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Lipo "C" ratings

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Old 03-28-2012, 08:45 AM
  #1
yakfish
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Default Lipo "C" ratings

I think I unerstand what "C" ratings are and how they work but I haven't really ever compared batteries with different "C" ratings to know how much of a difference they really make. I am just a basher and don't race. I know in racing you want that little bit of an extra edge so I higher "c" rating can give you a bit more punch. But for bashing will I really see a noticable difference? I have a Losi 8ight 2.0 buggy with a Mamba Monster 1515 2200 system and a 8ight-T 2.0 with a Mamba Monster 1520 1800 system. I run both 4s and 6s and all my batts are 40c. Will I see a noticable difference in power if I was to drop down to a 20c or 25c battery? I am relatively new to the electric side of things and I have heard people say to look at batteries as the fuel tank and the "C" rating as the fuel line. With larger fuel line (in this case "C" rating) for more punch. If this analogy is correct then wouldn't I see longer run times with a lower "C"rating? Or is there no advantage at all to run a lower "C" battery?
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Old 03-28-2012, 10:34 AM
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Default RE: Lipo

Lower C rating batteries will run hotter and cost a little less and weigh a little less.

I can't speak much for car applications specifially.
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Old 03-28-2012, 11:27 AM
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Default RE: Lipo

okay spo heres my laymens terms definition for lipo terminology,
we will relate everything to water

MaH: this is the abreviated form of "millionth ampres hours" basicly the more you have the longer you run, this is your resivour of water

volts: this is the amout of force behind your water in the pipe,

"c" rating: this is your pipe and how much pressure it can pump through at any given time.

so if your wanting to race and want to find out if your packs can handle your bl system you can actually find how many continuious amps you can drain from that pack at one time or a peak current for however long your pack can sustain it.

take you mah x your "c" / 1000

so with my favotire packs for 1/8 i use 5000mah x 40c / 1000= a continueus discrage rate of 200 amps at any time

most batteries have a burst current of double so my burst current is 80c for 10 seconds, to find my top draw i would take

5000mah x 80c / 1000 and get a burst current of 400 amps for 10 seconds. bl motors have a peak in power once there done accelerating and then they drop in amperage and sustain the degrease untill they level out to keep a constant speed.

lipo packs will swell up and get hot under discrage, as a racer 80 degrees F is perfect. i am betting the best performance my battery can give without damaging it. swelling happens when you draw too much and can cause the pack to explode. so watch this.
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Old 03-28-2012, 11:33 AM
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Default RE: Lipo

droping in c rating will decrease your runtime as well, due to the fact that your systems of choise are a very high draw. the motor is what pulls amperage not the esc. so being that the pack would be lower in c rating wit would have to draw more Mah to cover up the amperage that the c rating can't suply causing heat and lower run times. your 2200kv motor will want to pull around 120 amps once it's leveld out on 4s, on 6s it's going to want 220ish.

so getting a pack to run this kind of power is important.
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Old 03-28-2012, 11:37 AM
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Default RE: Lipo

Yakfish- Higher C batteries will only make a difference if the battery i the weakest link in your power set up. If your motor can only draw say 50amps you will not notice if you drop from 5000mah 25C (100amps max) to a 5000mah 15C (75amps) because the motor will max out at 50amps. The higher C battery will actually give you slightly longer run times despite the same mah because higher c batteries have lower internal resistance. Simply the closer you get to *****g a batteries amp output the less efficient it is due to heat loss. The higher C rating battery will lose less energy as heat. Weather you can notice the difference in not sure.

Another consideration is the easier you go on a battery the more cycles it TENDS to have.

Also higher C batteries TEND to have faster charge rates so I your charger is powerful enough you MIGHt be able to charge faster....

Simple?!!
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Old 03-28-2012, 12:43 PM
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Default RE: Lipo

godlenduff,
 
  i stated eirlier that the "c" rating does effect run time. if the motor doesn't need to pull the amperage from the c rating then it will last longer as it doesn't need the amperage to cover everything and thus resulting in a cooler battery and a longer run.
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Old 03-28-2012, 01:16 PM
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Default RE: Lipo

In your particular setup if we trust those numbers then you will notice a drop in power going to a 20C packs. There's no real advantage other than lower cost to using lower rated packs.
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Old 03-28-2012, 02:47 PM
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Default RE: Lipo

