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Old 10-07-2009, 05:13 PM   #46
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Originally Posted by devilchrist View Post
the internal resistance of each cell gets better during the initial break in cycles. 10-20 cycles seems to be the number being reported.

When i get new packs I use a p-touch to tape the IR on each pack and measure them during the course of it's life.
How do you measure the IR?

I used a 1-amp current regulator and digital meter that reads millivolts to measure voltage drop under modest regulated load, but the error of the meter in the millivolt range increases as the value goes toward zero.
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Old 10-07-2009, 05:35 PM   #47
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Originally Posted by linger View Post

Sorry, I didn't elaborate. I was referring to the method of testing a cell with the intent of determining the discharge C-rate. You start with a low enough C rate discharge that you know the cell will pass, and then you keep ramping up from there until the cell fails (usually temp, voltage cutoff or capacity). For example, you would start somewhere around 10C and go up in 2C increments. You gotta start low. There are "20C" labeled cells that can't handle 15C.

"time of discharge was used to calculate a C-rating" - no I think you are confusing C-rating with capacity, which ROAR does record for every pack that it tests.
Right - I did confuse capacity C and C-rate of discharge up there. So for proper C-rate testing, one would take the label capacity C from the maker of the pack. Charge and discharge the pack at constant rate 10C. Measure:

A. maximum pack temperature (what is the failure temperature?)
B. minimum pack voltage (what is the failure cutoff voltage?)
C. discharge time (failure time calculated from C rating and current C-rate)

Repeat this test in 2C increments until the pack goes into overtemperature, or hits the minimum cutoff voltage before reaching the rated discharge time.

This gives a process by which published C-rates would be valid comparables based on testing with the same temperature and cutoff voltage limits, no?
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Old 10-09-2009, 08:51 PM   #48
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Originally Posted by Pablo Diablo View Post
This is more to the point. "C" rating doesn't matter, voltage under load and punch/rip on the track does. While in general Higher C ratings indicate lower IR it doesn't guarantee better performance when racing rc cars that draw a fraction of the 40C the battery may be capable of.

After all this time using Lipo's you think someone would have done some good unbaised testing of the top brands, but no!

SMC say that their 6000 28C is a better performing battery than their 5200 40C in stock racing. What does that say about C ratings versus performance?

I have done discharge curves on quite a few different Lipo's and so far the Thunder Power/Fantom Pro Race 40C are the best. They are also the most highly regarded batteries on the track.

i wish rctvlive would get off their butts and do one finally.
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Old 10-09-2009, 09:28 PM   #49
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would there be a big diffrence on the track running a 13.5 motor in a 3200 7.4 28c and a 3200 7.4 25c the reasonI ask is I can get the 25c way cheaper and we only run this class that uses these in the winter>>>LMK Thanks
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Old 10-10-2009, 06:20 AM   #50
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would there be a big diffrence on the track running a 13.5 motor in a 3200 7.4 28c and a 3200 7.4 25c the reasonI ask is I can get the 25c way cheaper and we only run this class that uses these in the winter>>>LMK Thanks
The 13.5 isn't going to pull anywhere near the C rating capacity of either pack, go with the 25c pack if it's less expensive.
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