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Old 05-01-2009, 06:39 PM   #106
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Access-thanks for the post. Some numbers would be nice. Is the oscilloscope trace a full discharge. What was the average voltage?
John
It still won't let me post the URL. I know some forums don't like you posting the url of another forum they consider be 'competition'.

Here is the measured data. Basically the procedure was just to charge each pack to full on the charger, ie. until the open circuit voltage is exactly 4.2V (no 'trickle current'.) Then I set the scope to 5 seconds per division, and hooked up the resistor for 40 seconds, measuring the 5-second voltage and 20-second voltage from when the resistor was connected. I did this 4 times for each battery, consecutive, with no charging in-between.

All batteries were relatively new, they'd been through ~5 runs and not abused.

The venom 3s 4000mAH 20C pack tested the worst.
5 seconds: 10.6V, 20 seconds: 10.3V
(I gave up after one iteration here, not really worth testing anymore)

The thunderpower extreme v2 3s 3850mAH pack 20C:
iteration 1: 5 seconds: 11.25V 20 seconds: 11.0V
iteration 2: 5 seconds: 11.12V 20 seconds: 10.9V
iteration 3: 5 seconds: 11V 20 seconds: 10.8V
iteration 4: 5 seconds: 10.85V 20 seconds: 10.7V

The impulse power 4s 4000mAH pack 25C:
iteration 1: 5 seconds: 11.38V 20 seconds: 11.12V
iteration 2: 5 seconds: 11.25V 20 seconds: 11.00V
iteration 3: 5 seconds: 11.06V 20 seconds: 10.85V
iteration 4: 5 seconds: 10.88V 20 seconds: 10.7V

All numbers are approximate, measurement error +/- 0.1V roughly. I was really just trying to get a decent idea of the difference between a 'good' and 'bad' LiPo at the time. It should be noted that the impulse power cells are somewhat larger and heavier than the thunderpower pack, and the capacity is not an exact match either.

The load is a 0.25-ohm resistor, so the current draw was actually dependent on voltage. But it was the same resistor for each battery. The resistor used deans plugs, there was no real switch, I just plugged it in and let the scope trigger off the resistor voltage.

Notice how the voltages do move around, this is where the basic 'internal resistance' model fails. If it were true, each subsequent 5-second point would be less than the prior iteration's 20-second point, you'd just dropping voltages all throughout as the battery discharged.
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Old 05-02-2009, 10:24 AM   #107
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Access-Thanks. That screen makes a lot more sense now.

Voltage vs Amps
This data was requested. I will discuss how I obtained it. The data originated in two separate dyno runs of a Novak 13.5 motor, one with a 6 cell NiMH pack. This pack was not fresh. It was charged to a peak because this is the most consistent initial voltage that I could use with the given battery charger.

The second run is the same motor but with the Track Power LiPo. This LiPo is charged to 8.25 V which puts it at about 90% charged. This is a very consitent initial voltage and a green light starts to blink here. It is also a stable region of the battery discharge curve where voltage is not changing rapidly.

The blue line on the graph is higher than the red line; it shows more volts. This has more to do with the way I charged the packs than with a performance difference. Had I charged the LiPo full it would have a higher starting voltage than this particular NiMH.

I removed the first 3 high amperage points (on the right of the graphed lines) as there was a bad hook there. With these points we would not have a straight line. The rest of the graph is pretty straight until that flywheel reaches full speed. I removed the constant speed constant voltage part of the graph (at the left of the lines). What remains is pretty straight. I fit straight lines to the data so we could see the slopes which are related to the internal resistances. The LiPo line in red is less steep. This indicates less internal resistance.

The actual numbers for the slope may help System Theory with the Oval simulation or maybe not.
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-voltage-vs-amps001.jpg  
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Old 05-02-2009, 10:37 AM   #108
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Access,

URLs are blocked until you have a few more posts up. Same thing happened when I joined. I don't recall how many posts you need, maybe ten or twelve.

I added my comments on battery testing at the end of this thread:

http://www.rctech.net/forum/5505344-post131.html

If this is not clear I can add some description.

I attribute the gain in power on John's Dyno primarily to lower internal resistance, since nominal voltage is roughly the same for all 2s Lipos and he's using the same motor/ESC/Dyno.

The lower IR is typical of a higher capacity battery or a lower capacity battery optimized for very high discharge currents. This is a real advantage out on the track when geared properly for heat, as John explains.

Comment: the extra power due to lower battery resistance appears on the Dyno only if the speed control does not limit the extra current made available, based on peak current ratings in the transistors or thermal shutdown. This is fairly obvious but deserves explicit mention.

