R/C Tech Forums

Go Back   R/C Tech Forums > General Forums > Electric On-Road

Reply
 
Thread Tools Search this Thread
Old 11-11-2008, 01:55 PM   #1
Tech Master
 
Akhor's Avatar
 
Join Date: Feb 2008
Location: Alberta,Canada
Posts: 1,293
Trader Rating: 7 (100%+)
Default Lipo "C" difference

What is the difference in "C" rating on a lipo pack?

If you have two packs that are 3800 one with 25C and another with 35C the 35 is always more expensive. But what does it mean in terms of battery?
Akhor is offline   Reply With Quote
Old 11-11-2008, 02:01 PM   #2
Regional Moderator
 
CarbonJoe's Avatar
 
Join Date: Jul 2006
Posts: 2,661
Default

Quote:
Originally Posted by Akhor View Post
What is the difference in "C" rating on a lipo pack?

If you have two packs that are 3800 one with 25C and another with 35C the 35 is always more expensive. But what does it mean in terms of battery?
In theory, it is what the cell is rated at for continuous discharge. So for the example given, the 3800 25C is supposed to be able to supply 95 A continuous, and the 35 C is supposed to be able to supply 133A. If all you are running is a stock motor (17.5 brushless or 27 turn brushed), I really doubt that there will be any practical difference, since the peak current available is higher than the continuous Amp ratings, which is most likely way above what the motor could draw.

The electric motor will only draw peak current at zero RPM (from a dead stop) for a very small time. No way is a stock class motor going to draw anything near 100A continuous.
__________________
*** The Gate - Celebrating 8 years at the same location ***
The difference between genius and stupidity is that genius has its limits. - Albert Einstein
Understeer is when you hit the wall with the front of the car and oversteer is when you hit the wall with the rear of the car
CarbonJoe is offline   Reply With Quote
Old 11-11-2008, 02:05 PM   #3
Tech Champion
 
Krio's Avatar
 
Join Date: Mar 2008
Location: At dirt tracks in Michigan!
Posts: 5,087
Trader Rating: 150 (99%+)
Default

The "C" rating is how many amps the battery can deliver continuously while maintaining a proper voltage level. The effect of a higher "C" rating shows up during acceleration where maximum amps are required. A higher "C" pack of the same capacity will out accelerate a lower "C" pack.
__________________
www.kriostasis.com
Krio is offline   Reply With Quote
Old 11-11-2008, 02:16 PM   #4
Regional Moderator
 
CarbonJoe's Avatar
 
Join Date: Jul 2006
Posts: 2,661
Default

Quote:
Originally Posted by Krio View Post
A higher "C" pack of the same capacity will out accelerate a lower "C" pack.
This would only be true if the motor can sink more current than the battery/ESC can source. In other words, if a given motor can only draw 80 (or even 100) amps peak, either of the two batteries are sufficient. The internal resistance probably has more to do with acceleration than the continuous current rating. Packs with a lower internal resistance will almost always feel like they have more "punch", since they can deliver that current easier.
__________________
*** The Gate - Celebrating 8 years at the same location ***
The difference between genius and stupidity is that genius has its limits. - Albert Einstein
Understeer is when you hit the wall with the front of the car and oversteer is when you hit the wall with the rear of the car
CarbonJoe is offline   Reply With Quote
Old 11-11-2008, 02:24 PM   #5
Tech Champion
 
Krio's Avatar
 
Join Date: Mar 2008
Location: At dirt tracks in Michigan!
Posts: 5,087
Trader Rating: 150 (99%+)
Default

I tend to disagree. A higher C pack will ALWAYS be able to provide a higher voltage under ANY load. Both packs may still be in their usable C range, but a higher C will hold hold a better voltage, providing more current.
__________________
www.kriostasis.com
Krio is offline   Reply With Quote
Old 11-11-2008, 04:48 PM   #6
Tech Champion
 
skypilot's Avatar
 
Join Date: Mar 2007
Posts: 9,524
Trader Rating: 14 (100%+)
Default

this discussion will go on forever,
i'm using a core 5000 with 20C running stock truck with a 17.5 where most other either have a brushed or a 13.5 and have no problem keeping up.

