Pondering amperage...

[THEYTOOKMYTHUMB]

I'll try to keep this as brief as possible. I'm looking for clarification/confirmation on my view of lipo/esc combinations. I'm at the track last night racing the SC spec class. Most trucks are Slashes that have to use the XL-5 and 12t Titan motor. A conversation came up about the C rating on lipos. We were basically just comparing battery brands and different C ratings. The oval track we run is extremely short so laps are within the 4-5 second range in the SC spec class so it's really hard to tell a distinct advantage because the track is flat and the racing is tight. In certain situations more punch is certainly an advantage, but it's mostly just driving. The stock tires kinda limit how much you can really get on it coming out of the corners anyway. To kinda get to my point my argument to the guys stating that they want hotter batteries is that the XL-5/Titan combo is not really gonna benefit much from a higher C battery. Without looking up a bunch of numbers I'm gonna say that a 5000mah 20C run of the mill lipo can produce 100 amps fairly consistently. The same capacity 50-60C lipo will probably push 300 amps(burst) or more. The thing is though, the XL-5 is only gonna ask for 100 amps at the most. It's rated at 100/60, which is probably slightly inflated(like every other mfr esc or battery for that matter). So there's really no advantage with running a higher C pack with this particular esc, correct? Obviously a Mamba Max or something similar will take great advantage of the extra amps, but we're not running those. There was some debate. Basically, I'm the only one that saw it this way. I could tell though that it had them thinking maybe I was right. And I guarantee they will be thinking it over a little more. I'm not saying to run the cheapest lipo you can find, but there's no need to provide more power than your esc is gonna ask for. Am I missing something here? Please read all of this and post your opinions! I'm sure someone has definitive facts. Thanks

[Brandon G]

We were chatting about the same subject a while back when the spec Slash class opened up. Most were running 20-25C batteries. But when the 35C Reedy packs came out, they had noticably more acceleration. The back tabletop was being cleared by them while the rest of us were still landing on the top. So, even though the ESC's are the same, there was a noticable difference between the packs. In the hands of an experienced driver, it is an advantage, but the newer racers will find it will just make the truck more squirrely.

[civilguy]

I run a Trinity 20c lipo in my bone stock Slash. I raced a few weeks ago and was running a 6- or 7-pack-old Titan, so it wasn't 100% broken in but it wasn't green either. Another racer had a 40C lipo (who btw I consistently beat when we had relatively similar batteries). I could hold him off in the twistys, but on the straight it was like I was standing still. I have to say there is a difference, from personal experience.
And, no, I was not in training mode.

[THEYTOOKMYTHUMB]

On these 20-25C packs and the 20C pack do you happen to know the mah capacities? If they're under 5000mah they probably can't produce 100amp bursts which could account for some of the difference. I'm just trying to gather as much info as possible.

[civilguy]

Sure. Mine is a 4500mah Trinity Li-Pro (http://www.teamtrinity.com/pdf/4500_lipo_ins.pdf). I can find out what the other racer's was if you'd like.

EDIT: sent the guy a PM to get his info.

[THEYTOOKMYTHUMB]

This is good info guys. Real life accounts are a lot more reliable than any reading I can do. Anyone please add any relative info. I'm basically trying to decide how much to spend on my next lipo. I'm pretty sure I've looked at every option on the face of the Earth , so now I'm just going to decide which to get. Please no one turn this into which cheap lipo is best thread...we already have enough of those. Or, someone start a new one since it's been a while. I'm more interested in everyone's view on the amperage subject. I'm just trying to make myself comfortable with my first non Maxamps purchase. I love 'em, but they haven't run a sale in forever and I'd like to save some cash for some other stuff. My next lipo will just be 3rd for those nights where we have an M main.

[THEYTOOKMYTHUMB]

EDIT: sent the guy a PM to get his info.

Cool. thanks!

[jwscab]

So in a nutshell, the reason the higher C batteries are beneficial has to do with some physical and electrical properties.

