1st). What advantage would a lipo 5000mah 2s 100c have over a 5000mah 2s 50c in a Stock Slash 2wd running the XL5 and Titan 550 motor?
2nd). What advantage would a lipo 5000mah 2s 100c have over a 5000mah 2s 50c in a Slash 2wd with a Velineon or Castle 3800?
I know there is the capability of the higher output with the 100c but is the a point when it doesn't matter?
The extra capability doesn't matter when you are a good margin above what the power system is rated for.
I also hope the 100C was just an example... I have yet to see a pack that is actually 100C.
Whatever it is I just typed... could be wrong.
Just my opinion... but from what I understand on their testing alone that pack is not 100C.
Whatever it is I just typed... could be wrong.
Agreed. I am reasonably sure 100C is not possible with the state of LiPo technology.
Alt-248 on the number pad = °
The standard accepted value for C rating as I understand it is the capacity of the pack x the C rating. The pack pictured above is 5.6A x 100c = 560 amps. I cannot believe those bullet plugs or wires will handle that, much less the cells.
Realistic C ratings are usually in the 15 - 35C range from my experience. A 5000mah 25C battery for example can deliver 100 amps continuously without damage. Most batteries can also handle a 'burst' current of up to double the continuous rating, so 200 amps for a short duration. This depends on the pack's design and manufacturer however.
If your packs are adequate for your system C rating really isn't a huge factor. If your system pulls a max of 100 amps at burst load and you have packs that are rated for 100 amps continuous load then going to a pack rated for 200 amps continuous isn't going to do much, the system only pulls the power it needs.
One step further - The true definition of C rating as it applies to the cells should be viewed more as the maximum Amp discharge rate the cells can sustain before damage occurs. Think of the C rating as similar to the RPM redline limit in your full sized car. As the engine approaches the redline the likely hood of potential damage increases, which is also directly related to a potential decrease in total engine operating life hours. Taken to the next level and exceed the redline and damage will occur. The C rating of LiPo cells is very analogous, well at least to make it understandable.
As for higher C ratings not making much difference in performance, especially if the vehicle and motor system is not that demanding; well that is not completely true. First of all we have to assume (I know, I hate doing that) that when looking at packs available from a vendor and with each step up in C rating, there is truly a relative improvement. This improvement if present can be readily measured as a decrease in cell IR's. By the way, this is the real world and best way LiPo packs and cells should be performance-rated, by cell IR's. This would completely eliminate all the bogus C ratings. Anyhow, if the IR numbers improve as the relative C ratings go up, this means that for any given Amp load, the pack with the best (read lowest) IR's will hold voltage better under that same load. This higher voltage will immediately translate to more power generated by the motor.
Lastly, the vehicle does not "pull" Amp loads. All energy for a RC vehicle starts with the battery pack potential which is then released as the connected downstream impedance allows.
Last edited by Jakey; 02-27-2014 at 12:29 PM.
In a nutshell:
1- no noticeable difference
2- no noticeable difference
None of those systems can pull enough juice to hit the theoretical limits of those lipo specs. The quality of the lipo cells, and how high they hold their voltage under load, will make more difference. That's not represented by just the C rating.
I see your point about the voltage drop however, and if I'm racing and worried about my lap times down to 1/10 of a second then the high dollar packs are probably worth the coin. For myself and other bashers I would go for having more of the less expensive batteries and run longer overall.
In my last post I never meant to imply "high dollar packs were required", only that lower cell IR's do result in less voltage drop for any given Amp load, even if that Amp load is well below that in which the cells are capable of. Now if in fact the end user does not care or cannot tell the difference between a pack with cell IR's of 4.0mΩ per cell versus a pack with cell IR's of 1.8mΩ per cell, OK, that is fine, but I was just pointing out the EE side of the issue.
Last edited by Jakey; 02-27-2014 at 03:06 PM.
I don't have an engineering background so I'm spitting into the wind trying to talk intelligently about that kind of thing. My experience is all from running things and watching others run. It's neat to get input from people who have specific experience, information and hard data about how these things function internally.
No problem at all.
Thanks for the input.
I have to give some props, here! I used to race and bash several years ago. Bought my Stampede and Bandit back in 2002 and am in the process of rebuilding them. I just rekindled the flame for r/c...so I have a LOT to learn. For example, my NiCad batteries are not only toast, they've been left in the dust with the newer technology, today. Not only have I missed out on NiMh batteries, there's this new thing called "Lipo" (I know, short for Lithium Polymer). Anyway, I'm doing my homework...quickly trying to come up to speed with the new technology. Heck, brushless was just entering the market when I got out.
Oh...yeah. Back to my props. Thanks, guys, for this thread! I just learned a ton in a short amount of time! As I DO have a technical background (mechanical designer for an electric motor manufacturer), I appreciate the depth of this discussion. Thanks for helping a brother out!
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