Thanks for all the comments. Everything else in RC has a trade off and you need to find a balance. With gearing you are trading acceleration for top speed, with motor KV you are trading toque for rpms and trying to find a balance. everything has a trade off. I wondered if it was the same with the "C" rating as well. I guess the only trade off is performance and money. And in RC you always get what you pay for. What would be the minimum C rating for the setups I am running?
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Old 03-28-2012, 07:19 PM
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Default RE: Lipo

not so true on the "get what you pay for". i will never again buy a thunder power. blew 2 100 dollar packs in 1 month each because a cell failed.

gens ace are cheeper and perform better, 10 months and they haven't given me any issues.

last friday we had a race, the castle guys pulled off the track at 250 degrees and my hobbywing pulled off at 224. i am well aware of the 160 safe mark, and casltes 180 safe mark, new track and new dirt, nobody new we'd be running hot.

castles are great, don't get me wrong, and if you're a thunder power guy, do it. this is all about having fun.
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Old 03-28-2012, 09:56 PM
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Default RE: Lipo

Rc crazer I wrote my post as you wrote yours you posted 2secs before me
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Old 03-29-2012, 04:46 AM
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Default RE: Lipo

my opologies for the inconvieniance. sorry.
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Old 03-29-2012, 04:55 AM
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Dinnae be sit- no apology needed. I thought it funny two people practically thinking the same thing at the same time.
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Old 03-29-2012, 06:45 AM
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Default RE: Lipo

    Just to clarify, mAH is for milliamp Hours, or thousandths, not millionths, which is why you divide by 1000.
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Old 03-29-2012, 07:39 AM
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Default RE: Lipo

thanks, i have been doing that more and more resently, typing what my brain doesn't want me to. thanks

sorry all for the inconviniance.
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Old 03-29-2012, 08:20 AM
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Default RE: Lipo

"C" is the symbol for capacity for a battery cell, and by extension, the battery pack. When a pack is rated at "10C" for discharge, it means that it can experience discharge currents of 10X the cell's capacity. So if you're running a 2000 mAh battery pack, you can place a motor load on the battery that can result in a 20 amp continuous discharge without compromising performance of the pack. 2000 mAh means the battery pack is capable of providing 2 amps of current continuously for one hour.

If you were to place a load on the pack that exceeds 20 amps, at best you'd shorten the life of the pack and at worst you'd permanently damage the pack (and create a risk of fire upon attempting to recharge).

When selecting a pack, you should review the ratings of the motor you wish to use to determine it's ratings for continuous current draw and burst current draw. You would then select a battery pack that A) has the capacity that gives you the desired amount of run-time, and B) has a discharge rating that's above the continuous current draw rating for your motor. This will assure you're operating both the motor and battery pack withing the parameters they were designed for.

Basically speaking, a pack with a higher discharge rating will not result in a noticeably longer run time. The lower cell resistance of a high discharge pack will result in fewer electrical losses and thus will result in more energy being transferred to the motor. But if maximizing run time is critical, you'd want to look for the highest capacity pack that either fits or is of the maximum weight you'd want to use.

With battery technology, there's always trade-offs...
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Old 03-29-2012, 09:18 AM
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Default RE: Lipo


Quote:
ORIGINAL: rccrazer

thanks, i have been doing that more and more resently, typing what my brain doesn't want me to. thanks

sorry all for the inconviniance.
It would help if you knew what you were talking about - AND could properly convey it
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Old 03-29-2012, 09:25 AM
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Default RE: Lipo


Quote:
ORIGINAL: fly24-7

"C" is the symbol for capacity for a battery cell, and by extension, the battery pack. When a pack is rated at "10C" for discharge, it means that it can experience discharge currents of 10X the cell's capacity. So if you're running a 2000 mAh battery pack, you can place a motor load on the battery that can result in a 20 amp continuous discharge without compromising performance of the pack. 2000 mAh means the battery pack is capable of providing 2 amps of current continuously for one hour.

If you were to place a load on the pack that exceeds 20 amps, at best you'd shorten the life of the pack and at worst you'd permanently damage the pack (and create a risk of fire upon attempting to recharge).

When selecting a pack, you should review the ratings of the motor you wish to use to determine it's ratings for continuous current draw and burst current draw. You would then select a battery pack that A) has the capacity that gives you the desired amount of run-time, and B) has a discharge rating that's above the continuous current draw rating for your motor. This will assure you're operating both the motor and battery pack withing the parameters they were designed for.

Basically speaking, a pack with a higher discharge rating will not result in a noticeably longer run time. The lower cell resistance of a high discharge pack will result in fewer electrical losses and thus will result in more energy being transferred to the motor. But if maximizing run time is critical, you'd want to look for the highest capacity pack that either fits or is of the maximum weight you'd want to use.

