SystemTheory

Oval Car on Inside Line

Same model as above with two changes. If aC = 2.2 g's a car on the inside line starts at the acceleration point with initial velocity vI = 19{mph}. Gear ratio is G = 2.44 (rather than G = 2.00). So vI and G change in the model.

If full throttle is applied at the acceleration point (it is not), the graph below shows time moving from left to right, but operating points of the motor on same axis with kinematic quantities acceleration, velocity, and displacement.

This car could punch out at 1.3 g's but probably comes off below that due to throttle profile, it takes more time and displacement than shown, and it may wind out around 30-31{mph} before taking time and displacement for braking.

An accurate acceleration model for this car would require a throttle profile used to modulate the torque-speed source. An accurate lap time requires a better braking model too. However the basic principle of gear selection shows up in the graphs.

John Stranahan

iTrader: (27)

System Theory-Thanks for the comparison. I should be getting you a few more numbers. It may take some time. A throttle profile, total driveline inertia are possibles. No real brakes are used. Just a drop to zero power. The cars roll about 3/4 length of the track by inertia alone with no brakes.

More Battery Test Trackpower 5400 mA-h 16c gold brick LiPo vs SMC 28c 6000 mA-h hard case brick battery.

Note that the SMC has more capacity. This will always give you a voltage advantage even with the same chemistry. The difference in capacity is not huge, however.

Hacker brought up these SMC cells which are also available in 40c rating. I ran the cell in the same manner as my Thunderpower. I charged it to 8.25 volts (first blinking green light on my Scorpion Charger). I am almost embarrased at the result. There is a huge advantage to the SMC chemistry and extra capacity. The Novak GTB pushing the old Novak Velocity 3.5 R Puts out a best ever 673 W on the dyno. This compares to 524 W for the Trackpower Battery. Well Hacker all I can say is thanks. I ordered a second SMC pack to race open mod touring with 2s LiPo. The capacity will provide a cushion against dumping. Motor heat will be the limiting factor in the race requiring careful gearing to keep below the smoke range.

This is just a phenominal difference. I can hardly believe it. The data is obtained by two Sentry dynos.The track power on Matts. The SMC on mine. I use a 2.06 amp correction with the new sensor installed and calibrated as before with the turbo thirty. Since only flywheel acceleration data is used to calculate power, I have to believe it. I did use the same flywheel speed control and motor. I will rerun the Thunderpower on my dyno as I get time, though. Max RPM increased about 4000 to 64,000 RPM

We'll just keep this our little secret. (just kidding)

This SMC amp curve pegged at far left at 225 amps. Every point after that is lower but only slightly for the SMC battery. NICE! Every indication is that this amp curve is correct.

John Stranahan

iTrader: (27)

I reran the Trackpower (note I was in error on the brand) 5400 mA-h test with a Novak GTB speed control and Novak 3.5 motor (not the new ballistic series). I measured 528 W with my dyno. Originally I measured 524 W with Matts Sentry and connectors. That is very good agreement. I am very satisfied with the results in the previous post. There was no connector or wire problem contributing to lower results with the Thunderpower battery. The SMC battery just kicks ass in this test. The battery is about 1.5 ounces heavier than the thunderpower. I removed this much weight from my ballast on the TC.

You have to hand it to those DEANS connectors for performing this reliably at huge amp loads.

hacker

iTrader: (141)

John, nice to know the smc's are doing their job!

Now spend some more $$$ on the 40c 5200 version, and see how this compares to the 28c 6000

capacity v's potential output.......interesting

John Stranahan

iTrader: (27)

Hacker-Thanks for the post. Get Danny to send me one of those puppies and I will give it a test and return it. Link to the test above. I did check with him on this cell and it is ROAR approved and should appear on the next list, I believe in May.

petk

So now gearing not only depends on motor, track and ability to drive, but also the battery. The difference between the 2 batteries, means that the motor performs best at different rpm resulting in different optimum gear ratios.

Let's hope that batteries from the same manufacturer, doesn't differ too much.

Best,
Peter

ray_munday

iTrader: (4)

Great thread guys.

