Quote:
Originally Posted by Ether
I enjoyed reading your paper. Nice work.
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Same here!
Quote:
Originally Posted by Ether
Observation:
Figure4 on page 14 looks strange. The voltage at 30s at 200A is lower than the voltage at 15s at 400A.
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Quote:
Originally Posted by Ether
For equivalent drained Ah, the higher current drain rate should have a lower voltage.
200A*30s = 6000As = 1.66Ah
400A*15s = 6000As = 1.66Ah
So the voltage at 400A at 15s at should be lower than the voltage at 200A at 30s. But it's the other way around in the figure.
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I suspect that the high-current-draw tests didn't actually draw the intended current, largely due to the increased voltage drop across the battery internal resistance, and also partly due to heating, which increases resistance in conductors. If the resistors were selected to represent a 12V battery with no internal resistance, the 200A test would use a 60 mohm resistor, and the 400A test a 30 ohm resistor. According to the attached document, a new fresh battery has a nominal internal resistance of 11 mohm. Assuming that the fully charged battery is actually charged to 14V (for negligible load), the current draw for the 200A test case would be 14V / 71 mohm = 197A, not bad at about 1.5% low. For the 400A test case, it would be 14V / 41 mohm = 341A, or about 15% low. Of course, these numbers are only valid when everything's still at room temperature. As the resistors and batteries heat, the 400A test will get hotter than the 200A test, increasing the resistance disproportionately, and further decreasing the current ratio.
Again, let me speak through Ether's words:
Quote:
Originally Posted by Ether
This is just a minor observation and in no way detracts from the impressive accomplishment of the work and the report.
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I will certainly include a lesson strongly referencing this paper in this summer's robocamp. The test descriptions are well done, as are the summaries of the research done prior to testing.