Last edited by SystemTheory; 05-02-2009 at 10:56 AM. Reason: Add comment
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Old 05-03-2009, 08:44 AM   #109
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www dot rcuniverse dot com/forum/m_8310589/tm.htm

I just added a test for one of those 40C packs, specifically a 'THUNDER POWER RC 3200mAH Pro Race 40C' 2s pack. Best lipo I've tested yet.
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Old 05-03-2009, 08:50 AM   #110
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I suspect your web addy got abbreviated. The link did not work. leave out the www. maybe.

Test of the "Big Dogs" to come
LRP X12, 3.5 vs Novak Ballistic series 3.5
The Novak motor is in the mail. Should be here any day. I will test both on the SMC battery. We should break 700 Watts easy. 747 Watts = 1 Horse Power. Note the LRP 3.0 motor is really intended for 5 cell. It is not reccomended for use with 7.4 V using the newest sphere comp tc edition.
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Old 05-03-2009, 01:43 PM   #111
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Access battery tests:

http://www.rcuniverse.com/forum/m_83...tm.htm#8736389
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Old 05-03-2009, 03:45 PM   #112
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Measuring the Internal Resistance of Battery

Figure 1 shows a controlled current regulator attached to the battery through a switch. Standard capacity testing for NiMH used Ic = 30{A}. The clock starts when the switch closes, measures time in seconds, and it stops when measured battery terminal voltage Vt = Vc, the cutoff voltage. The standard cutoff Vc = 0.9 volts per cell in the series stack.

Notice source voltage Vs and source resistance Rs in the "lumped parameter" battery circuit model are variables. In the short run these values depend on the State of Charge / Depth of Discharge and battery temperature. In the long run there are also changes in the chemicals that impact State of Health.

Since both Vs and Rs can change independently during discharge inside the battery, and one only measures Vt and Ic, one cannot really measure source parameters during continous discharge, either on a test bench or out on the track.

To get a curve for Vs and Rs the technique is to chop the switch at specified time intervals, measure Vs at the terminals with zero current, then apply Ic again and measure voltage drop Vt. If you standardize this test and use good precision in the current regulator and voltmeter, you get a curve of Vs and Rs over the depth of discharge given the state of health of the unit under test (UUT - actual battery).

This link shows one effort to produce the Vs and Rs curves for older batteries:

http://www.slkelectronics.com/ecalc/gen4a.htm

John's curves of Voltage versus Amps compare Vt to Im during a Dyno run. The simplest equivalent circuit model appears in Figure 2.

Rm is the motor resistance, which primarily varies a function of temperature due to the resistivity of copper. Vbe is the reverse voltage which increases with motor shaft speed due to generator feedback in the coils. It also varies a bit due to changes in air gap temperature.

There are four variable circuit parameters which impact the measured Vt and Im, so it is very doubtful one is measuring Rs in any accurate way.

However, if the peak power on the flywheel is much greater with a battery swap, other factors kept equal, and both batteries have roughly the same initial open circuit voltage Vs, and the same motor/ESC/Dyno combination is used at roughly the same starting temperatures, then the gain in peak flywheel power is very likely due to a reduction in internal source resistance Rs for the battery that supplies more power to the flywheel.

This should show up as more raw electrical power output at the terminals, where John could multiply each line Pe = Vt*Im and get a battery power curve. The better battery would supply more electrical power at its terminals compared to the lesser battery as measured on the Dyno. This power crosses the air gap and produces a bit more peak mechanical power.
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-battery_test_ckt.png   Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-motor_ckt.png  

Last edited by SystemTheory; 05-03-2009 at 04:08 PM. Reason: added link; improve description
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Old 05-04-2009, 06:43 AM   #113
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guys,

I got the spreadsheet from John and made a few mods on the curve fitting to suit my light flywheel and adapt the initial start time to fit the acceleration curve best to the measured data. Overall it works okay, if you keep in mind with a light flywheel the Sentry 0.1s resolution is a problem for the initial start and then the RPM resolution gets you at the end near steady state. Anyway, here's some comparitive results for those interested in the new LRP gear.

LRP SPX Stock Spec ESC
LRP X11 10.5t
SMC 5000mAh 28c

I'm using the Fantom dyno mount and alum flywheel.

I did 3 runs of each profile and chose the best indicative result for each profile (all runs were very close). I also include a graph of the RPM and Acceleration curve fit so you can see where it gets shaky...

The current curves are very interesting (keep in mind the current reading on the Sentry is clamping above ~110A). You can clearly see the punch levels, the profile 1,2,3,4 increase in punch and it looks like (1,2,3) & (4,5,6) & (7,8) have similar timing profiles. You can also see the new Stock Spec profiles 5,6,7,8 increasing in punch, with 5,6 having a similar timing profile as profile 4 and profile 7&8 having large amounts of timing past the power peak. Any wonder you need to be careful on 7&8 with the motor pulling 30A at almost no load.

