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Old Feb 15, 2013, 10:03 PM
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hypothetical on "C" rating

I have a setup that pulls about 30 amps and initially holds 10.75v on a TP 25/50c (33.7/67.5a) 1300mah pack. If I get a 1350 65/130c (58/117a) pack about how much voltage should I expect it to hold?........roughly. 11.5 volts????
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Old Feb 15, 2013, 11:54 PM
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you will pretty much have to try it , just because it says 65c don't meen its so ...
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Old Feb 16, 2013, 12:10 AM
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I kinda figured it would be a try and see but thought someone might have some experience with it. I'm pretty confident the thunderpowers are gonna be on the top of the pile as far as realistic ratings judging by how strong their new 25c's are. ecalq says about .75v. will see tommorrow
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Old Feb 16, 2013, 08:51 AM
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Let's assume 65C is more than just marketing, and means that it is determined by the current that can be drawn while keeping the cell voltages safe, say 2.9V each (that may be too low for today's batteries). So, 65C would mean 87.75 amps at a drop of about 3.8V gives a total IR, including wire and solder connections, of about .043 ohms. That means, at 30 amps, there would be a drop of about 1.3V, so the predicted voltage would be about 11.2V, under ideal conditions. Take your measurements, and you will see how real that 65C number is in practice.
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Old Feb 16, 2013, 09:01 AM
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Huh?
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Old Feb 16, 2013, 09:49 AM
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"Holds a voltage", not exactly, it is continuously decreasing from the time the plane takes off to the time it lands.

Check out these discharge graphs, all at a 30A draw, packs ranging from 25-65C (If I read that correctly). http://www.rcgroups.com/forums/showp...15&postcount=1

Looks like the difference is about 0.2V(3s pack) between the highest and lowest pack voltage at any time during discharge.
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Old Feb 16, 2013, 11:10 AM
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Quote:
Originally Posted by bill clark View Post
I have a setup that pulls about 30 amps and initially holds 10.75v on a TP 25/50c (33.7/67.5a) 1300mah pack. If I get a 1350 65/130c (58/117a) pack about how much voltage should I expect it to hold?........roughly. 11.5 volts????
Just use the Discharge/IR calculator to see what you really have and what you need. Takes all the guesswork out of LiPo selection. http://www.jj604.com/LiPoTool/



.
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Old Feb 16, 2013, 11:48 AM
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Huh?
That was a thought experiment, assuming all the resistance of the battery system is accounted for in the C rating, which it is not. I made that assumption just to simplify the math, so I could illustrate just how small an effect that more than doubling the C rating would have on voltage drop. In this simplified world, his voltage drop was about 1.75V, and the estimated drop is 1.3V. In reality, the IR of the battery system is very dependent on quality of connections, the wires, and connector used, so the estimated drop should be much closer to his original reading, if all those factors were taken into account. Even that calculation would be ignoring the effects of the chemistry of the battery, as current is drawn from it.
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Old Feb 16, 2013, 03:32 PM
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I'm cycling the new 65c battery on the charger now (dont know if its needed but thats what i've always done) so I should have some data this evening. I usually dont use these high c batts but I need to put this little pig of an airplane on a serious diet so I'm reducing the capacity by a bunch which means my amp draw is getting close to the c rating with the 25c.
I have been flying electrics for about 6yrs and never had the ability to check IR. My chargers are doing fine so a stand alone unit would be nice. is there anything out there? something cheaper than a new charger with the feature.
thanks for the information above
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Old Feb 16, 2013, 06:06 PM
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Quote:
Originally Posted by bill clark View Post
I'm cycling the new 65c battery on the charger now (dont know if its needed but thats what i've always done) so I should have some data this evening. I usually dont use these high c batts but I need to put this little pig of an airplane on a serious diet so I'm reducing the capacity by a bunch which means my amp draw is getting close to the c rating with the 25c.
I have been flying electrics for about 6yrs and never had the ability to check IR. My chargers are doing fine so a stand alone unit would be nice. is there anything out there? something cheaper than a new charger with the feature.
thanks for the information above
Bill,

I build these as hobby. Not particularly cheap but accurate, very quick reading and capable of much higher resolution than most chargers.
PM me if you are interested.

http://www.rcgroups.com/forums/showp...&postcount=249

Wayne
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