh size at least to get same amp capability in 20C rating

But as in most things in life - theory does not always work out ... trying to run a pack at full output amp capability means you better have a big wallet and plenty of money in it !

There is another factor - most model spec's under state the real requirements for battery packs ... and I usually find an increase in C rated pack is needed. In this case where your model calls for a 30C ... I would be looking at a 35C or more ... just to get that full output it probably needs.

Nigel

kyleservicetech 04-19-2014 06:06 AM

Quote:

Originally Posted by solentlife (Post 945844)

Taking your post ... you ask if it's OK to use a lower C rated in a model. Yeh go ahead - BUT don't expect to have same performance ... Don't expect battery to last the punishment you will subject it to.

Out of curiosity, has anyone ever taken a hand full of batteries from a given supplier with the same mah rating, and different "C" ratings, and determined how much better the higher "C" rated batteries hold their voltage under load?

That would be very good information.

fhhuber 04-19-2014 06:11 AM

Well... you can fly using the recommended every time.

You will often be better off with higher C rating especially dealing with high demand applications (Like EDFs)

Yo can also do just fine with lower C than recommended in many cases. I'm only using 20C rated packs for my big EDF... 12S 5000 mah, pulling briefly 105 amps (last reading before the Wattmeter smoked), averaging about 60 amps (calculated from time vs what it takes to recharge) over the 4 minutes I fly it per charge.

fhhuber 04-19-2014 06:14 AM

Quote:

Originally Posted by kyleservicetech (Post 945845)
Out of curiosity, has anyone ever taken a hand full of batteries from a given supplier with the same mah rating, and different "C" ratings, and determined how much better the higher "C" rated batteries hold their voltage under load?

That would be very good information.

There have been people who were doing those tests... but it gets expensive to get all the batteries to test.

General results (with the same capacity and cell count) within any one brand showed higher C rated could deliver more current (lower internal resistance) but for a shorter time (because its the same total watt-hr capacity) The higher C rated packs didn't get as hot under the same load, as expected with the lower internal resistance.


Comparing one brand to another... the numbers on the packs were essentially not reliable.

solentlife 04-19-2014 06:18 AM

Quote:

Originally Posted by kyleservicetech (Post 945845)
Out of curiosity, has anyone ever taken a hand full of batteries from a given supplier with the same mah rating, and different "C" ratings, and determined how much better the higher "C" rated batteries hold their voltage under load?

That would be very good information.

Being a 'scrooge' .. I tend to not take my own advice !! I do try at times ti skimp on batterys ... trying to buy lowest C rated and size I can ... but it really does prove false economy.

I have found that Nanotechs may give their rated but not for long ... they soon grind down and become worse than their lowest rating (nanos of HK have dual C rates ie 25-50 ... etc.)

Rhinos - I find tend to be good and produce near enough what they label. They also seem to last reasonably well.

Turnigy blue - Generally I find are reasonable but C ratings can be a little optimistic, life ? generally if not called too often to full rate - similar to Rhino.

Zippy - same as Turnigy Blue

Zippy Compacts - same as Turnigy Blue except for one aspect - continued punishment will crap them out earlier than Blue. They puff up easily.

My best packs in fact are ones that I can no longer find ... E-Tech, I have a couple that getting on 3 years old and they have been abused something terrible ... and they still produce nigh on C rate and performance. They look battered and beaten up ... I reckon half the people on here would bin them - they do look beaten ! But boy do they put out ! Where did I get them ? Ebay !! I bought them because they had a different more square shape and fitted my T45 50mm better ... They now drag my Cessna and other models around since I upped my 50mm's to 4S.

I am a Rhino user except where I can't get the C and size I want .. then it's Compact or Blue.

