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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Frank Imbriaco on May 02, 2013, 05:42:50 AM
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I believe that measuring, understanding and applying knowledge of the I.R.(Internal resistance) of my batteries is the single greatest thing I need to learn in order to achieve long battery life and the performance I desire in my electric aircraft ;U/C and R/C.
I treat my batteries very well , but as expected,notice variations between
manufacturers, cell # s and size (mAh). Similar batteries of the same manufacturer are not created equal. Cells are not always matched, etc. , etc.
When does the IR tell us "times up" on this pack ? And no, I don't want to wait to the airplane won't lift of the ground.
I read the R/C forums all the time and very little is said there.
I'm hoping a person such as Dean can enlighten us. ???
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Hi Frank,
Yes, a much longer answer is forthcoming ... but,
1) different constructions/capacities/brands/ etc will "normally" show difference cell resistances.
2) cell resistances drop during the battery break-in period ...
3) assume a relatively stable or slowly rising value with number of charge-discharge cycles ...
4) and start to rise more rapidly at life's end.
5) high temps lower cell resistance and vice-versa. The resistance vs temp relationship is a strong one.
6) comparisons must be done at the same state-of-charge. i suggest the storage voltage.
7) Record keeping is the answer.
later,
Dean
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6a) comparisons must be made at the same internal temperature and state of charge?
It seems like a more pragmatic approach would be to note the charge needed at the end of each flight, along with pack temperature. If you're going to keep records anyway, noting down conditions for each flight ("windy", "calm", etc.) along with charge needed, and how the battery felt when hefted or how the temperature measured, would get you a lot of interesting information about trends (and a lot of other things) without needing sophisticated measurement gear.
Just start keeping a record of each flight, like a friend of mine that does indoor free-flight and notes the size and vintage of the rubber used, the torque wound to, and the flight duration.
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Hi Dean,
Thanks for sharing your insights and knowledge regarding the mysteries of I.R. .
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Ok I'll ask. How are you measuring the internal battery resistance? An ohm meter is out of the question because you'd blow the fuse or meter. You gotta be using ohms law to do the math would be my guess. But what voltage would you use ? The charged voltage or the discharged voltage?
R=EI
resistance is equal to the voltage multiplied by the current
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Hi Bob,
The better chargers do this by pulsing some current either into and then out of the battery (that is back and forth like a small AC current) and then measuring the voltage difference that results.
That AC current is imposed on top of the DC charging current, or in some cases, the test happens after the storage charge is achieved.
Comparing the fully charged resistances is also meaningful, but most often it's done at either half-charge or storage charge.
Yeah, Tim ... it's a drag if you want to do it to the nines.
Fortunately, we don't need to be anal: checking the resistance at room temp, at home after the day's flying, at storage charge, every so often will provide the battery health info we need.
You might also develop a good sense of what to expect for a known, often encountered, condition like reading whatever the charger gives you, after a charge that was started soon after a flight on a nice Summer day.
Comparing those readings will tell a story.
later, Friends ...
Dean