I'm going to get a heavier flywheel and repeat the tests later.

Chris...
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-lrp_spx_x11_10.5t_all.jpg   Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-lrp_spx_x11_10.5t_p5678.jpg   Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-lrp_spx_x11_10.5t_p1234.jpg   Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-lrp_spx_x11_10.5t_allcurrent.jpg   Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-curvefit_example.jpg  

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Old 05-04-2009, 09:38 AM   #114
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Very nicely done! You can really see how profile 1 and two are limiting power for slippery track use and how profile 7 and 8 are boosting the high RPM power at the expense of greater amp draw. Thanks.
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Old 05-04-2009, 11:59 AM   #115
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Very nicely done! You can really see how profile 1 and two are limiting power for slippery track use and how profile 7 and 8 are boosting the high RPM power at the expense of greater amp draw. Thanks.
As a general principle, is there a diminishing returns or inefficiency for high power levels, especially at lower RPMs? For instance, at 5000rpm, the strongest profile (3a) produces a little shy of 200W while the weakest (1a) produces 150W. But (1a) consumes only around 57A (2.63A consumed per watt delivered), while (3a) consumes 90A (2.22A consumed per watt delivered). Are the battery terminal voltages from all these tests similar enough so that amps are proportional to electrical power consumed? And what is going on with (8a)? Is it stepping up the timing advance too soon? Perhaps the majority of these power profiles are made for actual driving, where vehicles get up to speed much quicker than a several second dyno test.

What happens if you set it to the lowest profile and start the test at 50% throttle and walk it up to 100% as you hear the flywheel 'revving up' (increasing in pitch)? Granted the power delivered will be lowered, but does it become even more efficient? I know in real actual driving, you save fuel by driving with a 'light foot' and waste fuel with a 'lead foot' aka 'petal to the metal' driving. That is an ICE engine, is there a similar principle when it comes to powering vehicles with electric motors?
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Old 05-04-2009, 01:32 PM   #116
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I can answer maybe a few of your questions. It does seem from this test and the test that Matt did on boost that the extra power comes at the expense of efficiency. At high RPM the extra power is made by advancing the timing. This tends to kill the efficiency. At low RPM timing can be retarded to boost low RPM power. This probably improves efficiency. In addition current can be chopped and increased on a ramp like profile 8 to make a profile suitable for low traction or partially chopped to produce other profiles. I had to do this with my radio to make the LRP motors rev up smoothly on the dyno without stuttering. Note that the lower gearing on the track eliminates this problem.

My track experience with the hot motors tends to answer your question about part throttle use. It certainly saves a lot of power consumption and heat to roll on the throttle just fast enough to stay ahead of the speed. Full throttle needs to be used sparingly with hot winds. At part throttle the current is rapidly chopped. This restricts the current that can flow.(On brushed motors a recirculating current used to flow through the shottky diode in the off periods boosting efficiency. I don't think this applies to brushed). Also with hot winds a low gear is used to let the motor rapidly rev past its highest amp stage (at low RPM).

I have noticed in the stage of running past our dyno run that the motor will tend to become optimized and run more efficiently and pick up some extra RPM. This would probably occur during your 50% throttle run up.
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Last edited by John Stranahan; 05-04-2009 at 06:45 PM.
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Old 05-04-2009, 03:39 PM   #117
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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
I just want to make a note that this guy posting as "Hacker" has no association with Hacker brushless motors. I have had a few people ask if this was "Aero-Model/Hacker Brushless USA" I just wanted to make that perfectly clear.

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Aero-Model/Hacker Brushless USA
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Old 05-04-2009, 03:41 PM   #118
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I also wanted to note that the Thunder Power 5000mAh battery is by far the best battery I have used to date and the SMC guys have some work to do.
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Old 05-04-2009, 04:30 PM   #119
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I also wanted to note that the Thunder Power 5000mAh battery is by far the best battery I have used to date and the SMC guys have some work to do.
The 40C ones? I tested one late yesterday and also got the best results out of any battery I've ever tested. I wouldn't doubt that right now these are the best batteries out there. I don't think anyone can beat 7.769V after 30 seconds of 10C discharge.
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Old 05-04-2009, 06:17 PM   #120
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The 40C ones? I tested one late yesterday and also got the best results out of any battery I've ever tested. I wouldn't doubt that right now these are the best batteries out there. I don't think anyone can beat 7.769V after 30 seconds of 10C discharge.
What about the reedy 5000 35C packs or Trinity 35C 5000mah packs?
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