Nigel

garyp1029 04-19-2014 03:42 PM

Many thanks to all of you for your responses. I still have a lot to learn and will give serious consideration to all. Considering the Electrifly S.S. rxr, Tower Hobbies says both the motor's kV rating and RPM are unknown. If so, how did they determine that the plane requires an 18 amp ESC and a 1300A 30C battery? I have seen that some use a 1500A battery (and one person even an 1800) so I am tempted to try a 1500--assuming it fits and can be located so the CG remains the same. Thanks again. Gary

fhhuber 04-19-2014 04:03 PM

Difference in weight/size of a 1300 ma vs 1500 ma is small. That should be very easy. The 1800 might take a little thought but still shouldn't be too hard s there is usually some space all around the battery pack.

We don't just stuff in the biggest battery that fits... we consider first the minimum needs to fly the model (voltage and current delivery) and the desired minimum duration. Then we consider the weight and its effects on the model.

If added weight won't affect performance excessively THEN the larger capacity battery is appropriate.

Doubling battery capacity will not double flight time. More weight requires more minimum airspeed to maintain flight and that means more power is required, draining the battery faster. (You can even reduce flight time by adding too much battery.)

More weight generally means a lower top speed and reduced aerobatic performance.

More weight can mean more damage in a minor crash.

xmech2k 04-19-2014 05:08 PM

Quote:

Originally Posted by garyp1029 (Post 945894)
... If so, how did they determine that the plane requires an 18 amp ESC and a 1300A 30C battery? I have seen that some use a 1500A battery (and one person even an 1800) so I am tempted to try a 1500--assuming it fits and can be located so the CG remains the same. Thanks again. Gary

With an amp meter. You need to get one too if you want to stay in the hobby. It's an essential tool so you know how many amps your motor/prop is pulling, so you don't get a smoke leak. And as long as you keep the same C rating, notwithstanding weight and cg considerations, it's always safe to go with a bigger mAh rated battery, since it will have a higher discharge rate.

kyleservicetech 04-19-2014 05:09 PM

Quote:

Originally Posted by fhhuber (Post 945895)
Difference in weight/size of a 1300 ma vs 1500 ma is small. That should be very easy. The 1800 might take a little thought but still shouldn't be too hard s there is usually some space all around the battery pack.

We don't just stuff in the biggest battery that fits... we consider first the minimum needs to fly the model (voltage and current delivery) and the desired minimum duration. Then we consider the weight and its effects on the model.

If added weight won't affect performance excessively THEN the larger capacity battery is appropriate.

Doubling battery capacity will not double flight time. More weight requires more minimum airspeed to maintain flight and that means more power is required, draining the battery faster. (You can even reduce flight time by adding too much battery.)

More weight generally means a lower top speed and reduced aerobatic performance.

More weight can mean more damage in a minor crash.

Very good points.

One of those computer programs such as www.motocalc.com will help in the selection of your battery. Problem with motocalc though, it's only as accurate as the specifications on the motor you are using.

Some of those specs are way the heck off, resulting in those computer programs giving inaccurate results.

kyleservicetech 04-19-2014 05:12 PM

Quote:

Originally Posted by xmech2k (Post 945904)
With an amp meter. You need to get one too if you want to stay in the hobby. It's an essential tool so you know how many amps your motor/prop is pulling, so you don't get a smoke leak. And as long as you keep the same C rating, notwithstanding weight and cg considerations, it's always safe to go with a bigger mAh rated battery, since it will have a higher discharge rate.


Agreed on that.

If you're playing with different prop sizes, or different batteries, a wattmeter is in my humble opinion (IMHO!) absolutely mandatory. If the wattmeter saves burning up just one motor, it has paid for itself.