I have not tried any other battery i bought these when they first came out and they are still going strong. I race a average of 2-3 times a week on and off road
__________________
semi retired..
skypilot is offline   Reply With Quote
Old 11-11-2008, 07:14 PM   #7
mok
Tech Master
 
mok's Avatar
R/C Tech Elite Subscriber
 
Join Date: Jul 2007
Location: Perth, Australia
Posts: 1,075
Trader Rating: 9 (100%+)
Send a message via MSN to mok Send a message via Skype™ to mok
Default

I cant say I've seen a difference in punch between the IP3800 25C and LRP 5300 28C packs.
What I have seen is reduction in pack temperature, especially when geared hard on a large outdoor track.

3.8 * 25 = 95A continous
5.3 * 28 = 148.4A continuous

As far I see it, when a motor pulls 120A then the IP3800 pack wont be delivering and hence the pack heats up.
At 148.4A, the LRPs provide ample overhead (even for low wind brushless motors) and remains cooler.
And I believe you can never have enough overhead, so the biggest capacity and highest C rated packs are always your best bet

I've also noted that there is no difference in capacity usage between difference C rated batteries.
When geared the same using the same motor, you're still going to take xx mAh out of the packs, regardless of ratings (give or take).

Now introduce the differences between 2S1P and 2S2P packs and yes, this discussion will go on forever :P

-Mark
mok is offline   Reply With Quote
Old 11-11-2008, 11:38 PM   #8
Tech Legend
 
Wild Cherry's Avatar
 
Join Date: Aug 2001
Location: TRCR Modified Driver
Posts: 22,595
Default

Quote:
Originally Posted by Krio View Post
I tend to disagree. A higher C pack will ALWAYS be able to provide a higher voltage under ANY load. Both packs may still be in their usable C range, but a higher C will hold hold a better voltage, providing more current.

A winner !!!


More C = more speed
Wild Cherry is offline   Reply With Quote
Old 11-12-2008, 12:41 AM   #9
Tech Fanatic
 
Join Date: Jun 2007
Posts: 947
Trader Rating: 1 (100%+)
Default

So would that work if a pack was 1900mah rated at 60C?
Sabin is offline   Reply With Quote
Old 11-12-2008, 01:22 AM   #10
mok
Tech Master
 
mok's Avatar
R/C Tech Elite Subscriber
 
Join Date: Jul 2007
Location: Perth, Australia
Posts: 1,075
Trader Rating: 9 (100%+)
Send a message via MSN to mok Send a message via Skype™ to mok
Default

yes it would.. but you'd have a lower capacity with shorter runtimes.
mok is offline   Reply With Quote
Old 11-12-2008, 01:55 AM   #11
Tech Fanatic
 
Join Date: Jun 2007
Posts: 947
Trader Rating: 1 (100%+)
Default

I posted the last message as a bit of a joke to see if anyone was watching.

for starters, there are no 60C continuous discharge lipos. and if you looked at the various discharge graphs for lipos in general you will see that there's a distinct dip at 30C. a 60C discharge would effectively destroy a lipo.

the thing about a C rating now is it' s being pushed as a marketing gimmick. I started with lipos way before they became popular with RC cars by shoehorning old packs in for speed runs.

Looking at the only discharge graph for these orion slpb cells, it leads me to believe that a 30 and 35c continuous discharge will cause the voltage to drop very close to cutoff within the first five minutes of pushed that hard. To me, that says 'danger! you are overdriving the pack! back off!'. I'd hate to think what a 40C discharge will do.

