Got a question for y'all...
 

If my dragster discharges about 185mAH in 1.4 seconds on a given run down the drag strip, what do you think is the burst amp draw of the system?

not enough info.

I will say a maxcramps lipo could put out 17 kilajoules of power in a single dragstrip pass, underwater and unbalanced

I take it this is no help?

.185Ah (3600sec/hour) /1.4sec = 475Amps average

(at this rate, you would discharge a 5000mAh battery in 38sec)

475A? :neutral:

Whats the mah capacity of the pack- you could work out how many Cs that is then. Assuming 5000mah though, thats... 95C burst. Wowzers...

Is hard to tell though. You would need a high speed high amp logger to really tell. Highest Ampdraw is at 0 rpm and then you start to move and ampdraw goes down.

All I can say is: :lol:

It also gives 475.78 A, but I think that's the average draw.

I think that's right, but the burst would be cool to know.

40C/80C 4200mAH 3S Thunder Power lipo. So the burst rate might be higher still. About 113C.

Agreed. But in the absence of high frequency and high amp data loggers we can guestimate till the cows come home :intello:

So if the average is 475 A, could the burst be about 950 A? I think it might be possible, or more even. :surprised:

Just looked at the link that pinkpanda3310 posted, and it does list burst current.... at 2,854.69 A :surprised:
I used the pack voltage as 9V because that's about where it's at, at the end of the run (going on EagleTree data from the past, pack voltage as low as 5.5V at the initial throttle hit). Based off that, the link also lists:

Average Power: 4,282.04 W (5.74 hp)
Estimated "Burst" Power: 25,692.24 W (34.44 hp)

Is that (burst current and burst power) even in the realm of possibilities? :oh:

If that amp reading is right, I know the cells will not stay at 3.7volts per, but using that, it says you hit 5272.5watts:gasp::gasp:. divide that by 750W(roughly 1HP electric) you get 7.03HP:surprised::surprised:. I'm impressed, wow

Edit: Just read sunny's post before mine, wow!! I believe some of you are really starting to maximize the power of brushless and starting to get the true power out of it

I think so too. But hard to say what'll happen if you sustain that level of performance for more than 2 or 3 seconds.

If indeed you are getting 25KW peaks, we're talking microseconds here. Any longer and everything would go POOF! 475A seems like the real peak A draw, average I would guess is around 60A, sustained maybe 40-50A.

BrianG if you could confirm this I'd appreciate it. :lol:

Personally, I think that 185mAh in 1.4 sec IS burst for the most part.

That power calc page is meant to calculate average draw over a much longer run (statistics work better with a larger sample). Using it in that 1.4 sec duration will result in pretty skewed results.

Note that 5275 Watts is the input power, you must mulplicate it by the efficiency of the system to get the real output, and during bursts, efficiency drops, doesn't it ?
Lets say 70% efficiency, that gives how many output HP ?

I think so too (skewed). I just wanted a ballpark number for the peak current in that 1.4 seconds. My conservative guesstimate is 950A like I posted above, even for a milisecond or two.

Have you tried this calculator? This one is more specific to a given set up but will still only average out the power per run.

By the time your finger gets to full throttle, the motor is already building up revs. So my guess is the peak draw on the system is in the first 0.2-0.5 seconds. Data loggers sample every 0.2 sec don't they?

Just realized the data logger thing I said ><

950A. I don't think so. You maybe could with a bigger motor and dual 5A 50c batts in parallel, but a single pack with a 1/10 motor+MMM, no.

That calculator is also very general. And the sample rates of the data loggers available for our little RC budgets are not enough to get any sort of usable data in 1.4 seconds. Very soon, that number will go below 1.3 seconds for me.

@snellemin: but that is also speculative isn't it? So what do you think the peak is if the average over 1.4 seconds is 475A?

475A sounds about right, as posted before. Why don't you do a 30 second burnout and datalog the ampdraw. You could then see the corresponding voltage under load. You can get a pretty good idea what your battery is capable off with your particular setup.
Do you still warm up your batteries before a run to reduce the IR of your batts? Do you cool down(spray it with something) the ESC at the Tree, so you get the most out of your ESC in that 1.4 second run? Do you have caps added to the MMM?

That's a good idea. How about a home made dyno? Get a trolley wheel or something the same weight as your car and rig it to the back wheels. Add a brake to it to simulate wind drag. Might not be worth the head ache to make but just a thought. I'm not an electrical buff (I know, surprising isn't it :oops:) but there must be a more accurate power analyzer you could use then. The weight of it is no longer an issue.

A burnout won't put the same load as a fully hooked launch. As for battery warming, I only do it on days that ambient temps are in the 70s or lower. I like my packs to be around 95 - 100 degrees where I seem to get the best results. And no special cool down of the ESC and no extra caps either, even in the 1.3 second passes I've done.

They could've been 1.2 passes Sunny. Fets are rated at 25C and not the Texas 37C heat.
A burnout will tell you voltage drop under a certain load. Keep adding pressure to the rear pod for an increase in resistance/ampdraw. See if you can reach 170A with your eagletree in live mode.
Then use your eagletree in a live run and record the voltage drop at startup. There is no point in recording ampdraw at that point as the 10hz recording speed isn't fast enough. The voltage however doesn't rise as fast and the eagletree will catch it. That Voltage at near 0 rpm will tell you what your battery is made out off.

I might not have the fastest reaction time or be as good of a driver, but I can setup your electronics for max performance. That is why there are drivers and engineers in F1.