With battery technology, there's always trade-offs...
Thank you, thank you. One of the best descriptions I have seen on these forums. The only thing you didn't metion is to find an ESC that will handle the current withouta large power loss.
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Old 03-29-2012, 09:52 AM
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Default RE: Lipo

You need to remember that the C rating is all about the ability of the battery to maintain output voltage under a load. If you are only pulling small % of the rated C amperage, a higher C rating battery will not give you noticeably increased performance. The other thing is this has NOTHING TO DO with the ability of the battery to stay cool to avoid puffing. A 50c battery will still puff up if you don't give it enough cooling.- even if you are only loading it at 20C.. In fact what I have seen in practice are higher C packs tend to puff easier than lower C packs..
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Old 03-29-2012, 10:20 AM
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Default RE: Lipo

Quote:
ORIGINAL: rgburrill


Quote:
ORIGINAL: fly24-7

''C'' is the symbol for capacity for a battery cell, and by extension, the battery pack. When a pack is rated at ''10C'' for discharge, it means that it can experience discharge currents of 10X the cell's capacity. So if you're running a 2000 mAh battery pack, you can place a motor load on the battery that can result in a 20 amp continuous discharge without compromising performance of the pack. 2000 mAh means the battery pack is capable of providing 2 amps of current continuously for one hour.

If you were to place a load on the pack that exceeds 20 amps, at best you'd shorten the life of the pack and at worst you'd permanently damage the pack (and create a risk of fire upon attempting to recharge).

When selecting a pack, you should review the ratings of the motor you wish to use to determine it's ratings for continuous current draw and burst current draw. You would then select a battery pack that A) has the capacity that gives you the desired amount of run-time, and B) has a discharge rating that's above the continuous current draw rating for your motor. This will assure you're operating both the motor and battery pack withing the parameters they were designed for.

Basically speaking, a pack with a higher discharge rating will not result in a noticeably longer run time. The lower cell resistance of a high discharge pack will result in fewer electrical losses and thus will result in more energy being transferred to the motor. But if maximizing run time is critical, you'd want to look for the highest capacity pack that either fits or is of the maximum weight you'd want to use.

With battery technology, there's always trade-offs...
Thank you, thank you. One of the best descriptions I have seen on these forums. The only thing you didn't metion is to find an ESC that will handle the current without a large power loss.
True. And I though about that after I finished my response. [:@] Of course, to complete the "system," you need a speed control that properly sized for handling the continuous and burst currents for the application and the motor selected needs to be sized for the application.

My background is airplanes, so I only know about motor sizing as it relates to planes. However, one thing I've found over the past few years is that the manufactures are getting better about providing more plain-spoken metrics for helping users select the properly sized equipment. For planes, you have a RPM per volt metric. For example, if a motor is 1000 Kv, it means that for every volt your pack is, you'll get 1000 RPM. So, for a 3 cell LiPo (11.1V rated), the motor will spin 11,100 RPM no-load. What would drive the current draw would be the load generated by the propeller. Manufacturers will generally provide propeller recommendations that will provide the desired thrust without exceeding the continuous current rating of the motor. There are also computer programs that calculate this.

I'm not sure how sizing works with car motors given that the load is not continuous as would be the case with the thrust of a plane. But, I'd expect that manufacturers provide recommendations for speed control and battery ratings needed to avoid damage to any of the components.

As has been expressed by others, cooling is an important consideration. Heat is big enemy of electronics. Whatever choices are made, You've got to keep things cool and ventilated.

Once last point. Particularly with LiPo cells, you should always use a balancer when charging. This is especially important with lower cost packs. One thing that's typical of higher cost packs is that the manufacturer will make a greater effort to bundle cells with very similar internal resistances. If this is not done and you have a pack with cells that have a wide spread of internal resistances, they will charge a different rates. What happens is the charger is looking at the average voltage for all the cells and will cut the charge cycle off when the average reaches the limit. If you have cells with varying internal resistances, it can result in some of the cells becoming overcharged and some being undercharged. From there, the imbalance grows greater and greater with each charge cycle. Eventually, things get so screwed-up that a cell can be overcharged to the point of reaching thermal run-away and catching fire.

The balancer will take the cells of higher voltage and divert charge into those of lower voltage to keep things - balanced. In the end, this will prolong the life of packs and greatly reduce the risk of a pack overcharging and catching fire.
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