-the higher the C, the more current the battery chemistry can deliver. This is mostly determined by the resistance(or impedance) of the internal cell structures. the higher the resistance, the lower the output. the reason being, is that the resistance is in series with the output wires, an unavoidable physical property, so by ohms law, any current passing through a resistance will deliver a voltage drop. this means less voltage delivered to the destination. this also the same phenomenon that you see when looking for current ratings on FET's in esc's. the lowest 'on resistance' wins.

now if there was a current limited load, then the C wouldn't be as noticeable, however, when you nail the throttle, and the motor shaft is sitting at 0 rpm, 2 things happen. the motor windings will draw a huge instantaneous current, and the motor will deliver the highest torque it can because of this large current.

the higher C battery can sustain that higher peak without dropping in voltage as much, thus the magnetic field strength is higher, thus more torque. this applies at any rpm, but is most noticable at 0 rpm, or in transitional periods where the current is applied progressively.

[THEYTOOKMYTHUMB]

But wouldn't the current be limited by the esc between the motor and battery? Good info. Not arguing, just asking.

[jwscab]

it can be limited, if the esc has a current limiter. But's only limited in current otherwise by the on resistance of the FET's. and that only becomes a problem when the voltage developed across the FET becomes high enough that it damages the junction, which is why they have ratings in current and breakdown voltage(FETs). The junction can also be damaged from heat, which is a byproduct of the resistance.

the current is actually somewhat limited by the physical nature of the conductors(wires, etc). so in effect, everything has a current limiter because current can't change instantaneously, which is good. Otherwise everything would pretty much get destroyed when you would turn it on by having infinite current. But you can easily see 100, 200 amp surges on a lower power circuit(say, something rated at 20 amps), based on the impedance of the conductors and voltage present. since this inrush is very very short, usually there is no damage.

all this is basic, (OK some not-so-basic) electrical theory, so you can find tons of info on the web about it, or you can ask me more. I do know some, as it's my profession.

[THEYTOOKMYTHUMB]

You clearly know a lot more than me! This is great stuff. So in your opinion what would be the maximum lipo rating that the XL-5 can take advantage of? In other words is there a point at which you reach overkill with an esc like this?

[jwscab]

as far as C rating goes, the better or higher the C, the higher quality the cells and pack are, which means more expensive, so you are only limited on how much money you want to spend on the C rating. they probably sort the cells by C rating coming off the assembly line, much like computer processors are sorted by highest attainable clock speed. there aren't different Intel chips of the same model that are 1.8Ghz vs 2.1Ghz, you are basically getting a cherry picked processor that can sustain the performance, but they all come from the same fab.

A good way to look at it is to figure out at what point is that tiny itty bitty fraction of a perfomance advantage needed, and do you want to pay for it.

technically speaking, you could dyno load the car, and capture waveforms to see what the maximum current the vehicle would ever see, then select the battery that will give you the least amount of voltage drop at that current. I would bet that it would be the highest C battery that you find, but at that point, it might be the difference of a couple tenths of a volt or less, and would not really be too noticeable.

so I guess all that just boils down to buy the highest C you can afford or justify, based on your desired capacity(4500, 5000, etc).

[THEYTOOKMYTHUMB]

So basically, I was wrong last night... Nothing new for me! Big time thanks for the info. At least I can rest easy knowing that a few more bucks might be well spent.

[jwscab]

no, you weren't completely wrong. the system will only pull so much current. at some point, going up in C with the same cell capacity just means that you will drop slightly less voltage. So while going from 20C to 35C might be a huge improvement for your particular system, going to 45C from 35C won't add too much to the performance. dimishing returns, if you will. Does that make sense?

so figure out what your system can pull, and then figure out what battery makes the most sense, and then upsize your choice by 1 step in C rating if you can afford it. again, this is all for performance too, if you are just bashin around, it doesn't make financial sense, or if your classes/drivers are pretty variable. in a stock spec race, you might get an advantage, as you have seen.

[THEYTOOKMYTHUMB]

Makes sense to me. Thanks again for all the info. Now can you explain how a flux capacitor works?