Thread: Discussion Odd Motocalc C rating opinions
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Old Nov 19, 2012, 03:32 PM
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Hastings, New Zealand
Joined Jan 2001
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Originally Posted by Andy120 View Post
Hi, Flopped over from batteries.
Im trying to understand why if i enter a lipo set at say 20C, motocalc will be happy temperature etc wise. If i change the cells to a higher C rating eg. 25C, motocalc returns temperature alerts. I would expect the opposite.
Just changing the C rating should have no effect. It is cell impedance which determines heating (according to the equation: Power = Current squared times Resistance).

When you changed to cells with higher C rating, did they also have lower impedance? They should have. If not then either one of the cells has inaccurate specs in Motocalc, or its C rating is suspect. In a marginal situation it might not take much to trigger the warning, but if one is OK and the other says 'greater than 200F' then its specs are probably wrong.

Real cell impedance varies with current, temperature and state of charge, so a single impedance number cannot be accurate in every situation. Modern High C Lipos can put out higher voltage under load. Manufacturers don't generally provide impedance specs, so where did Motocalc's numbers come from and how accurate are they? I ignore them, set the chemistry to custom (blank), change voltage to 3.85V per cell, and adjust cell impedance to get 3.5V out at the rated current (impedance = 0.35V / capacity * C rating). I also reduce the capacity to 90% (eg. 2000mAh for '2200mAh' cells) because the battery probably can't deliver that last 10%.

Motocalc's battery model is quite simplistic, so in reality the results could be quite different from what it predicts. Provided you keep well within the battery's C rating and keep it cool, it should be OK (assuming the battery is accurately rated!).
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