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Old 03-22-2012, 09:23 PM   #16
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I have found the higher C packs are better for the high grip applications and lower C rating packs work on low grip applications. Maybe slower is faster for me.
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Old 03-22-2012, 09:47 PM   #17
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Originally Posted by martin660 View Post
Maybe slower is faster for me.
Slower is faster for most people. Slower laps with fewer mistakes usually beats faster laps with a handful of errors.
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Old 03-22-2012, 09:50 PM   #18
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Originally Posted by wingracer View Post
Never heard of a thermocouple I guess
Do you have one on your rc? Give me a break.
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Old 03-23-2012, 12:48 AM   #19
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OK time to chime in I suppose, First thing to look at is the capacity not the C rating, the capacity is in effect how much go juice is in the tank, you can buy some pretty sweet lipos cheap with around 5000mAh, the C rating is its discharge rate and sorry to everyone without degrees in electronics, but with the standard cabling in RC you would be in trouble with a 40C 5000mAh burst.

Now lets do some maths, say if you completely drained a 5000mAh pack in a 5 minute heat, that means it can sustain a constant current of 5 amps per hour so lets average this out over a heat, 5 mins is 1/12th of 60, so to drop a pack completely in 5 mins you would be drawing 5000mAh x12 which is 60 amp continuous.
Then we will double that just to be generous and you will have bursts of around 120Amps. now 120 Amps at say an average of 7Volts will create 840 Watts of energy, now go around your house and start comparing all your appliances and you will get where I'm comming from.

Also increased temperature in packs reduces voltage due to increased resistance , google temperature coefficients, unless the pack was completely made from carbon.

All in all when it comes to buying a pack start by getting a 5000mAh pack and then as long as its over 20C then you will be fine, unless you are super pro and drive on rails with absolutely no variation in consistency.

RC mushroom has the zeppin lipos for $27 usd, I have a couple and run 13.5T super stock, I am competitive and usually have a consostency of around 0.3 which isn't bad considdering the super low traction we race on.
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Old 03-23-2012, 04:06 AM   #20
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Originally Posted by CarbonJoe View Post
The theory is that the lower C rated packs heat up more internally, which generates higher voltage.
Assuming this theory could be true... The advantage would need to be taken during the charging process correct? Once a battery's usage begins it's voltage certainly would not increase under exhaustion.?.? since max. V is tech'd at big races, nobody using any type of pack has an advantage here unless they are not able to maximize their voltage @ a 1C charge rate.

Through all of what I've read, battery performance is really about sustainability, not power.
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Old 03-23-2012, 05:15 AM   #21
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Some measured data, instead of speculation and heresay. Notice that the middle line is the pack at "room termperature". Remember a couple of years ago when people were heating their packs? This is why.

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Old 03-23-2012, 05:35 AM   #22
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Some measured data, instead of speculation and heresay. Notice that the middle line is the pack at "room termperature". Remember a couple of years ago when people were heating their packs? This is why.

Your example is for the same pack and in no way supports the theory that a lower c rated pack will become "more excited" and thus releases more power. It also doesn't account for any temperature rise during use. How much does a lipo heat up when it is used in a rc car. Controlled temperature tests may produce these results but it does not reflect a real use situation. Furthermore it shows absolutely no comparison of a cooler high c rated battery vs a warm low c rated battery. The pack may make more power at temp but that doesn't mean its going to outperform a higher c rated pack.
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Old 03-23-2012, 08:07 AM   #23
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Quote:
Originally Posted by CarbonJoe View Post
Some measured data, instead of speculation and heresay. Notice that the middle line is the pack at "room termperature". Remember a couple of years ago when people were heating their packs? This is why.

Blue line = heated pack during charge, v does not increase during discharge
Pink line = nominal temp during charge, v does not increase during discharge
Yellow line = below nominal temp for lipo operation, v does increase during discharge

The supplied charts do not support your theory, unless the argument is that lipo v will increase in sub-par temp conditions of (40f-5c)

Missing information
Would be useful to see indicated temps during discharge
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Old 03-23-2012, 08:33 AM   #24
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The only way to establish any performance benefit is to discharge on a test righ and use that to measure the internal resisitance.
Over a 5 min race you want the voltage to remain as high as possible and the resistance to remain as low as possible to maximise the punch and power.

Lipo batteries have the lowest internal resistance when warmed, that's why so many people were doing it.
Pre heating has sisnce been banned in the UK by the BRCA.

I notice that my packs heat up by around 10degC by the end of a race (in cold temperatures) and I also notice no difference betwwen my 1yr old Thunder Power 65C 5300 packs and my 3yr old Thunder Power 40C 5000 packs.
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