Originally Posted by Andy120
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 200°F' 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!).