On the other hand, if you have to small of a prop on your motor, that wattmeter will let you know that the motor can handle a bigger prop, within the ratings of the motor/esc/battery pack.

solentlife 04-19-2014 05:19 PM

Quote:

Originally Posted by garyp1029 (Post 945894)
Many thanks to all of you for your responses. I still have a lot to learn and will give serious consideration to all. Considering the Electrifly S.S. rxr, Tower Hobbies says both the motor's kV rating and RPM are unknown. If so, how did they determine that the plane requires an 18 amp ESC and a 1300A 30C battery? I have seen that some use a 1500A battery (and one person even an 1800) so I am tempted to try a 1500--assuming it fits and can be located so the CG remains the same. Thanks again. Gary

capacity size of the pack is not a problem if the model can take the physical size. My Cessna is designed for 1300 3S packs - but I fly her on anything from 1300 up to 2200 3S packs.
If the model balances near on CoG ... the pack fits in the slot ... then fine.

The factory would have given Tower Hobbies the specification of what battery to use etc. TH do not manufacture - they are distributors only.

What is important is to stay with the correct VOLTAGE ... I assume from the figures you post that it's most likely a 3S LiPo ? Then as long as you use the same S rating ... size is what you want and can fit in.

Nigel

JetPlaneFlyer 04-19-2014 05:21 PM

As a rule of thumb for trainers and sports models 20c-25c should be fine, for fast aerobatics and 3D models then 25-30c is enough. Higher c ratings are usually only an advantage the likes of racers, EDFs and helis that have very high current demand and short flight times.

But higher c rating doesn't do any harm other that costing more and usually being a few grams heavier.

solentlife 04-19-2014 05:22 PM

Quote:

Originally Posted by xmech2k (Post 945904)
With an amp meter. You need to get one too if you want to stay in the hobby. It's an essential tool so you know how many amps your motor/prop is pulling, so you don't get a smoke leak. And as long as you keep the same C rating, notwithstanding weight and cg considerations, it's always safe to go with a bigger mAh rated battery, since it will have a higher discharge rate.

Should that not be S rating ... ie VOLTAGE ?

C rating makes no odds - you can use as high a C as you like for no difference. As long as you don't go lower than factory spec.

Nigel

xmech2k 04-19-2014 05:39 PM

Quote:

Originally Posted by solentlife (Post 945910)
Should that not be S rating ... ie VOLTAGE ?

C rating makes no odds - you can use as high a C as you like for no difference. As long as you don't go lower than factory spec.

Nigel

True about the voltage. I was thinking if you went with a higher mAh but lower C rating, you could risk winding up with a battery not being able to supply enough amps in some cases.

solentlife 04-19-2014 05:45 PM

Quote:

Originally Posted by xmech2k (Post 945913)
True about the voltage. I was thinking if you went with a higher mAh but lower C rating, you could risk winding up with a battery not being able to supply enough amps in some cases.

True ... then needs a bit of maths !! ;)

Nigel

Wrongway-Feldman 04-20-2014 02:50 PM

I have found in recent years that plane manufacturers are listing higher and higher C requirements for their models. And I can't for the life of me figure it out.
There are models on the market today that have not changed in years that originally required a 15C battery that now require a 30C pack. Specs on the plane have not changed. ESC rating has not changed.
I wish I could remember the name of the manufacturer but I had a large J3 cub that a few years ago required a 4S 3300 at 20C. Plane had a 30amp ESC and a 36xx motor.
A friend bought that exact same model still sold in the same packaging last fall. Still has the same motor and ESC but now requires a 4S 3300 30c. I'm starting to think that for many manufacturers it is all just a bit of hype.
Parkzone still uses all 15c packs, I think we can agree that parkzone is no slouch in this industry.
I have used batteries from all makes, except the expensive ones cause I'm cheap, and have virtually no difference in performance between any of them. I treat mine poorly, I forget to set them to a storage charge, I charge them and then forget I charged them and leave em laying around months.
I've never had one puff. Had a chicklet 1S go poof on the charger once but that's it.
I don't even know how many cycles on my packs cause I don't log it.

The only thing I look at is what the amp draw from a motor ESC combo will be.
I then look at weight capacity of said model and room for the pack. I then go with the biggest pack that will realistically fly. It is rare that I've ever required more than 20C out of a pack. Other than a 1300 watt 70mm edf I have. But that's another story.


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