I take a different approach to lipos. I look at the C rating AND the total mah. A high C rating might look nice but if the battery does not have the mah to back it up, you are essentially overdriving the pack. Sure it would be nice to say you have a 35 or 40C pack but can the pack itself stand up to those extremely high discharges?

so no, more C does not equate more speed. more C equates coming too close to cutoff point. a higher voltage under load on the other hand, DOES equal more speed. that's where the higher mah rated packs come in.
Sabin is offline   Reply With Quote
Old 11-12-2008, 01:58 AM   #12
Tech Addict
 
MTX3-B's Avatar
 
Join Date: Feb 2005
Location: Sepang Home of Malaysia's Formula One
Posts: 684
Trader Rating: 5 (100%+)
Default

Quote:
Originally Posted by Krio View Post
I tend to disagree. A higher C pack will ALWAYS be able to provide a higher voltage under ANY load. Both packs may still be in their usable C range, but a higher C will hold hold a better voltage, providing more current.
I am a bit confused with all these C rating stuffs, as i know there is yet a standardised definition (backed with the relevant testing to back up the C rating claims). Anyhow, would appreciate if you could enlighten me on the following:-

I use 3200 20C and 4000 25C in my Tamiya M-Chassis running on a sport tuned (23T equivalent or approximately 11.5T brushless equivalent). Both packs are relatively new.

Based on the discussion:

1) A higher capacity will yield a higher average voltage during the run?
2) Does the C rating really matter (power output ie. punchy packs) for a lower load motor such as Sport Tuned?

Based on my reading from the several threads, the true C rating is basically the capacity of the pack to deliver X amps thruout the duration i.e. capacity of the battery without droping the recommended threshold level of 3.0V per cell and without the pack reaching anything above a certain temperature.

Since i am purely a simple racer and based on my testing of the two packs, i can safely say that i couldnt feel the difference between the two save for the higher pack gives me more runtime. Perhaps a pro driver could tell the difference.

For low amp draw applications, in my case, is it correct to say that the C rating does not really matter? Given a choice between 4600 20C vs. 4200 30C, which one would u choose to run the 23t brushed motor?

Thanks in advance.
__________________
4WD -- TT01-E; TA-05; HMF 416 WE
FWD -- FF03 PRO
2WD -- M03-M; M05 PRO; X-EVO
MTX3-B is offline   Reply With Quote
Old 11-12-2008, 03:20 AM   #13
Tech Fanatic
 
Join Date: Jun 2007
Posts: 947
Trader Rating: 1 (100%+)
Default

Quote:
Originally Posted by MTX3-B View Post
1) A higher capacity will yield a higher average voltage during the run?
from start to finish, yes.

Quote:
2) Does the C rating really matter (power output ie. punchy packs) for a lower load motor such as Sport Tuned?
For a 23t motor, you could probably get away with a 15C pack, since the amp draw is so low.

Quote:
Based on my reading from the several threads, the true C rating is basically the capacity of the pack to deliver X amps thruout the duration i.e. capacity of the battery without droping the recommended threshold level of 3.0V per cell and without the pack reaching anything above a certain temperature.
the standard formula for C is: (MAH of battery x C value)/1000 = max discharge amp rating.

Marketing may say different but it's never a good idea to run the battery at the max discharge rating all the time.
Sabin is offline   Reply With Quote
Old 11-12-2008, 12:53 PM   #14
Tech Elite
 
raffaelli's Avatar
 
Join Date: Sep 2001
Location: Land of the Free, Home of the Brave
Posts: 2,924
Trader Rating: 13 (100%+)
Default

Quote:
Originally Posted by Sabin View Post
from start to finish, yes.

For a 23t motor, you could probably get away with a 15C pack, since the amp draw is so low.

the standard formula for C is: (MAH of battery x C value)/1000 = max discharge amp rating.

Marketing may say different but it's never a good idea to run the battery at the max discharge rating all the time.

1900 mah at 60c is 114A

5300 mah at 25C is 132.5A


5300mah more better?
__________________
24ITA
raffaelli is offline   Reply With Quote
Old 11-12-2008, 02:11 PM   #15
Tech Master
 
Akhor's Avatar
 
Join Date: Feb 2008
Location: Alberta,Canada
Posts: 1,293
Trader Rating: 7 (100%+)
Default

Quote:
Originally Posted by raffaelli View Post
1900 mah at 60c is 114A

5300 mah at 25C is 132.5A


5300mah more better?
According to your formula it is. As well it will have a longer run time.
Akhor is offline   Reply With Quote
Reply



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off



All times are GMT -7. It is currently 06:22 PM.


Powered By: vBulletin v3.9.2.1
Privacy Policy | Terms of Use | Advertise Content © 2001-2011 RCTech.net