One question John - were you logging the battery voltage through these tests? Plotting the voltage vs. current draw will give a line showing the internal resistance of the battery (as well as pure voltage). I have done this test on the car with Nimh vs an IB3800 25C Lipo and the difference was marked (see graph attached) but it would be really cool to compare between different Lipo packs. Note that this data was taken with an Eagle tree unit (I have both Sentry and Eagle Tree loggers).

Regards

Ray

John Stranahan

iTrader: (27)

Ray-I have that data for the Track Power. I'll see if I can get a plot up later.

PetK-Agreed. In all my racing though I have always attemted to have a least a couple of identical packs. I can see where mixing in the trackpower with the SMC might cause timing errors on the track as well as the gearing problem you talked about.

Access

Internal resistance is not necessarily a straight line, it's affected by the battery's internal temperature and other factors, even if you neglect charge state it is not a straight line. It's just modeled that way for simplicity's sake. In reality even if a battery is half discharged, just letting it sit for a while will cause the voltage to go back up a bit.

I did some testing of 3s LiPos at the beginning of this year and found the difference between a 'good' LiPo and an 'okay' one to be as much as 0.78V or 0.26V per cell with a 0.25 ohm load (roughly 10C) as measured 5 seconds and 20 seconds into a 40 second discharge. Using the internal resistance model, these differences are going to get even larger as you pull more and more current.

I'd post the url for my testing and all the plots/graphs but right now I am getting an error from the forums "You are only allowed to post URLs to other sites after..."

The resistors used in this test have a low but not unmeasurable temperature coefficient, ~200ppm/C (2% per 100C). While I was doing the test the resistors melted through a polycarbonate shield, if I were to do the test again I'd have to cool them down between runs to keep it totally canon.

I don't think anyone can dispute the importance of good batteries especially if you are racing. Charge method can also affect performance, I did another experiment to show the effects of overcharging, which unlike with the other chemistry batteries (NIMH or NICD) is beneficial to performance with LiPos. Though not without other potential drawbacks.

ThunderPowerRC

John In your post review of two batteries you make reference to Track Power and Thunder Power. Which is it. I think if your ran your test with a true Thunder Power 40C you would see a different result. We have the lowest IR and highest voltage/current draw of any battery. Our test and track results are prof of this even in the 21.5 spec oval class TQ's and wins.

Access

In my tests the "Thunderpower Extreme V2 25C" was was roughly equal to Hobbypeople's "Impulse Power 20C" brand -- both tested very well.

John Stranahan

iTrader: (27)

Thunderpower- my apologies. The pack I tested was a Trackpower. It was described correctly initially. I had made corrections by post #93 but not in time. I would be happy to test a 40c Thunderpower on this dyno. Send me one and I will test and mail it back. That test with a 3.5 motor seems to be unusually discriminating of battery performance under high drain. Give me a PM. I did test an early Thunderpower pack with good results in another thread. Again sorry for the mixup.

Access-thanks for the post. Some numbers would be nice. Is the oscilloscope trace a full discharge. What was the average voltage?
John

mattnin

iTrader: (44)

John, did you ever finish the new mathematical model? I would like to replicate it on the open office version of the spreadsheet.

John Stranahan

iTrader: (27)

Matt-The latest Sentry Dyno Master version is on the way. You can get the spreadsheet by e-mail now at [email protected]. A couple of guys have done so already. It should work for other data aquisition devices as well. Two readers are adapting it to their device. If you have luck please post. Matt may post a dowloadable version in the near future in excel and open office. I only have the excel version.

I look forward to a Battle of the 40c packs. I can run them on 17.5 and then on a 3.5 motor. I can't really afford to buy packs just for a test. I can afford to mail one back promptly after this light use.

ray_munday

iTrader: (4)

Ok thanks. I now have a set of IB4200 saddles so I can do some testing with the 4wd - Ill post up my results when I have them. The 4wd allows you to pull a much higher current draw (the 2wd is limited as it wants to wheelstand).

Sure. My on vehicle testing shows it to be roughly a straight line, which makes it easy to compare from one pack to another. When i get the chance, Ill dissect the results of a full run and see if the slope of the I/V curve changes over the discharge.
If you want to post the url, just type dot instead of the . and we will be able to work it out Sounds like some useful results you have.

Ray